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