Bacterial Physiology (Micr430)
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Transcript of Bacterial Physiology (Micr430)
Bacterial Physiology (Micr430)
Lecture 11Protein Transport
(Text Chapter: 17)
Protein Transport: 3 modes
Translocation – transport of proteins into or through a membrane
Export – the protein is translocated into the periplasm of a Gram negative bacterium
Secretion – a protein is transported to the extracellular medium, into another cell, or to the bacterial cell surface.
The Sec System
Proteins destined for cell membrane, periplasm or outer membrane use Sec system.
Sec System has four components: A leader peptide; A chaperone protein; A membrane-bound complex of three
proteins (SecYEG) A cytoplasmic ATPase (SecA)
Leader Peptides of Exported Proteins
Fig. 17.1
A model
Fig. 17.2
Translocation of membrane-bound proteins
For some proteins destined to be residing in cell membrane, the transport is also Sec dependent and occurs in similar fashion.
Internal hydrophobic regions of the protein stop “total” translocation and anchor the protein into the membrane.
A model
Fig. 17.3
Extracellular Protein Secretion
A variety of different proteins are secreted out of the cell: Hydrolytic enzymes: proteases,
nucleases Nonhydrolytic enzymes; such as
cholera toxin, diphtheria toxin. Structural proteins such as pilin and
flagellar proteins Virulence proteins secreted directly
into host cells
Extracellular Protein Secretion
There are six major secretion systems known for Gram negative bacteria; we will concentrate on Type I, Type II and Type III.
Type II system is Sec dependent Types I and III are Sec independent
An Overview
Fig. 17.4
The Type I Pathway
Examples of proteins secreted using this pathway: hemolysin and proteases
Protein characteristics: No leader sequences but carboxyl
terminus of the proteins is recognized by the secretion apparatus
Do not require sec genes Proteins are directly secreted outside the
cell without ever entering the periplasm
The Type I Pathway
Three proteins form the secretion apparatus in this pathway:
An ABC transporter (also known as ABC exporters) (example, HlyB) has ATPase activity to generate energy for secretion
The membrane fusion protein (MFP) (example, HlyD) is anchored in the inner membrane and has a periplasmic domain that is believed to connect to the OMP.
The outer membrane protein (OMP) (example, TolC)
The Type III Pathway
Type III (three) secretion systems (TTSS) are also Sec-independent.
These systems form an apparatus used for the injection of virulence proteins into eukaryotic host cells in some Gram negative bacteria such as Yersinia spp., Salmonella spp., Pseudomonas spp.
A Model for Type III pathway
Fig. 17.5
TTSS and Flagellin Secretion
Type III appears to have evolved from flagellin secretion system based on homology of many proteins of these two systems.
Fig. 17.6
The Type II Pathway
Type II pathway appears to be the major route by which proteins are secreted by Gram negative bacteria.
This pathway secrete proteins in two stages: Stage 1, proteins are translocated into
periplasm; Sec pathway is used in this stage Stage 2, proteins from periplasm are
secreted across the outer membrane to the external environment
A Model for Type II pathway
Fig. 17.7