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

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

Sec-dependent pathway 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

Sec-dependent pathway Chaperones include SecB and signal recognition particle (SRP) Chaperones release preprotein which binds to SecA

Sec-dependent pathway 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

Type I secretion systems Used for the secretion of various enzymes and toxins including -hemolysin Secreted proteins contain a C-terminal (noncleavable) signal sequence

Type I secretion systems Are composed of only three different proteins 1. ATP -binding inner membrane protein 2. Outer membrane channel-forming protein 3. Membrane fusion protein

Type I secretion systems

Type I secretion systems 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

Type III secretion systems 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

Type III secretion systems 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

Type III secretion systems

Type III secretion systems

Type III secretion systems

Type III secretion systems 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

Type IV secretion/ autotransporters

Type IV secretion/ autotransporters OM IM ATP ADP Sec Y E G SecA SecB N C

Gram negative protein secretion systems