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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
IMATP
ADP
SecYEG
SecYEG
SecA
SecB
N
C
Gram negative protein secretion systems