Fusion proteins containing a minimal GPI-attachment signal are apically

expressed in transfected MDCK cells

MICHAEL P. LISANTI1, INGRID W. CARAS2 and ENRIQUE RODRIGUEZ-BOULAN1*

^Department of Cell Biology and Anatomy, Cornell University Medical College, New York, NY 10021, USA2Genentech, Inc., 460 Point San Bruno Boulevard, South San Francisco, CA 94080, USA

* Author for correspondence

Summary

We have shown that addition of the C -terminal 37amino acids of decay-accelerating factor (DAF) tosecretory proteins leads to glycosyl-phosphatidyl-inositol (GPI) anchoring and apical surface ex-pression in MDCK cells. Theoretically, transferredapical sorting information may reside in the glyco-lipid-anchor moiety or the DAF sequence (9 aminoacids) that remains after signal cleavage and GPIattachment. We show here that removal of eight ofthese nine remaining amino acids, thereby creating a

minimal GPI-attachment signal, results in apicalexpression of GPI-anchored human growth hor-mone. These data argue that the apical sortinginformation conveyed by the C terminus of DAF isrelated to its ability to direct GPI attachment, ratherthan to a specific sequence that remains in the fusionprotein.

Key words: fusion proteins, GPI attachment, MDCK cells.

Introduction

Most integral membrane glycoproteins are anchored to thecell surface via a strech of relatively hydrophobic aminoacids. This transmembrane domain also acts as a stop-transfer signal during translocation of the protein to thelumen of the endoplasmic reticulum (ER). An alternativemechanism for anchoring proteins to the plasma mem-brane involves covalent attachment of a glycophospholi-pid, glycosyl-phosphatidylinositol (GPI; Ferguson andWilliams, 1988; Low and Saltiel, 1988; Lisanti etal. 1990),to the C terminus of the protein ectodomain. GPI-anchorattachment is directed by C-terminal hydrophobic se-quences of the precursor protein (Caras et al. 1987a,6;Caras and Weddell, 1989; Caras et al. 1989) in a processthat involves coordinated proteolytic removal of 17-31C-terminal residues prior to anchor addition. Removal ofthe C-terminal peptide and replacement with a GPI occursin the ER rapidly after translation, suggesting that apreformed GPI moiety is added en bloc (Ferguson andWilliams, 1988; Low and Saltiel, 1988; Lisanti etal. 1990).

Polarized epithelial cells are characterized by theasymmetric distribution of plasma membrane proteinsand lipids into two distinct domains, apical and basolat-eral (Rodriguez-Boulan and Nelson, 1989; Simons andWandinger-Ness, 1990). This asymmetry is generated byintracellular sorting of membrane components at thetrans-Go\gi network and is kept by the tight-junctionalfence and specific recycling mechanisms. We recentlyreported that GPI-anchoring is strongly associated withapical expression in epithelial cells. All endogenous GPI-anchored proteins of five independent epithelial cell lineswere apically polarized (Lisanti et al. 1988, 19906; Wilsonet al. 1990). Furthermore, GPI anchoring of threeindependent secretory proteins via recombinant transfer

Journal of Cell Science 99, 637-640 (1991)Printed in Great Britain © The Company of Biologists Limited 1991

of GPI-attachment signals from DAF (37 C-terminalresidues) or Thy-1 (53 C-terminal residues) led to apicalexpression in transfected MDCK cells (Brown et al. 1989;Lisanti et al. 1989, 1990a). These experiments suggestedthe hypothesis that GPI acts as an apical sorting signal inepithelial cells (reviewed by Lisanti and Rodriguez-Boulan, 1990); however, the possibility that amino acidsremaining after proteolytic processing of the transferred Cterminus (9 for DAF, 22 for Thy-1) might contribute to theapical distribution of these fusion proteins was notformally excluded.

To this end, we examined the polarity of GPI-anchoredproteins containing a minimal DAF GPI-attachmentsignal. Here, We demonstrate that removal of eight of thenine remaining amino acids still results in apicalexpression of GPI-anchored human growth hormone(hGH), consistent with the idea that sorting informationlies within the GPI moiety. This is the first demonstrationthat the same molecule (hGH) can be directed to the apicalsurface by several distinct GPI-attachment signals.

Materials and methods

Materials / reagentsPhosphatidylinositol-specific phospholipase C (PI-PLC), purifiedfrom Bacillus thuringiensis, was a generous gift from Martin Low(Columbia University College of Physicians and Surgeons, NY).Purified rabbit IgG against hGH was provided by the MedicinalAnalytical Chemistry Department at Genentech, Inc. (South SanFrancisco, CA). pMV6 tk(neo), conferring resistance to theantibiotic G-418 (Maddon et al. 1985), was a gift from Dr MosesChao (Cornell University Medical College).

ConstructscDNAs were inserted into a mammalin expression vector between

637

a retroviral promoter and an SV40 polyadenylation signal (Caraset al. 19876). Constructs for the expression of fusion proteins(hGH-DAF 29 Ser and hGH-DAF 29 Gly) contained a minimalDAF signal for GPI attachment (the 28 C-terminal amino acids ofDAF plus a single upstream amino acid, either Ser or Gly) asdescribed previously (Moran et al. 1991).

We and others found that native GH is secreted in significantquantities from both surfaces of MDCK cells. Although in earlierstudies it was found to be secreted in roughly equal quantitiesfrom the apical and basolateral surfaces of total transfectedMDCK populations (Gottlieb et al. 1986), we found that individualclones may vary in their expression (from 45% to 75% apical;Lisanti et al. 1989). This suggests that different MDCK cell clonesmay differ in the amount of bulk constitutive secretion deliveredto a given surface. In contrast, GPI-anchored fusion proteins werealways highly polarized (90-95% apical) in all MDCK clonesexamined (the polarity of at least three clones was examined for agiven construct).

Cells and cell cultureMadin-Darby Canine Kidney (MDCK) cells, type n, weremaintained in DME supplemented with 10% (v/v) fetal bovineserum (FBS) (Gibco) and antibiotics (Rodriguez-Boulan, 1983).Cells from a single confluent 75 cm2 flask were trypsinized andtransferred to six Transwell filter chambers (24.5 mm, 0.4 /an porepolycarbonate, tissue cultured treated; Costar Inc, Cambridge,MA) and medium was changed every 2 days.

Transfection, selection and screeningMDCK cells were transfected as described previously using amodification of the calcium phosphate precipitation procedure(Lisanti et al. 1989). Briefly, ~106 subconfluent MDCK cells weretrypsinized, resuspended in lml Dulbecco's modified Eagle'smedium (DME) containing 10 % FBS, and transferred to a 100 mmdish. Cells were overlayed with 10 fig/dish of the non-selectableplasmid and 1 //g/dish of pMV6(neo) and incubated for 6-9 h at37 °C. After a 15% glycerol shock, cultures were allowed torecover for 2 days in DME containing 10% FBS. Cells weretrypsinized, diluted 1:8 and replated in 160 mm dishes in DMEcontaining 10% FBS and 500 /ig ml"1 G418 (Gibco, Grand Island,NY). After 10-14 days resistant colonies appeared. Individualclones were obtained by limiting dilution. Duplicate plates werethen screened for the presence of the transfected gene productafter a 12-16 h incubation in DME containing 10% FBS and10 mM sodium butyrate to elevate expression (Gorman andHoward, 1983; Gottlieb et al. 1986). Positive clones were detectedafter fixation and permeabilization by incubation with theappropriate polyclonal rabbit antiserum (1:100 dilution in PBSC/M containing 0.2 % BSA) as described previously (Lisanti et al.1989), except that an anti-rabbit ABC Vecstain kit (peroxidase;Vector Labs, Burlingame, California) was used instead of 126I-labeled protein A. Approximately 10-30 % of the G418-resistantcells expressed the transfected gene products.

ImmunofluorescenceCells grown on glass coverslips (2-3 days confluent) wereincubated 12-16 h in DME containing 10% FBS and 10 mMsodium butyrate, washed (3x) with PBS C/M and processed forantibody staining. Monolayers were fixed with 2% paraformal-dehyde, permeabilized with saponin, and allowed to bind anti-hGH (diluted 1:100) for 90 min as described previously (Lisanti etal. 1989). Bound antibodies were visualized using the appropriaterhodamine-conjugated IgG (Cappel, Cooper Biomedical Inc.,Malvern, PA).

Domain-selective labelingCell surface labeling with sulfo-NHS-biotin was as describedpreviously (Lisanti et al. 1988; Sargiacomo et al. 1989). Briefly,filter-grown monolayers were washed (2x) with ice-cold PBScontaining 0.1 mM CaCl2 and 1 mM MgCl2 (PBS-C/M) beforeaddition of the labeling reagent. One ml of sulfo-NHS-biotin(O.SmgmP1 in ice-cold PBS-C/M) was added to either the apicalor the basolateral compartment; the other compartment received

an equivalent volume of PBS-C/M (10,23). After 30 min of gentleagitation at 4°C, the medium was removed and excess reactivebiotin was quenched by incubation with Hepes-buffered serum-free DME (10 min at 4°C). After labeling, cell monolayers werewashed with PBS-C/M (2x) and processed for immunoprecipi-tation. Biotinylation is confined to either the apical or basolateralcell surface and is uniform up to the level of the tight junction(Sargiacomo et al. 1989).

PI-PLC treatmentPI-PLC treatment was similar to a method described previously(Conzelmann et al. 1986; Lisanti et al. 1988, 1990). Briefly, aftercell surface biotinylation, live cells were treated with or withoutPI-PLC (8 units ml"1 in 20 mM Hepes-buffered saline, pH7.3,containing 0.1 mM CaClj and IIDM MgCy for l h at roomtemperature. After treatment, cells were extracted with thedetergent Triton X-114. Detergent phases, as well as theincubation media, were subjected to immunoprecipitation. Incu-bation media samples were adjusted to 1% Triton X-114 beforeimmunoprecipitation. Activity of PI-PLC is measured by hydroly-sis of PH]PI at pH7.0 in 0.1% sodium deoxycholate (i.u.=l/nnolmin"1).

ImmunoprecipitationBiotinylated filter-grown monolayers were extracted for 45 min at4°C with lml of Tris-buffered saline (TBS: 10mM Tris-HCl,pH7.4, 0 .15 M NaCl) containing 1% Triton X-114 and proteaseinhibitors and centrifuged (14 000# for 10 min). After tempera-ture-induced phase separation, detergent phases were collected,re-extracted (2x) and diluted to 1 ml with the above TBS (lackingdetergent). After preclearing with Sepharose CL-4B, anti-hGHIgG prebound to protein A-Sepharose were added and incubatedovernight at 4°C, with continuous rotation (Lisanti et al. 1989).Immunoprecipitates were washed (5-6x) with TBST (TBS,pH 8.0, 1 % Triton X-100) and eluted with Laemmli (1970) samplebuffer (100/d/sample) and boiling (1-3 min).

Detection of labeled proteinsSamples were subjected to SDS-PAGE (Laemmli, 1970) underreducing conditions (15% acrylamide gels) and transferred tonitrocellulose (Towbin et al. 1979). After incubation with 12SI-labeled streptavidin, blots were washed and dried as describedpreviously (Sargiacomo et al. 1989). Biotinylated proteins werevisualized by autoradiography on Kodak XAH-5 film (—80°C withan intensifying screen). Afr markers are as indicated and areexpressed x 10 : myosin heavy chain, 200; phosphorylase b, 97.4;bovine serum albumin, 68; ovalbumin, 43; carbonic anhydrase,29; beta-lactoglobulin, 18.4; and lysozyme, 14.3.

Results and discussion

DAF-GPI attachment signaKs)DAF has served as a model protein for study of the signaldirecting GPI addition. Characterization of fusion proteinscontaining C-terminal sequences of DAF have demon-strated that successful GPI-anchoring requires both aC-terminal 17 amino acid hydrophobic domain and asecond element located 10-20 amino acids N-terminally(Caras et al. 1989). This second element, the cleavage/attachment site, must remain in the GPI-linked fusionprotein after proteolytic processing and GPI addition. Inaddition, it was shown that only a single amino acid(either Gly, Ser, Ala, Asp or Asn), the cleavage/attachment site, in combination with the 28 residueC-terminal domain that is cleaved, can direct efficient GPIattachment (Moran et al. 1991).

We have shown previously that several GPI-anchoredfusion proteins containing the 37 C-terminal amino acidsof DAF are apically expressed in transfected MDCK cells(Lisanti et al. 1989), suggesting that GPI-anchoring may

638 M. P. Lisanti et al.

..QCRSVEGSCGFCJ. GTTRLLSGHTC FTLTGLLGTLVTMGLLT hGH-DAF 29Gly

.QCRSVEGSCGFS. GTTRLLSGHTC FTLTGLLGTLVTMGLLT hGH-DAF 29 Ser

QCRSVEGSCGF PNKGSGTT^ GTTRLLSGHTC FTLTGLLGTLVTMGLLT

I319

hGH iPfii—GPI-attachment signal

hGH-DAF 37 Fi£- 1. Schematic diagramillustrating the hGH-fusionproteins. The DAF sequenceincluded in the fusion proteins isbracketed. The cleavage/attachment site (Ser or Gly) andthe C-terminal hydrophobicdomain are underlined. Theright-hand column indicates thename of the fusion protein. Forexample, hGH-DAF 29 Gly is anhGH-DAF fusion protein thatcontains the 29 C-terminalamino acids of DAF and glycineas the cleavage/attachment site.

convey apical localization. However, we could not excludethe possibility that the transferred DAF sequence thatremains after signal processing (9 amino acids) might bedirectly involved in apical sorting. To address thisquestion, we expressed modified hGH-DAF fusion proteins(hGH-DAF 29 Ser and hGH 29 Gly) that contain aminimal DAF GPI-anchoring signal, leaving only a singleamino acid in the fusion protein (Fig. 1).

Apical expression of hGH-DAF fusion proteinscontaining a minimal GPI-attachment signalcDNAs were introduced via co-transfection with a plasmidexpressing neomycin resistance. After G418 selection,resistant cells were screened for the transfected geneproduct. Positive clones expressing hGH-DAF 29 Ser orhGH-DAF 29 Gly were initially examined by immuno-fluorescence and displayed a typical punctate, microvillarpattern observed with other apical antigens (Fig. 2). Todetermine conclusively whether the transfected proteinswere apically polarized, we employed a biochemicalpolarity assay: domain-selective biotinylation coupledwith immunoprecipitation (Lisanti et al. 1988; Sargiacomoet al. 1989). Confluent monolayers grown on polycarbonatefilter chambers (Transwell, Costar), were apically orbasolaterally biotinylated with the membrane/tight-junction impermeant marker, sulfo-NHS-biotin (see Ma-terials and methods). After labeling, cells extracts wereimmunoprecipitated and biotinylated protein visualizedby 126I-labeled streptavidin blotting. Both hGH-DAFfusion proteins were detected almost exclusively at theapical surface (Fig. 3); approximately 90-95% of biotinyl-ated hGH-DAF 29 Ser and hGH-DAF 29 Gly were confinedto the apical surface.

To confirm that the transfected gene products were GPI-anchored, biotinylated monolayers were subjected to PI-PLC treatment to induce release to the incubation media.Both hGH-DAF fusion proteins were sensitive to PI-PLCtreatment, as has been shown previously for expression ofthese constructs in COS cells (Fig. 4; Moran et al. 1991).

Fig. 2. Immunofluorescent localization of hGH-DAF 29 fusionproteins in transfected MDCK cells. Cells were seeded at highdensity on coverslips. After fixation and permeabilization,cells were incubated with anti-hGH antibodies. Bound IgGswere visualized with the appropriate rhodamine-conjugatedsecondary antibody.

Apical expression is independent of the primary sequencethat remains after GPI-attachmentGPI-attachment signals, although very different in pri-mary structure, are commonly recognized by the sameuncharacterized processing enzyme, the 'GPI-transpepti-dase'. These sequences possess at least two commonproperties, hydrophobicity and an appropriate cleavage/attachment site (Gly, Ser, Ala, Cys, Asp or Asn) thatpermits their recognition as GPI-attachment signals. As

hGH-DAF29

Ser

Apical sorting of GPI-linked proteins 639

SerA B A B

GlyA B A B

1 2 3 4 5 6 7 8Fig. 3. Apical localization of hGH-DAF 29 fusion proteins intransfected MDCK cells. Filter-grown monolayers expressinghGH-DAF 29 Ser (lanes 1-4) or hGH-DAF 29 Gly (lanes 5-8)were apically (A) or basolaterally (B) biotinylated andsubjected to immunoprecipitation with affinity-purified anti-hGH IgG. Biotinylated proteins were visualized via 125I-labeledstreptavidin blotting/autoradiography. The polarity of twoindependent clones is shown for each fusion protein.

PI-PLC - + - +

1 3 4

Fig. 4. hGH-DAF 29 fusion proteins are sensitive to release byPI-PLC. Monolayers expressing hGH-DAF 29 Ser (lanes 1-4)were apically biotinylated and incubated in the absence (—) orpresence (+) of PI-PLC. After treatment, both cell extracts(lanes 1,2) and incubation media (lanes 3,4) were subjected toimmunoprecipitation with affinity-purified anti-hGH IgG.Biotinylated proteins were visualized via 126I-labeledstreptavidin blotting/autoradiography. Similar results wereobtained with clones expressing hGH-DAF 29 Gly.

GPI anchoring cannot occur without the appropriateattachment site (six of 20 possible amino acids), theC-terminal amino acid may be considered as part of theGPI-anchor structure. We expressed hGH-DAF fusionproteins in which only the attachment site (either Ser orGly) remains in the fusion protein. Both fusion proteinswere GPI-anchored and apically expressed in MDCK cells.As Gly is the simplest amino acid and lacks a carbon side-chain, it is highly unlikely that this single C-terminalamino acid, the GPI-attachment site, could act by itself asan apical targeting signal. Therefore, our results indicatethat apical sorting information conveyed by GPI-attach-ment signals depends on their ability to direct GPIaddition, rather than on a specific sequence that remainsin the fusion protein. These results provide furtherevidence for the hypothesis that the GPI-anchoringmechanism functions as an apical targeting signal inpolarized epithelial cells.

We thank Dr Kathleen S. Perrego for critically reading themanuscript. M.P.L. is supported by a Medical Scientist Traininggrant from the Cornell University MD/PhD program; I.W.C byGenentech, Inc. and E.R.B. by grants from the NIH (GM-34107and GM-41771) and the American Heart Association (grant inaid, NY branch).

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(Received 8 March 1991 - Accepted 5 April 1991)

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