Bacterial secretion systems Flashcards
in eukaryotes proteins were targeted to organelles by the SRP (p54 subunit) the baterial homologue is called
Ffh in ecoli
interacts with the signal receptor in the alpha subunit of FtsY (membrane protein)
When Ffh and FtsY come together they form a GTPase which pushes the protein through the membrane through membrane complex SecYEG
Co translational transport
polypeptide is translocated as soon as it leaves the ribosome (fed through the channel) doesn’t need that much energy as the energy of the ribosome pushes it through the open channel.
The SecYEG translocon (sec dependent secretion)
heterotrimeric protein complex in the inner membrane, SecY forms the pore, SecE forms the hinge
SecY is a homologue of Sec61 in eukaryotes
There is no SecG homologue in eukaryotes - it interacts with SecA
SecA is used when the proteins are in the cytoplasm (have not been pushed into the pore directly from the ribosome - POST translational rather than co translational). SecB binds the free protein and SecA mediates the interaction of SecB with SecY
After the protein goes through SecYEG the signal peptide is
cleaved
by a signal peptidase
How does SecYEG get into the membrane in the first place?
YidC
Sometimes the presence of a hydrophobic domain in the tranlocase pore can cause lateral translocation into the membrane by the chaperone YidC (pulls the protein sideways into the membrane)
SUMMARY of sec dependent secretion
Sec dependent secretion:
SRP mediated co translational
Sec mediated post translational
Proteins that do not embed in the membrane pass into the periplasm, from there
bacteria secrete them outside using secretion systems
Sec independent SS
TypeIV, TypeIII, Type I
1 step: Go straight through from cytoplasm to outer membrane and beyond
Sec dependent SS
TypeV, TypeII
2 step: Go through SecYEG into periplasm then into outer membrane
TypeV is called an AUTOTRANSPORTER
TypeII is the most important G-ve SS. Also called a GSP: general secretory pathway. Type II SS push the protein through the channel after it has gone through SecYEG to exit the outer membrane
Type I:
Trimeric complex
no periplasmic intermediate
Sec independent.
Comprised of modules:
the ATP Binding cassette (ABC)
membrane fusion protein (MFP, in the periplasm part), outer membrane protein (OMP)
Typically used in secreting proteins but also in drug efflux pumps. Secrete many classical toxins such as hemolysin. Prominent in G-ve bacteria.
An outer membrane protein like TolC can pair up with many MFPs and many differnt ABCs.
Type II:
Often called the general secretory pathway (GSP).
Once in the periplasm, a complex macromolecular structure employing a piston like mechanism for secretion. Piston is built of Pilin
A ‘secretin’ in the outer membrane which is gated.
Cholera toxin is secreted by the T2SS. Will have an amino terminal hydrophobic groove (signal peptide) that is picked up by Fth and FtsY.
Type III:
SS predominant in pathogens not commensals. Another macromolecular structure, often referred to as an injectisome.
SPI (salmonella pathogenicity island) is a TIII SS. 50 protein structure.
Ancestrally related to flagella
Involved in the virulence of many intracellular Gram negative bacteria
Type IV:
Another macromolecular structure. As with the T3SS’s the genes encoding them are often on pathogenicity islands
Ancestrally related to conjugation apparatus - and is still involved in the transfer and uptake of DNA. Can also pull proteins from the periplasm (put there by SecYEG) and pass them through the channel.
Involved in the virulence of many intracellular Gram negative bacteria
N.b. the pertussis toxin goes through a T4SS through a sec dependent mechanism.
Type V:
Much simpler in concept. Often referred to as autotransporters. Have passenger and transpoter domains
Important for localising proteins to the outer membrane
These do have a periplasmic intermediate
Secreted effectors and virulence
Salmonella’s effectors mediate uptake through macrophages via a T3SS
1,Salmonella cells attach to the intestinal epithelium by means of adhesins, such as those encoded within SPI-3 and SPI-4 (salmonella pathogenicity islands 3 and 4)
2, Invasion of bacteria follows, and engulfment by M cells is mediated by virulence factors encoded within SPI-1 (T3SS) and SPI-5.
Salmonella does this by secreting the effector SipA into the host which causes actin rearrangment leading to salmonella uptake
5, Once taken up by phagocytes inside the cytoplasm, Salmonella replicates within the SCV. Factors encoded within SPI-2 and the pSLT plasmid are essential for survival in the phagosome (SifA prevents maturation of the phagolysosome).
7, Bacteria are internalized within phagocytes and located again within an SCV, where SPI-3, in addition to SPI-2 and the pSLT plasmid, play an important role
T3SS and T4SS:
Translocon at the tip of the injection apparatus forms a pore in host cell membranes and injects the effectors directly, to mediate manipulation of the immune response (e.g. siderophores) Approx 13 SPIs discovered, activated at different stages of host cell invasion.
*PhoP/Q represses SPI 1!
Virulent salmonella also have a plasmid that encodes for factors that are important for surviving inside the macrophage. A spiC (effectors) mutant of Salmonella shows reduced survival in macrophages due to interference with vesicular trafficking and lyzosome fusion.
