Lecture 7 - Bacterial secretion systems Flashcards
What is the role of translocation and secretion in bacteria?
1/3 of proteins = PM and OM by translocation, external by secretion
How are proteins moved in G+ve vs G-ve bacteria?
- G+ve = translocated across PM thru porous CW or embedded in CW
- G-ve = translocated across CW or secreted thru OM via 11 systems
What is the Sec system? Which bacteria use it?
Major pathway for translocated unfolded proteins across PM in both G+ve and G-ve
What is co-translational translocation in the Sec system?
Inserting proteins into PM
Describe the 4 step process of co-translational translocation
- PM protein’s translation ribosome coupled to translocation by Sec
- Signal sequence at N-terminal of 1st protein recog by signal recog particle (SLP)
- SRP recruits docking FtsY = delivers protein to SecYEG transmembran channel
- Protein escapes by translation into membrane during translocation
What is post translational translocation in the Sec system?
Moving unfolded preproteins across PM to periplasm/OM/external
Describe the 5 step process of post-translational translocation
- Signal peptide at N terminal recog by SecB chaperons
- SecB binds signal = delayed folding and delivery to SecA
- SecA guides protein to channel then acts as ATPase to translocate preprotein across PM
- SecY, SecE and SecG form membrane channel
- Signal peptidase removes signal peptide = folding
What is the Tat pathway used for? Which bacteria use it?
- Proteins to be secreted in folded form
- Both G+ve and G-ve
Compare the Sec-dependent and Sec-independent secretion systems and which types are in each
- Sec dependent = needs Sec/Tat for transport into periplasm, types 2, 5
- Sec independent = doesn’t need Sec/Tat, types 1, 3, 4, 6
Describe how the T2SS works and give example/s of the proteins transported by it
- Pseudopilus in periplasm connects PM proteins to OM channel
- Piston model = pseudopilus extends/retracts by ATP to push protein thru OM channel
- AB cholera toxins, proteases, lipases
Describe the process of T5SS and example/s of proteins transported by it
- N terminal signal peptide of unfolded protein recog by SecB = translocated across PM to periplasm
- Unfolded protein autotransported via translocator domain as OM pore and passenger domain to pass thru
- Autoproteolytic cleavage to release passenger domain
Eg. virulence proteins IgA protease of N. gonorrhoeae and Ics of Shigella flexneri
Describe the T1SS and example/s of proteins transported by it
- Bypass Sec and periplasm straight to surface
- Protein complex = ATP binding cassette (ABC) transported in PM, TolC pore forming in OM, connecting protein
- Eg. virulence and resistance eg E. coli’s alpha-haemolysin and multidrug efflux
- G-ve bacteria esp
Describe the T3SS, important bacteria it’s used in, and example/s of proteins transported
- Injectisome needle extends beyond OM to directly inject virulence factors
- Evolved from flagellar assembly proteins bc base = basal body
- Eg. E. coli, Salmonella, Shigella, Pseudomonas
- Toxins, phagocytosis inhibitors, invasins
Describe the T4SS and example/s of proteins transported by it
- 12+ complex forming pilus/tunnel that grows and binds to host integrins = direct transfer
- Eg. E. coli’s F plasmid
- Eg. Agrobacterium’s Ti plasmid for oncogenic T-DNA into plants
- H. pylori’s CagA inflammatory effector
Describe the T6SS and example/s of proteins transported by it
- Related to phage injection system = same phage tail structure of baseplate sheath inner tube, same secretion by contraction, VrgG forming sharp tip
- Effector proteins assemble and loaded on tip in cytoplasm then contraction and ejection to puncture target cell
- Eg. E. coli’s T4 lytic phage
- Eg. peptidoglycan hydrolases in interbacterial warfare
- V. cholerae’s effectors into macrophages to inactivate actin
What secretion systems are common in G-ve?
- Pilus adhesins for attachment and colonisation
- Delivery of toxins/effectors to host by secretion systems
What is antivirulence and how are secretion systems related to it? What system is currently under study?
- Antivirulence = reduce virulence without antibiotics
- Block common secretion systems in bacteria eg pilus adhesins and delivery of toxins/effectors
- T3SS inhibitors identified
