Bacterial protein secretion systems Many proteins need to be transported across the bacterial...

Bacterial protein secretion systems

Many proteins need to be transported across the bacterial membrane

These include flagella and pilus subunits

Gram negative bacteria have evolved several systems for the secretion of proteins to the external environment

Bacterial protein secretion systems

Different systems are named according to the order in which they were discovered

i.e. Type I, II, III, IV, etc.

Gram negative protein secretion systems

Sec-dependent pathway

Also known as the general secretory pathway (GSP)

Most commonly used system to transport proteins across plasma membrane or integrate them into the membrane

Used by both gram positive and gram negative bacteria


System composed of both cytosolic and membrane-bound proteins

Secreted proteins contain N-terminal signal sequence

Chaperone protein binds to sequence and guides protein to transport machinery or translocon


Chaperones include SecB and signal recognition particle (SRP)

Chaperones release preprotein which binds to SecA


SecA translocates preprotein through SecYEG complex

Hydrolysis of ATP drives translocation

After emerging from the membrane, signal peptide is removed by signal peptidase

Type II secretion systems

Found in many gram-negative bacteria, including species of Erwinia, Pseudomonas and Vibrio

Used for the secretion of proteins through the outer membrane

Composed of as many as 14 different proteins (both inner and outer membrane-localized)

Type II secretion systems

Outer membrane protein forms a channel through which protein can pass

Family of proteins referred to as secretins

Secretins

Type I secretion systems

Also known as ABC protein secretion pathway

ABC = ATP-binding cassette

Secretion is independent of the sec pathway


Used for the secretion of various enzymes and toxins including -hemolysin

Secreted proteins contain a C-terminal (noncleavable) signal sequence


Are composed of only three different proteins

1. ATP -binding inner membrane protein

2. Outer membrane channel-forming protein

3. Membrane fusion protein


Proteins transported across both membranes in a single step

Energy for translocation derived from hydrolysis of ATP

Type III secretion systems

Found in a number of gram-negative bacteria

Bacteria with TTSS always exhibit intimate contact with host cells

Systems designed to both secrete and translocate or “inject” proteins into host cells


Composed of at least 20 different proteins

Include cytosolic, membrane-bound and secreted proteins

Membrane proteins form a structure similar to the flagella basal body named the needle complex


Outer membrane component of needle complex belongs to the secretin family of proteins

Proteins transported across both membranes in a single step

Hydrolysis of ATP provides energy for translocation


Many protein required for secretion through needle complex are similar to flagella assembly proteins

TTSSs likely evolved from bacterial flagella systems

Type IV secretion systems

Can be used to transfer proteins or DNA from one cell to another

Transfer proteins to eukaryotic host cells

Transfer plasmids to bacterial cells (conjugation)

Type IV secretion systems

Composed of several proteins that span the entire cell wall

Hydrolysis of ATP provides energy for transport

Found in many bacterial pathogens including species of Legionella, Bordetella and Helicobacter

Conjugal pilus

Several morphologically distinct pili

Appear to function as adhesins rather than transfer conduits

Retraction of pilus pulls cells close together

Type IV secretion/ autotransporters

Transport through inner membrane is sec-dependant

All substrates carry sec-signal sequence

Proteins divided into three domains i) sec signal sequence, ii) passenger domain, iii) -barrel domain


OM

IMATP

ADP

SecYEG

SecYEG

SecA

SecB

N

C

Gram negative protein secretion systems

Bacterial protein secretion systems Many proteins need to be transported across the bacterial...

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