The Lipoprotein LpqW Is Essential for the Mannosylation ofPeriplasmic Glycolipids in Corynebacteria*□S

Received for publication, April 18, 2012, and in revised form, October 14, 2012 Published, JBC Papers in Press, October 22, 2012, DOI 10.1074/jbc.M112.373415

Arek K. Rainczuk‡§, Yoshiki Yamaryo-Botte¶, Rajini Brammananth‡§, Timothy P. Stinear�, Torsten Seemann§**,Ross L. Coppel‡§**1, and Malcolm J. McConville¶1, and Paul K. Crellin‡§1,2

From the ‡Australian Research Council Centre of Excellence in Structural and Functional Microbial Genomics, Monash University,Victoria 3800, Australia, the §Department of Microbiology, Monash University, Victoria 3800, Australia, the ¶Department ofBiochemistry and Molecular Biology, Bio21 Institute of Molecular Sciences and Biotechnology, University of Melbourne, Parkville,Victoria 3010, Australia, the �Department of Microbiology and Immunology, University of Melbourne, Parkville, Victoria 3010,Australia, and the **Victorian Bioinformatics Consortium, Monash University, Victoria 3800, Australia

Background: LpqW regulates synthesis of mycobacterial cell wall lipoglycans via unknown mechanisms.Results: A Corynebacterium glutamicum lpqW mutant has a global defect in lipoglycan synthesis, and LpqW is functionallylinked to the mannosyltransferase MptB.Conclusion: LpqW activates lipoglycan synthesis pathways in C. glutamicum by directly regulating MptB.Significance: These results highlight the regulatory role of lipoproteins in glycolipid biosynthesis.

Phosphatidylinositol mannosides (PIM), lipomannan (LM),and lipoarabinomannan (LAM) are essential components ofthe cell wall and plasma membrane of mycobacteria, includingthe human pathogenMycobacterium tuberculosis, as well as therelated Corynebacterineae. We have previously shown that thelipoprotein, LpqW, regulates PIM and LM/LAMbiosynthesis inmycobacteria. Here, we provide direct evidence that LpqW reg-ulates the activity of key mannosyltransferases in the periplas-mic leaflet of the cell membrane. Inactivation of theCorynebac-terium glutamicum lpqW ortholog,NCgl1054, resulted in a slowgrowthphenotype andaglobal defect in lipoglycanbiosynthesis.The NCgl1054 mutant lacked LAMs and was defective in theelongation of the major PIM species, AcPIM2, as well as a sec-ond glycolipid, termed Gl-X (mannose-�1–4-glucuronic acid-�1-diacylglycerol), which function as membrane anchors forLM-A and LM-B, respectively. Elongation of AcPIM2 and Gl-Xwas found to be dependent on expression of polyprenol phos-phomannose (ppMan) synthase. However, the �NCgl1054mutant synthesized normal levels of ppMan, indicating thatLpqW is not required for synthesis of this donor. A spontaneoussuppressor strain was isolated in which lipoglycan synthesis inthe �NCgl1054 mutant was partially restored. Genome-widesequencing indicated that a single amino acid substitutionwithin the ppMan-dependent mannosyltransferaseMptB couldbypass the need for LpqW. Further evidence of an interaction isprovided by the observation that MptB activity in cell-freeextracts was significantly reduced in the absence of LpqW. Col-lectively, our results suggest that LpqW may directly activateMptB, highlighting the role of lipoproteins in regulating key cellwall biosynthetic pathways in these bacteria.

Bacteria of the Corynebacterineae, a suborder of the Actino-bacteria, include a number of important human pathogens,includingMycobacterium tuberculosis,Mycobacterium leprae,and Corynebacterium diphtheriae (1). These bacteria synthe-size a distinctive multilaminate cell wall composed of pepti-doglycan, complex polysaccharides, and both covalently linkedand free glycolipids and lipoglycans (2). The structure andhydrophobic properties of the mycobacterial cell wall contrib-ute to the intrinsic resistance of these bacteria to an array ofhost microbiocidal processes, many antibiotics, and steriliza-tion conditions (3). Many of the cell wall components of path-ogenic mycobacteria species are essential for pathogenesis andin vitro growth, hampering efforts to characterize the functionof individual genes in their assembly (4–7). In contrast, a num-ber of non-pathogenic Corynebacterineae such as Corynebac-terium glutamicum can tolerate the loss of major cell wall com-ponents, making them useful model systems for delineatingprocesses involved in the assembly of core cell wall structures(8–18).All Corynebacterineae synthesize a family of glycolipids

termed phosphatidyl-myo-inositol mannosides (PIMs)3 thatare important components of both the cell membrane andouter wall layers. Polar PIM species can also function as mem-brane anchors for the lipomannans (LM) and lipoarabinoman-nans (LAMs). These lipoglycans are essential for both the via-bility and in vivo survival of pathogenic mycobacterial speciesand have been shown to have potent immunomodulatory prop-erties (19–22). Many of the steps of PIM/LM/LAM biosynthe-sis have been elucidated (see Fig. 1, and recently reviewed inRefs. 7 and 15). InM. tuberculosis, PIM synthesis is initiated bythe mannosylation of phosphatidylinositol by PimA (Rv2610c)to form PIM1 (23, 24). Next, O-6-mannosylation of the myo-* This work was supported by the Australian Research Council Centre of

Excellence in Structural and Functional Microbial Genomics (COE562063)and the National Health and Medical Research Council of Australia (ProjectGrant ID1007676).

□S This article contains supplemental Fig. 1.1 These authors should be considered as equal senior authors.2 To whom correspondence should be addressed. Tel.: 61-3-9902-9148; Fax:

61-3-9902-9222; E-mail: paul.crellin@monash.edu.

3 The abbreviations used are: PIM, phosphatidylinositol mannosides; LM, lipo-mannan; Gl-A, glucopyranosyluronic acid diacylglycerol; Gl-X, mannose-�1– 4-glucuronic acid-�1-diacylglycerol; ppMan, polyprenol phospho-mannose; BHI, brain heart infusion; HPTLC, high-performance thin layerchromatography.

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inositol ring is performed by PimB� (Rv2188c) (16, 24) resultingin the formation of PIM2. Both enzymes are cytoplasmic�-mannosyltransferases that require GDP-mannose (GDP-Man) as the sugar donor. PIM2 accumulates in the cell envelopemainly in its acylated forms AcPIM2 and Ac2PIM2, the formerproduced by the acyltransferase Rv2611c (25). Acylated PIM2species can be further mannosylated to form more polar PIMs(Ac1/2PIM4-Ac1/2PIM6) and their hyperglycosylated forms(LM and LAM) (26). These reactions are performed by a num-ber of glycosyltransferases that require a lipid sugar donor,C35/C50-polyprenylphosphomannose (ppMan) (26–31). Inmycobacteria, Ac1/2PIM4 is proposed to be a branchpoint lead-ing to the synthesis of polar PIMs and LM/LAM, respectively.PimE (Rv1159) has been shown to elongateAcPIM4with one ormore �1–2-linked mannose residues to form AcPIM6 (31).Alternatively, a subpopulation of AcPIM4 is extended withchains of �1–6 linked mannose to form LM that is furthermodified with a number of single �1–2-mannose side chains(12, 32, 33). Deletion of the C. glutamicum gene encoding theppMan-dependent mannosyltransferase, MptB, prevents ini-tial AcPIM2 elongation, indicating that this mannosyltrans-ferase can prime the synthesis of the LM �1–6-mannan back-bone (17). Interestingly, loss of MptB in Mycobacteriumsmegmatis has little effect on LM synthesis, suggesting redun-dancy in this priming step in more distantly related mycobac-teria (17). The short lipomannans generated by MptB arefurther elongated and side chain-modified by related ppMan-dependent mannosyltransferases MptA (Rv2174) (12, 33) andMptC (Rv2181), respectively (7, 32, 35, 36). Finally, LM is con-verted tomature LAM following the addition of arabinose unitsby EmbC (Rv3793), AftC (Rv2673), AftD (Rv0236c), andunidentified �1–5 arabinofuranosyltransferases (5, 15, 29, 37).In addition to the conserved phosphatidylinositol3 PIM3

LM 3 LAM pathway, some corynebacteria utilize a secondpathway of lipoglycan biosynthesis in which a subpopulation ofLM lipoglycans is assembled on a glucopyranosyluronic aciddiacylglycerol (Gl-A, GlcADAG) glycolipid anchor (see Fig. 1)(16, 18, 38). In this pathway, Gl-A is first mannosylated byMgtA (NCgl0452 in C. glutamicum) forming mannosyl-glucu-ronic acid diacylglycerol (Gl-X, Man-GlcA-DAG), which isextended with further �1–6 (backbone) and �1–2 (side chain)linked mannose residues to produce a distinct population ofLM molecules (38). In species of corynebacteria that have thissecond pathway, the phosphatidylinositol-anchored LMpool istermed LM-A,whereas theGl-A-anchored LM is termed LM-B(39). The extension of Gl-X and PIM species involves commonppMan-dependent mannosyltransferases. In particular, dele-tion of the C. glutamicum gene encoding MptB (NCgl1505)results in the accumulation of AcPIM2 andGl-X, and a block inboth LM-A and LM-B biosynthesis (17).LpqW is a putative lipoprotein that is highly conserved in

members of Corynebacterineae. The M. tuberculosis lpqWgene (Rv1166) is essential for viability (40), whereas loss of theM. smegmatis ortholog (MSMEG_5130) results in a defect inLM/LAM biosynthesis (41–43). AnM. smegmatis lpqW trans-poson mutant had a reduced capacity to synthesize LM/LAMbut had a normal spectrumof polar PIMs (41). Thismutant wasunstable and readily accumulated secondary mutations in

pimE, which resulted in a block in synthesis of polar PIMs andrestored synthesis of LMandLAM(43). The crystal structure ofM. smegmatis LpqW contains a putative AcPIM4 bindingpocket, raising the possibility that LpqW may function as aglycolipid chaperone, regulating access of AcPIM4 to eitherPimE and/or the elongating ppMan-dependent mannosyl-transferases (42, 44).To further investigate the function of LpqW here, we have

deleted the lpqW ortholog, NCgl1054, in C. glutamicum.C. glutamicum is an excellent experimental system for investi-gating the potential regulatory function of LpqW in LM/LAMsynthesis as (i) it is more accepting of loss of cell wall compo-nents than Mycobacterium spp., (ii) it lacks polar PIMs and apimE ortholog (43), simplifying analysis of its role in LM versusPIM biosynthesis, and (iii) this species synthesizes both LM-AandLM-B, allowing dissection of the role of LpqW in regulatingdifferent classes of glycolipid anchor into the common LMpathway (Fig. 1). We show here that deletion of NCgl1054results in a global defect in the elongation of both AcPIM2 andGl-X. Although similar to the biochemical defect induced bydisruption of ppMan synthesis, we show that LpqW is notrequired for synthesis of this donor. Significantly, loss of LpqWcan be bypassed by substitution of a single amino acid residue inthe ppMan-dependent mannosyltransferase MptB, leading toreactivation of lipoglycan pathways in the NCgl1054 mutant.Loss of LpqW was also associated with loss of MptB activity ina cell-free cell envelope assay. Collectively, our findingsstrongly suggest that LpqW directly regulates the activity ofMptB thereby controlling the elongation of both the AcPIM2and Gl-X membrane anchors.

EXPERIMENTAL PROCEDURES

Bacterial Strains, Culture Conditions, Transformation, andGenetic Manipulation—Escherichia coli DH5� strain wasgrown in Luria-Bertani (LB) medium at 37 °C with aeration.

FIGURE 1. Pathways of glycolipid biosynthesis in C. glutamicum. Early PIMintermediates are assembled on the cytoplasmic leaflet of the cell membraneby GDP-Man-dependent mannosyltransferases before being transported(“flipped”) to the periplasmic leaflet where they are further elongated by afamily of ppMan-dependent mannosyltransferases, that includes MptB, toform LM-A. LM-A is further modified by a second family of arabinosyltrans-ferases to form LAM. The ppMan donor is synthesized by the cytoplasmicallyorientated enzyme, Ppm1, and must also be transported to the periplasmicleaflet. An alternative family of lipomannans, termed LM-B, are assembled ona structurally distinct glycolipid anchor that contains the core structure Gl-A.Gl-A is initially extended with a single mannose residue by the cytoplasmi-cally orientated enzyme MgtA and subsequently elongated by MptB andother ppMan-dependent mannosyltransferases to form LM-B. PI,phosphatidylinositol.

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C. glutamicumATCC 13032 was grown in brain heart infusion(BHI)medium (Oxoid) or LBHIS (LB, brain heart infusion, sor-bitol) (45) at 30 °C with aeration. When necessary, ampicillinwas added to a final concentration of 100�gml�1 and kanamy-cin at 50 �gml�1. E. coli plasmid DNAwas isolated from 10mlof an overnight culture using the High Pure plasmid isolationkit (Roche) and C. glutamicum genomic DNA was extractedfrom �0.5 g of cells using the Illustra DNA extraction kit (GEHealthcare), according to the manufacturer’s instructions.When necessary, DNA was purified using an UltraClean 15DNApurification kit (MoBio). The concentration and purity ofDNA was assessed using a NanoDrop ND-1000 spectropho-tometer (Nanodrop Technologies). PCR reactions were per-formed in a PTC-200 thermal cycler (MJ Research) using Taqpolymerase (Roche) or ProofStart DNA polymerase (Qiagen).Initial denaturation of template DNA was done at 95 °C for 5min, followed by 35 cycles of denaturation at 94 °C for 1 min, a1-min primer-specific annealing step, and a 1-min per kbextension step at 72 °C. The program included a 10-min finalextension step at 72 °C. PCR products were purified by extrac-tion from 1% agarose gels using an UltraClean 15 DNA purifi-cation kit (MoBio). Endonucleases, T4 ligase, polynucleotidekinase system, and alkaline phosphatase were obtained fromNewEngland Biolabs and used according to themanufacturer’sinstructions.Bioinformatic Identification and Analysis of Corynebacterial

LpqW—The corynebacterial ortholog of mycobacterial LpqWwas found using the tBLASTn (46) algorithmon theNCBIweb-site using the protein sequences of Rv1166 andMSMEG_5130.The Genome Region Comparison tool located on JCVI websitewas used to visualize synteny in the region and to identifyorthologs in related species.Construction of C. glutamicum�NCgl1054 and Complemen-

tation Studies—The ortholog of lpqWwas deleted using a two-step recombination strategy previously used successfully in ourlaboratory (11, 16). A 2.4-kb fragment containing the entireNCgl1054 gene was amplified using ProofStart DNA polymer-ase (Qiagen) and the primers NCgl1054For (5�-GCTC-TAGACGTATTCCTGCTCGTGGCCTG) and NCgl1054Rev(5�-CCCAAGCTTATGCGTTGCTCGCCGGCTGC) and clonedinto the XbaI/HindIII sites (underlined) of pUC19 (47). A1.1-kb EcoRI fragment internal to the gene was excised, theremaining plasmid was religated, and the deletion cassettewas subcloned into XbaI/HindIII-digested pK18mobsacB, asuicide plasmid in C. glutamicum (48), which contains kana-mycin and sucrose selection markers. The resultant plasmid,pK18mobsacB:�NCgl1054, was sequenced then electroporatedinto electrocompetent C. glutamicum cells, prepared asdescribed previously (45), using an ECM 630 electroporator(BTX). Clones resulting from single homologous recombina-tion events were selected on kanamycin. These were grownovernight without antibiotic selection and then serially dilutedand plated onto LBHIS plates containing 10% sucrose to selectfor a second crossover event. Small, sucrose-resistant and kana-mycin-sensitive colonies were screened by PCR usingNCgl1054For and NCgl1054Rev primers. Southern blothybridization (see below) was used to confirm the initial singlecrossover and NCgl1054 deletion strains.

To complement the �NCgl1054 strain, the entire NCgl1054gene together with 113 bp of upstream sequence was PCR-amplified using primers NCgl1054CompF (5�-ATATGGAT-CCAAGGAGATATAGATTTGGGGGTGAGAATAAGGTT)and NCgl1054CompR (5�-ATATGAGCTCAGATCATTCTT-CAACATCGT) and cloned into BamHI/SacI sites (underlined)of pUC19, followed by subcloning into the unique PvuII site ofpSM22 (49), which contains the corynebacterial origin of rep-lication repA and kanamycin resistance gene aphA3. Asequenced complementation plasmid (pSM22:NCgl1054) andpSM22 control plasmid were then electroporated into theC. glutamicum �NCgl1054 deletion strain, followed by selec-tion on kanamycin-supplemented BHI plates.Construction of C. glutamicum �ppm1 and Complementa-

tion Studies—A ppm1 knock-out strain was generated using asimilar strategy. In this case, the deletion cassette was con-structed by cloning two gene flanking regions obtained usingprimers ppm1_L_F (5�-CTAGGATCCTTGCTATCGGCGC-GGTGTCATCC) and ppm1_L_R (5�-ATCTCTAGAACC-AGCGTCGTAGCATCTACTGC), and ppm1_R_F (5�-ATCT-CTAGAACTGTCCAAGGAAATGGTCG) and ppm1_R_R(5�-TACAAGCTTGGCTGCAGTATTTCC) primer pairs.Purified PCR products were cloned sequentially into theBamHI/XbaI/HindIII sites (underlined) of pUC19 followed bysubcloning of the BamHI/HindIII fragment into pK18mobsacB.The resultant plasmid. pK18mobsacB:�ppm1, was electropo-rated into C. glutamicum, and the �ppm1mutant was derivedusing the method described above for the �NCgl1054 strain. Adeletion mutant was detected by PCR analysis using primersppm1_F (5�-ATTGATATCAAGGACACTGTCACCATCGC)and ppm1_R (5�-ATTGATATCACCGGCAAGCAGTTTAG-AGC) and then confirmed by Southern blotting.Southern Hybridization—For Southern blot analysis, 2 �g of

genomic DNA was digested with selected restrictionenzymes under optimal conditions for 16 h. To ensure com-plete digestion, additional units of endonuclease were added,and incubation was continued for another 3 h. Purified sam-ples, and digoxygenin-labeled, HindIII-digested � DNA mark-ers, were separated on a 1% agarose gel followed by depurina-tion, denaturation, neutralization, and capillary transfer onto anylon membrane. The membrane was then hybridized at 65 °Cwith a gene-specific probe prepared by digoxygenin labeling aPCR product obtained using primers NCgl1054CompF/NCgl1054CompR for the �NCgl1054 strain.Compositional Cell Wall Analysis—Wild-type (WT) and

mutant cells were harvested by centrifugation at logarithmicgrowth phase (A600 of between 1 and 3) from 100 ml of cultureinBHI. Pelletswere freeze-dried,weighed, and stored at�20 °Cuntil needed. Cells were delipidated by agitation in 6ml of chlo-roform:methanol (2:1, v/v) for 2 h at room temperature twicefollowed by extraction in chloroform:methanol:water (1:2:0.8,v/v). Supernatants were collected, dried under a N2 stream, and1-butanol:water partitioned as follows. The dried lipid fractionwas resuspended in 400 �l of water-saturated 1-butanol and200 �l of 1-butanol-saturated water followed by vortexing andbrief centrifugation. The top (organic) phase was collected,and the process was repeated. The final extract was dried under

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vacuum and resuspended in 5 �l/0.01 g (dry weight) of water-saturated 1-butanol.The delipidated cell pellet was then subjected to ethanol

reflux by suspending in 5 ml of 50% (v/v) ethanol and incubat-ing at 100 °C for 2 h, with occasional vortex mixing. Sampleswere centrifuged at 300 � g for 5 min, the supernatant wascollected, and the procedure was repeated twomore times. Pel-lets were used for corynomycolic acid extraction. The superna-tantwas pooled, and ethanolwas blownoff under a streamofN2followed by freeze-drying. The resultant samples were resus-pended in 400 �l of water containing 0.02% (w/v) CaCl2 and 10units of proteinase K followed by incubation at 37 °C for 2 h.The digest was diluted with 50% propan-1-ol (50 �l) and 1 M

ammonium acetate (50 �l) and loaded onto a column of octyl-Sepharose (1 ml) equilibrated in 5% 1-propanol and 50 mM

ammonium acetate. After washing the column, lipoglycanswere elutedwith 30, 40, 50, and 60% 1-propanol (1-ml volumes)and carbohydrate-containing fractions identified by spotting5-�l aliquots on high-performance thin layer chromatography(HPTLC) sheets and stained with orcinol-H2SO4 (50). Lipogly-can-containing fractions (in 30% to 40% propan-1-ol fractions)were pooled, dried under vacuum, and resuspended in 30%(v/v) propan-1-ol for further analysis.HPTLC Analyses—Extracted lipids were analyzed on alumi-

num-backedHPTLC silica sheets (Merck) in chloroform:meth-anol: 13 M ammonia:1 M ammonium acetate:water (180:140:9:9:23, v/v). Glycolipids were detected with orcinol in HCl andcharring at 110 °C. Glycolipid bands were purified by HPTLCand eluted from silica scraping with 500 �l of chloroform/methanol/water (3:3:1, v/v/v) three times. Pooled extracts weredried under N2 stream, resuspended in butanol, and stored at�20 °C.Polyacrylamide Gel Electrophoresis of LM/LAM—Purified

lipoglycan fractions were mixed with PAGE sample buffer andincubated at 100 °C for 5 min. Samples were then separated ona 15%polyacrylamide gel followed by fixing for 45min in 100mlof 40% (v/v) methanol and 10% (v/v) acetic acid, incubation for10 min in 100 ml of 40% (v/v) methanol, 10% (v/v) acetic acidand 0.7% (v/v) periodic acid, followed by a 10-min wash in 100ml of 5% (v/v) methanol and 7% (v/v) acetic acid, and 5 min in2.5% (v/v) glutaraldehyde. The gelwas then rinsed in purewaterfour times and incubated for 10 min with DTT solution fol-lowed by staining with the SilverSnap Kit (Pierce). The stainingreaction was stopped by washing the gel in 5% (v/v) acetic acidsolution. Sugar content was also quantified by GC-MS aftermethanolysis and trimethylsilyl derivatization (50).Mass Spectrometric Analysis—HPTLC-purified lipids were

resuspended in 1-butanol and diluted in 10 mM ammoniumformate inmethanol (1:10, v/v). Samples were flow injected (0.2ml/min) with the solvent system H2O/MeOH/THF (7:10:33,v/v/v) into the electrospray source of anAgilent 6460 triplequa-drupole LC-MS (Agilent Technologies). A capillary voltage of4000 V, gas temperature of 300 °C, and flow rate of 7 liters/minwere used. The sheath gas temperature and gas flow was set to220 °C and 7 liters/min, respectively. The nebulizer was set at15 psi. Total ion scan was performed in positive mode at frag-mentator voltage 120 V. For MS/MS experiments, Gl-X [M �NH4]� m/z � 950.1,m/z � 1112.9, andm/z � 1274.9 are frag-

mented with a fragmentator voltage of 135 V or 220 V andcollision energy of 55 eV.Metabolic Radiolabeling of Whole Cells—Mid-log phase cul-

tures were harvested by centrifugation, gently resuspended inprewarmed PBS (800 �l), and incubated at 30 °C for 30 min.Cells were pulse labeled by adding [2-3H]-mannose (50�Ci/ml;PerkinElmer Life Sciences). After 5 min (30 °C), an aliquot ofsample (200�l) was removed, centrifuged at 14,000� g, and thesupernatant was frozen in liquid nitrogen. The remaining sam-ple was centrifuged (14,000 � g, 30 s), the supernatant wasdiscarded, and cell pelletswere gently resuspended in 1500�l ofprewarmed BHI. Sample aliquots (500 �l) were removed andfrozen after 1-, 5-, and 30-min incubation in unlabeledmedium(chase). Lipids were extracted from the frozen pellets and ana-lyzed by HPTLC as described above. Labeled species weredetected either with a TLC linear analyzer (Berthold) or aftercoating HPTLC sheets with EN3HANCE Spray (PerkinElmerLife Sciences) and exposure to Kodak BioMaxMR film (Sigma-Aldrich) at �80 °C.Mannosidase Treatment—Samples were dried under vac-

uum, resuspended in 2 �l of 1% taurodeoxycholate, and incu-bated with 20 �l of jack-bean �-mannosidase (Sigma) in 0.1 M

sodium acetate, pH 5.0, at 37 °C. An additional 20 �l of manno-sidase was added after 24 h, and samples were incubated foranother 24 h. Lipids were extracted with 80 �l of chloroform:methanol (2:1, v/v) and recovered by butanol partitioning.Cell Envelope Cell-free Assay—Wild-type andmutant strains

were grown in BHI medium at 30 °C to mid-log phase (A600 �4–6), and cell pellets were washed twice in 25 mM HEPES-NaOH (pH 7.4) prior to being suspended at 0.15 g wet pellet/mlin lysis buffer containing 50 mM HEPES-NaOH (pH 7.4), 2 mM

EGTA, and protease inhibitor mixture (Roche Diagnostics).The cell suspension was sonicated on ice with a taperedmicrotip 1⁄4 using anUltrasibuc processor (750W,Cole-Palmer;1-s pulse, 1-s pause for a total of 10min at 40% amplitude). Theresultant lysate was centrifuged twice at 1000 � g, (4 °C, 10min) to remove cellular debris. The supernatant was ultracen-trifuged at 100,000 � g at 4 °C for 1 h using a Beckman coulterTLA 120.2 rotor. The resultant pellet, containing both cell walland cell membrane, was resuspended in a one-tenth volume ofthe lysis buffer and recovered as the cell envelope fraction (26).Protein concentration was measured using Bradford reagent(Bio-Rad).The cell envelope fraction (0.5 mg protein) was supple-

mented with 5 mM MgCl2 and 5 mM �-mercaptoethanol andpreincubated for 5 min at 30 °C prior to addition of GDP-[3H]-mannose (230,000 cpm; �2 �Ci) and further incubated for 180min. The reaction was stopped by adding a 20 times volume ofchloroform/methanol 2:1 (v/v). Total lipid was extracted asdescribed above.Genome Sequencing and Bioinformatics Analyses—Whole

genome sequencing of C. glutamicum isolates was achievedusing Ion Torrent semi-conductor sequencing on the PGMplatform, with a single 316 chip with 100 bp chemistry for eachisolate, following the manufacturer’s instructions (Invitrogen).Resulting sequence reads were mapped to aC. glutamicum ref-erence genome (accession no. NC_003450.3) in the NCBI col-lection, using an in-house Python utility called Nesoni, which

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uses SHRiMP2 (51). A global variant analysis was then per-formed and allelic variability at any nucleotide position wastallied to generate a list of differences for each genome com-pared with the reference. Sequence reads have been depositedin the NCBI Sequence Read Archive under accession no.SRA057127.

RESULTS

Identification of Corynebacterial lpqW—An ortholog of themycobacterial lpqW gene was identified in C. glutamicumATCC13032 using a tblastn search of LpqWprotein sequencesfrom M. tuberculosis H37Rv (Rv1166) and M. smegmatismc2155 (MSMEG_5130). The bestmatchwasNCgl1054, whichencodes a protein 502-residues-long, sharing 21.2% identityand 32.4% similarity with mycobacterial LpqW. Syntenic anal-ysis revealed that corynebacterial lpqW is found in the samegenetic context as in other analyzed mycobacterial genomes(Fig. 2), further supporting NCgl1054 as the most likelyortholog of lpqW in C. glutamicum.Inactivation of the NCgl1054 Gene—A two-step recombina-

tion strategy was used to inactivate NCgl1054. Deletion of a1.1-kb internal fragment was achieved using the suicide vectorpK18mobsacB (49), carrying a kanamycin resistance gene (aph)and Bacillus subtilis sacB gene conferring sensitivity to sucrose(Fig. 3A). Successful deletion ofNCgl1054was detected by PCR(data not shown) and then confirmed by Southern blottinganalyses (Fig. 3B).Growth Characteristics and Complementation of C. glutami-

cum �NCgl1054—The mutant was observed to form relativelysmall colonies (�1 mm) on BHI plates after 24 h incubation at30 °C.A slow growth phenotypewas also observed in liquidBHI(Fig. 4). The NCgl1054 mutant grows at about half the rate ofthe WT parent (doubling time of �100 min) under these cul-ture conditions. To complement themutation,�NCgl1054wastransformed with pSM22:NCgl1054, a plasmid carrying anintact NCgl1054 gene with 113 bp of the upstream sequencethat was expected to include the native promoter. UnmodifiedpSM22was also introduced into�NCgl1054 as a control. Intro-duction of pSM22:NCgl1054 but not pSM22 restored colonysize and growth rate to nearlyWT levels (Fig. 4), demonstratingthat the growth defect was due to deletion of the NCgl1054gene.Cell Wall Analyses of C. glutamicum �NCgl1054 and Com-

plementation Strains—Cell wall components of mutant andcomplemented strains were analyzed and compared with the

parental WT strain C. glutamicum ATCC 13032. No changeswere found in the levels or composition of mycolic acids or cellwall arabinogalactan (data not shown). However, the mutantwas shown to have an altered glycolipid profile when total lipidextracts were analyzed by HPTLC (Fig. 5A). First, the mutantstrain accumulated higher levels of AcPIM2 and Gl-X. Thesespecies run as a doublet in the solvent systemused in Fig. 5A butcould be resolved in an alternative solvent system, revealingaccumulation of both species relative to trehalose corynomyco-lates that serve as internal controls (supplemental Fig. 1). Thesespecies co-migrated with authentic standards, and their identi-ties were confirmed by LC-MS/MS analysis. Specifically, theHPTLCband assigned asAcPIM-2/Gl-X contained a glycolipidspecies with a [M � NH4]� ion ofm/z 950.1, corresponding toGl-X (Man-GlcA-DAG). MS/MS of this ion produced a frag-ment ion atm/z 577.4 corresponding to a diacylglycerol (C34:1DAG) species after neutral loss of a Hex-HexA disaccharide.Secondly,WTbacteria expressed two additional glycolipid spe-cies, termed glycolipid A and B, which were not detected in�NCgl1054. LC-MS analysis of HPTLC fractions enriched forglycolipids A and B gave [M � NH4]� ions of m/z 1112.9 and1274.9, which correspond to ammonium adducted molecularions for Hex2HexA-DAG and Hex3HexA-DAG, respectively(Fig. 5B). Both species contained the same DAG moiety (34:1)as Gl-X, because MS/MS of these ions gave fragment ions at577.6/577.3 (Fig. 5B). These species, designated Gl-Y and Gl-Z,weremetabolically labeled with [14C]acetate and were sensitiveto Jack bean �-mannosidase digestion (Fig. 5C), suggesting thatthey correspond to Man2GlcA-DAG and Man3GlcA-DAG,respectively. The absence of Gl-Y and Gl-Z species in themutant indicated that NCgl1054 is required for the elongationof Gl-X with mannose residues.PIM species have been shown to function as membrane

anchors for LM-A and LAM lipoglycans, whereas mannosy-lated forms of Gl-X function as membrane anchors for LM-B(18). To further investigate the consequences of LpqW disrup-tion on lipoglycan biosynthesis, total lipoglycans were purifiedfrom WT, �NCgl1054, and the complementation strains andanalyzed by PAGE and GC-MS (Fig. 6). As expected, WT bac-teria produced two lipoglycan populations corresponding toLM (LM-A and LM-B) and LAM (Fig. 6, A and B). In contrast,the �NCgl1054 strain and �NCgl1054 bearing a control plas-mid lacked both lipoglycan classes. LM/LAM biosynthesis wascompletely restored in the �NCgl1054 strain carrying a func-

FIGURE 2. Comparison of the lpqW gene region of M. tuberculosis, M. smegmatis, M. leprae, and C. glutamicum, based on protein sequence similarity.Orthologs of Rv1166 (lpqW) are shown in black, regions containing no matches are indicated by white arrows, orthologs of Rv1165 (a gene with a proposed rolein chemotaxis and motility) are indicated by dark gray arrows, orthologs of Rv1170 (involved in mycothiol biosynthesis (34)) shown by light gray arrows.Sequence similarity scores are given in (% identity)/(% similarity) of protein sequences when compared with Rv1166 (not to scale).

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tional copy of the gene. These results suggest that loss of LpqWresults in a global lipoglycan defect arising from loss of synthe-sis of mannosylated Gl-X and polar PIM glycolipids that arerequired for cell wall lipoglycan biosynthesis.

Synthesis of Polar LM-B Precursors Is Dependent on ppManSynthase—The synthesis of Gl-A and Gl-X is thought to occuron the cytoplasmic face of the cell membrane (Fig. 1). In partic-ular, the conversion of Gl-A to Gl-X is catalyzed by the GDP-Man-dependent mannosyltransferase, MgtA (Fig. 1). WhetherGl-X is further elongated to Gl-Y or Gl-Z by MgtA or otherGDP-Man-dependent mannosyltransferase or by ppMan-de-pendent mannosyltransferases, such as those involved in LMbiosynthesis (Fig. 1) has not been defined (12, 17, 31, 32). Toinvestigate whether loss of ppMan synthase also leads to dis-ruption of mannosylated Gl-X biosynthesis, we generated aC. glutamicum mutant lacking Ppm1 (27, 52) by deleting a748-bp internal fragment of the ppm1 gene using the same two-step recombination strategy that we used to create the�NCgl1054 strain. The integration event and disruption of thegenewas detected by PCR analysis using ppm1-specific primers(data not shown) and then confirmed by Southern hybridiza-tion (Fig. 7). The resultant strain, termed �ppm1, formed smallcolonies on agar plates and had a reduced growth rate compa-rable with the�NCgl1054mutant (Fig. 4), suggesting that thosestrains had similar fitness.WT,�NCgl1054, and�ppm1 strains were pulse labeled with

3H-mannose for 5 min and label-chased over 30 min. Sampleswere taken with rapid quenching, and purified lipids were ana-lyzed by HPTLC (Fig. 8). [3H]Mannose was rapidly incorpo-rated into ppMan and a spectrum of PIM species (PIM1, PIM2,AcPIM2) in wild type bacteria. Subsequently, label was chasedout of ppMan and into mannosylated Gl-X species. Althoughlabel in PIM1 and AcPIM1 was also rapidly chased, label inAcPIM2 was only partially chased (Fig. 8), consistent with thisspecies being ametabolic end-product as well as a precursor forLM-A/LAM synthesis. In marked contrast, label was onlyincorporated into PIM1, AcPIM1 and AcPIM2 in the �ppm1mutant (Fig. 8,middle panel). The mannosylated Gl-X species,Gl-Y and Gl-Z were not detected, and only label in PIM1 spe-cies was chased. The same PIM species were labeled in the

FIGURE 3. Disruption strategy and analysis of �NCgl1054. A, diagramshowing the arrangement of genes in the NCgl1054 region of wild-type C. glu-tamicum. Below, the fragment amplified for cloning into pK18mobsacB Xba/HindII sites is shown with the EcoRI sites used to delete the 1.1-kb fragmentinternal to NCgl1054. Above, SalI digestion sites, sizes of expected bands onSouthern blot, and position of the probe used are shown. NCgl1054 and sur-rounding sequences were amplified by PCR and cloned into pUC18, then a1.1-kb EcoRI internal fragment of the gene was removed, and the deletioncassette was subcloned into a suicide plasmid pK18mobsacB carrying thesacB gene. Kanamycin-sensitive and sucrose-resistant clones were tested forthe second crossover event by PCR and Southern blot. B, Southern blot anal-ysis of SalI-digested DNA of C. glutamicum WT, a single crossover strain andthe NCgl1054 mutant. SalI endonuclease has a restriction site within thedeleted section of the gene; expected band sizes are shown in A. Lanes: Mark-ers, digoxygenin-labeled � DNA standards digested with HindIII; WT, wild-type C. glutamicum ATCC 13032; sco, single cross-over strain of C. glutamicumcarrying a copy of pK18mobsacB:NCgl1054 integrated into the NCgl1054locus; �NCgl1054, C. glutamicum NCgl1054 deletion mutant.

FIGURE 4. Growth characteristics of �NCgl1054 and complementationstrains. WT and NCgl1054 mutant strains were inoculated in BHI and bacterialgrowth at 30 °C monitored using A600 over 24 h. Results from three independ-ent cultures were plotted, and error bars represent S.E. Growth rate and col-ony size were fully restored in the cells carrying pSM22:NCgl1054, a shuttleplasmid containing the NCgl1054 gene and upstream sequences expected tocontain its native promoter.

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�NCgl1054 strain, indicating a defect in the synthesis ofppMan-dependent PIM and Gl-X species. Significantly, the�NCgl1054mutant synthesized similar levels of ppMan to wildtype bacteria, although the rate of chase of label out of ppMan,as well as PIM1 and AcPIM1, was dramatically slowed in the�NCgl1054 strain (Fig. 8, far right panel). Collectively, theseresults provide further evidence that LpqW is required for the

elongation of both Gl-X and AcPIM2 species. Although thesereactions are dependent on the presence of ppMan, LpqW isnot required for the synthesis of this mannose donor, suggest-ing that it regulates Gl-X/AcPIM2 elongation via an independ-ent mechanism.A Mechanism for Bypassing lpqW in C. glutamicum—We

have previously shown that secondary mutations in the

FIGURE 5. C. glutamicum �NCgl1054 lacks two glycolipids detected in wild type bacteria. A, total glycolipids were analyzed by HPTLC and detected withorcinol-H2SO4 staining (left panels). Lipids with a slower HPTLC mobility than AcPIM2/Gl-X (boxed) were purified and reanalyzed (right panels). TL, total lipids; Fr,fractions. B, individual glycolipid species were HPTLC-purified and analyzed by LC-MS in positive ion mode. The identity of Gl-X was confirmed by detection of[M � NH4]� at 950.3 and fragment ions at m/z 577.4 (loss of Hex.HexA). Glycolipid species A and B (present in WT but missing in �NCgl1054) had [M �NH4]� � 1112.9 and [M � NH4]� � 1274.9, respectively, consistent with the presence of Gl-A (Hex.HexA.DAG) and Man-Gl-A (Hex2.HexA.DAG). All of theselipids are based on diacylglycerol (34:1) and contain both 18:1 and 16:0 fatty acids. The mass difference from Gl-X m/z � 950.1 to m/z � 1112.9 and m/z 1112.9to m/z � 1274.9 are 162.9 and 162.0, respectively. C, C. glutamicum WT and C. glutamicum �NCgl1054 mutant were labeled with [14C]acetate, and extractedglycolipids were treated with Jack Bean �-mannosidase (JB�M) (�). �, untreated controls prior to HPTLC analysis. Arrows indicate Jack Bean �-mannosidase-sensitive glycolipids that are not present in the mutant strain.

FIGURE 6. Analysis of lipoglycans extracted from delipidated cells. A, total sugar content of purified lipoglycan fraction was determined by GC-MS. Valueson the y axis represent combined levels of mannose, rhamnose, and arabinose. Error bars represent S.E. from two independent experiments. Labels on x axis areas follows: WT, C. glutamicum wild-type; �NCgl1054, mutant lacking 1.1-kb internal fragment of NCgl1054; �NCgl1054�pSM22, mutant complemented withempty pSM22 vector; �NCgl1054�pSM22:NCgl1054, mutant complemented with a functional copy of NCgl1054; �ppm1, ppm1 deletion mutant. B, PAGEanalysis of LM/LAM fraction extracted from various strains. Polar glycolipids were extracted from delipidated cells by ethanol reflux and purified on octyl-Sepharose column, followed by proteinase K treatment. Samples were separated on 12% PAGE and stained with SilverSNAP kit. Strains are labeled as in A. Std.,protein molecular weight standard (Invitrogen). NCgl1054 mutant fails to synthesize detectable levels of lipoglycans, whereas complementation with afunctional copy on the gene restored lipoglycan synthesis to near wild-type levels.

FIGURE 7. Construction of a �ppm1 mutant of C. glutamicum. A, NCgl1423 (ppm1), encoding one component of the polyprenol-mannose synthase, wasdisrupted by homologous recombination between the locus and pK18mobsacB:�ppm1, which carries �1-kb sequences (gray blocks) flanking a 748-bpfragment internal to ppm1. B, Southern hybdridization analysis of the �ppm1 mutant. Genomic DNA of wild-type C. glutamicum, a single crossover strain (sco),and the �ppm1 mutant was digested with BamHI and subjected to Southern blotting using a ppm1-specific probe (dashed line). Expected bands were 4.8 kbfor the wild-type and 4.0 kb for the mutant. Markers, digoxygenin-labeled � DNA standards digested with HindIII; WT, wild-type C. glutamicum ATCC 13032;�ppm1, C. glutamicum ppm1 deletion mutant; sco, single crossover strain of C. glutamicum carrying a copy of plasmid pK18mobsacB:�ppm1 integrated at theppm1 locus.

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M. smegmatis pimE gene can restore LM/LAM synthesis fol-lowing disruption of lpqW in this species (43). AlthoughC. glu-tamicum lacks a PimE ortholog, we were interested to deter-mine whether spontaneous suppressor mutants could arise inthe �NCgl1054 strain leading to restored LM synthesis andgrowth rate. To promote the out-growth of suppressor strains,�NCgl1054 bacteria were diluted 1:10 in 10-ml BHI broths at30 °Cwith shaking for 24 or 48h for a total of 10 passages.Whenaliquots from each passage were plated on BHI agar, putativesuppressor strains (based on wild type colony size) were iden-tified after passages 4–6. Analysis of one of these colonies, col-lected after four passages, revealed partial restoration of lipo-glycan synthesis (Fig. 9A). Specifically, this suppressor/bypassstrain expressed appreciable level of LM and LAM as well aswild type levels of polar glycolipid species (Fig. 9B). Wholegenome sequencing and comparison to the parental�NCgl1054 strain identified a single base pair change (A to G)within themptB gene (NCgl1505), encoding the ppMan-depen-dentmannosyltransferase,MptB. This mutation is predicted toproduce a tyrosine to cysteine substitution at residue 507 ofMptB that is located in the middle of the second last predictedtransmembrane domain of the polytopic membrane protein.These data suggested that LpqW may either regulate the

activity of MptB or the access of glycolipid precursors to MptBand that the suppressormutation removes this requirement. Todistinguish between these possibilities, cell-free assays wereperformed using cell envelope extracts from WT cells, the�NCgl1054 mutant and the bypass mutant. Following incuba-tion withGDP-[3H]mannose,MptB activity was inferred by theappearance of labeled Gl-Y/Gl-Z. If LpqW is involved in regu-lating substrate accessibility to MptB, we would predict thatloss of LpqW would have little effect on MptB activity in theabsence of compartmentalization. If, on the other hand, it isdirectly involved in regulating MptB activity, loss of LpqWshould result in loss of activity in the mutant and restoration inthe bypass strain. As shown in Fig. 9C, Gl-Y/Gl-Z biosynthesiswas detected in cell envelope preparations of WT and the

bypass mutant but was absent in the �NCgl1054mutant prep-aration. These findings support a role for LpqW in the activa-tion of MptB and indicate that the Y to C substitution in thebypass mutant renders MptB partially independent of LpqWactivation.

DISCUSSION

We have previously shown that the lipoprotein LpqW has arole in the biosynthesis of hyperglycosylated PIM species suchas LM and LAM in mycobacteria. Specifically, disruption oflpqW in M. smegmatis leads to reduced LM/LAM synthesiswithout affecting the synthesis of PIMs (41). This mutant israpidly overgrown by suppressor strains in which LM/LAMsynthesis is restored at the expense of polar PIM biosynthesisdue to mutations in the ppMan-dependent mannosyltrans-ferase, PimE (43). Based on these findings, we hypothesizedthat LpqW functions to regulate either polar PIM and/orLM/LAM biosynthesis. In this study, we extend these obser-vations to show that LpqW has a direct role in regulating theelongation of diverse glycolipid anchors by ppMan-depen-dent mannosyltransferases.We investigated the function of the lpqW ortholog in C. glu-

tamicum because of the following: 1) the organism is more per-missive to loss of cell wall components than Mycobacteria spp,increasing the likelihood that the knock-out would have amorestable phenotype, 2) the organism has a relatively simple PIMprofile, composed primarily of PIM2/AcPIM2 and lacks a rec-ognizable PimE ortholog, and 3) the organism accumulates asecond family of glycolipids that are structurally distinct fromthe PIMs, allowing analysis of the selectivity of LpqW func-tion. The C. glutamicum lpqW ortholog exhibits significantsequence similarity (�32%) to theM. tuberculosis andM. smeg-matis lpqW genes and is located in the same genetic context.Interestingly, five of the six residues proposed to constitute abinding pocket in the M. smegmatis LpqW (42) are conservedin the C. glutamicum protein, despite its reduced size and rela-tively low sequence similarity with a mycobacterial ortholog.

FIGURE 8. Glycolipid biosynthesis in C. glutamicum WT, �NCgl1054, and �ppm1. Bacteria were pulse labeled with [3H]mannose for 5 min (pulse) and thensuspended in BHI medium and sampled at 1, 5, and 30 min (Ch1, Ch5, Ch30). In WT bacteria, [3H]Man is rapidly incorporated into ppMan and subsequentlychased into PIM and Gl-A species. In the ppm1 mutant, labeling is only incorporated into early PIM and Gl-X species. Gl-X species modified with mannose werenot detected. In the NCgl1054 mutant, ppMan was strongly labeled during the pulse, whereas labeling was slowly chased from this species. No labeling wasdetected in the polar Gl-X species.

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Disruption ofNCgl1054 resulted in a slow growth phenotypethatwas complemented by episomal expression ofNCgl1054.Asimilar slow growth phenotype was observed following trans-poson-disruption of M. smegmatis lpqW (41), supporting thenotion that this protein has similar functions within theCorynebacterium genus. Biochemical analysis of the mutantindicated that loss of NCgl1054 was associated with a block inthe elongation of the major PIM species, AcPIM2, to formLM-A (the precursor to LAM), as well as the elongation of Gl-Xto form LM-B. Specifically, HPTLC analysis and LC-MS/MSrevealed that themutant lacked twomannosylatedGl-X speciesthat corresponded to Gl-Y (Man2GlcA.DAG) and Gl-Z(Man3GlcA.DAG). These analyses suggest that LpqW isinvolved in regulating a process or enzyme that is common to

the synthesis of structurally distinct PIM and Gl-glycolipids,rather than just regulating fluxes in the PIM pathway.The biochemical phenotype generated by disruption of

NCgl1054 was very similar to that generated by deletion of theC. glutamicum gene encoding ppMan synthase, ppm1. Specifi-cally both mutants exhibited a defect in the elongation ofAcPIM2 and Gl-X to more polar glycolipid species andLM/LAM.These analyses suggest thatGl-X, aswell as AcPIM2,are initially assembled on the cytoplasmic leaflet of the cellmembrane and subsequently flipped to the periplasmic side ofthe membrane where they are elongated by ppMan-dependentmannosyltransferases. ppMan is synthesized on the cytoplas-mic leaflet of the cell membrane (Fig. 1), and LpqW could, inprincipal, be required for the synthesis of this mannose donor

FIGURE 9. Analysis of a suppressor mutant of C. glutamicum �NCgl1054 with the capacity to produce LM-A/LAM. A, PAGE analysis of LM/LAM fractionextracted from various strains. Lipoglycans were purified on an octyl-Sepharose column, proteinase K-treated, separated by 12% PAGE, and detected using aSilverSNAP kit. WT, C. glutamicum wild-type; �NCgl1054, lpqW mutant; bypass mutant, a �NCgl1054 derivative with partially restored lipoglycan synthesis; Std.,protein molecular weight standard (Invitrogen). B, HPTLC analysis of C. glutamicum total lipid extracts. Glycolipids were visualized using orcinol/H2SO4. Arrowindicates a modified PIM species that is found exclusively in the bypass mutant. C, cell-free assays using cell envelope fractions from WT C. glutamicum,�NCgl1054, and the bypass mutant. Cell envelopes were incubated with GDP-[3H]mannose for 3 h and labeled lipids analyzed by HPTLC and visualized byfluorography. The mobility of [14C]acetate-labeled C. glutamicum lipids is provided as standard. D, a model for the mode of action of LpqW in C. glutamicum. Byanalogy with the E. coli Lpo proteins, LpqW may regulate the activity of MptB that initiates the elongation of AcPIM2 and Gl-X with �1– 6-linked mannose chainsleading to the formation of LM-A/LAM and LM-B, respectively. Disruption of NCgl1054 leads to inactivation of MptB and a global defect in LM/LAM biosynthesis.In the bypass mutant derived from the �NCgl1054 strain, a Y to C mutation at residue 507 of MptB causes a conformational change that partially activates MptBin the absence of LpqW. This strain can now synthesize LM/LAM in the absence of functional LpqW activation.

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and/or its transport to the periplasmic leaflet of the cell mem-brane. However, we show here that ppMan synthesis is normalin the NCgl1054 mutant. This is consistent with our previousanalysis of the M. smegmatis �lpqW mutant and derived sup-pressor strains that synthesize normal levels of polar PIMs orLM/LAM, respectively, indicating ongoing synthesis of ppManin both the initial and derived suppressor strains. Collectively,these results suggest that LpqW does not play a role in regulat-ing either the synthesis or periplasmic transport of ppMan.M. smegmatis mutants lacking LpqW initially exhibit a

defect in LM/LAM biosynthesis but are rapidly overgrown bysuppressor/bypass mutants when cultivated in minimalmedium (41). These strains have restored LM/LAM synthesisat the expense of polar PIM biosynthesis. Genetic analysis ofthese suppressor mutants demonstrated that the restoration ofLM/LAM synthesis was dependent on mutation of PimE, appMan-dependent mannosyltransferase that is involved inconverting AcPIM4 to AcPIM5 and AcPIM6. C. glutamicumlacks an ortholog of PimE and does not accumulate polar PIMspecies. Nonetheless, suppressor strains were readily generatedafter 4–6 passages when the C. glutamicum �NCgl1054mutant was incubated in BHI broth. In common with theM. smegmatis LpqW suppressor strains, the �NCgl1054 sup-pressor strains exhibited restored LM/LAM biosynthesis.Intriguingly, whole genome sequencing of one of these suppres-sor strains revealed a single base mutation corresponding toone of the transmembrane domains of the ppMan-dependentmannosyltransferase,MptB. TheC. glutamicumMptB has pre-viously been shown to catalyze the elongation of both AcPIM2andGl-X, and aC. glutamicummutant strain lackingMptB hasthe samebiochemical phenotype as�NCgl1054, i.e. an accumu-lation of AcPIM2/Gl-X and loss of all downstream LM andLAM lipoglycans (17). Collectively, our observations suggestthat LpqW regulates MptB function, either directly or througha, as yet uncharacterized, complex.Despite our evidence for direct regulatory effects on MptB

function, our findings could not entirely rule out an alternativescenario in which LpqW functions to enhance the rate of trans-membrane flipping and/or regulates the transport of AcPIM2/Gl-X precursors to MptB. Indeed, LpqW shares overall struc-tural similarity to a large family of substrate-binding proteinsthat are commonly associated with ABC transporters in the cellmembrane. The latter have been shown to translocate diversemetabolites including cell wall components (53). Substrate-binding proteins are thought to initially bind transporter sub-strates in the periplasmic space and to be responsible for thesubstrate specificity of the cognate transporter (54, 55).Although substrate-binding proteins are generally involved inimport processes, it is possible that LpqW could associate witha flippase or ABC transporter involved in the transmembraneexternalization of AcPIM2 or Gl-X. Such a role is consistentwith the presence of a possible AcPIM-binding cleft in theLpqW crystal structure (42). Apart from regulating fluxes intodifferent enzyme complexes, a glycolipid chaperone might alsobe required to regulate themovement of intermediates betweendistinct subdomains of the cell membrane (52). However,results from our cell free assays did not support such a trans-port/substrate accessibility role for LpqW because the loss of

compartmentalization did not restore Gl-Y/Gl-Z synthesis incell envelopes from the �NCgl1054 strain, despite synthesis ofthese species in cell envelopes extracted from WT and thebypass mutant.Having obtained evidence against a transport/substrate

accessibility function for LpqW, we strongly favor our originalhypothesis that LpqW regulates the activity of the AcPIM2/Gl-X mannosyltransferase MptB in C. glutamicum (Fig. 9D).Such a model would be consistent with recent studies showingthat E. coli lipoproteins have a role in regulating enzymesinvolved in peptidoglycan biosynthesis (56–58). The E. colilipoproteins, LpoA and LpoB, are N-terminally anchored intothe outer membrane bilayer but can bind to and stimulate theenzymatic activity of the transpeptidases that are involved inthe assembly of new peptidoglycan. LpqW, similar to LpoA/B,is predicted to be secreted into the periplasmic space, althoughits precise location in the cell wall has not been defined. How-ever, it is conceivable that it is anchored into the outer mem-brane of C. glutamicum orMycobacterium spp that are rich inmycolic acids and, in the case of C. glutamicum, cardiolipin(59). In addition to regulating the activities of key enzymesinvolved in cell wall assembly, outer membrane lipoproteinsthat span the periplasmic space may also be involved in medi-ating the insertion of (lipo)polysaccharides into the outermem-brane.Our proposal thatMptB and LpqWmay directly interactin vivo would provide the first example of a cell wall polymer-ase/lipoprotein activator pair in mycobacteria.

Acknowledgments—We thank Jessica Porter for technical expertisewith IonTorrent sequencing, Julie Ralton for technical assistance, andSvetozar Kovacevic for critical reading of the manuscript.

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A Lipoprotein Activator of Cell Wall Lipoglycan Synthesis

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Torsten Seemann, Ross L. Coppel, Malcolm J. McConville and Paul K. CrellinArek K. Rainczuk, Yoshiki Yamaryo-Botte, Rajini Brammananth, Timothy P. Stinear,

Glycolipids in CorynebacteriaThe Lipoprotein LpqW Is Essential for the Mannosylation of Periplasmic

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