7 - Bacterial secretion systems Flashcards
1
Q
Where is one third of bacterial proteins found
A
- PM, OM and periplasm (by translocation)
- Or external environment (by secretion)
2
Q
Secretion in gram positive bacteria
A
- Proteins must be translocated across PM
- Either pass through porous CW (secreted) or become embedded and attached to CW
2
Q
Secretion in gram negative bacteria
A
- CW as for Gram positive, and in addition may be transported to OM or through OM (secreted)
- Gram –ve protein secretion systems are more numerous and complicated than Gram +ve
2
Q
The Sec System (General secretion pathway)
A
- Major pathway for translocating proteins across the plasma membrane
- Common to G+ and G- bacteria
- Transports proteins in unfolded state
3
Q
Co-translational translocation in the sec system
A
- Used to insert proteins into the PM
- Signal sequence is recognised by SRP signal recognition particle
- SRP then recruits docking protein FtsY which delivers protein to SecYEG transmembrane channel for transport into PM
- During translocation through channel, driven by translation of the protein, the (hydrophobic) protein escapes through side of channel into membrane where it stays
4
Q
Post translational translocation in the sec system
A
- SecB binds to signal peptide, delays protein folding as it exits ribosome, delivers protein to SecA
- SecA guides protein to channel, and acts as
ATPase motor to translocate the preprotein across the PM - SecY, SecE and SecG form a channel in the membrane
- Signal/leader peptidase removes the signal peptide and the protein folds to its active conformation
5
Q
Tat pathway
A
- Some proteins need to be secreted in folded form
- Materials for posttranslational modifications of certain proteins are not available in the periplasm or extracellularly, so they are folded and modified in the cytoplasm
- Common to G+ and G- bacteria
- Tat secreted proteins in G- can either remain periplasmic or be secreted by T2SS
6
Q
Secretion systems across OM of gram -ve bacteria
A
- At least nine different mechanisms (type I to IX)
- Sec dependent or sec independent
7
Q
Sec dependent systems
A
- Depends on general sec (or tat) system for transport from cytoplasm into periplasm
- Eg Type II and type V
8
Q
Sec independent systems
A
- DO NOT depend on general sec or tat systems for transport
- Eg Type I, III, IV, VI
9
Q
Type II secretion system (T2SS)
A
- Proteins use general Sec or Tat systems to reach periplasm
- Pseudopulis in the periplasm connects PM proteins to OM channel
- Proteins cross OM through channel made by special pore forming proteins
10
Q
What does T2SS secrete
A
- Numerous enzymes (e.g. proteases, lipases
- Some AB toxins (e.g.
cholera toxin)
11
Q
Pseudopilus
A
- Related to type IV pilus and to systems for DNA uptake in transformation
- The “piston” model suggests that pseudopilus extension and retraction,
driven by ATP, pushes the folded protein through the OM channel
12
Q
Type V secretion system (T5SS)
A
- Uses general sec system to reach periplasm
- One domain of the unfolded protein is recognised by SecA and translocated across PM
- Protein transports itself across the OM (autotransporter, with 2 domains)
13
Q
What does T5SS secrete
A
Virulence proteins (e.g. IgA protease of N. gonorrhoeae destroys host antibodies
14
Q
2 domains of autotransporter
A
- The translocator domain of the protein inserts in the OM to form a pore
- The passenger domain of the proteins passes through the pore
- In some cases autoproteolytic cleavage releases the passenger domain and it is secreted
15
Q
Type I secretion system T1SS
A
- Proteins pass directly from cytoplasm to cell surface, bypassing sec and periplasm
- Complex of three proteins spans PM, periplasm and OM
- Common to G+ and G- bacteria
16
Q
Three protein complex of T1SS
A
- the PM component is an ATP-binding cassette ABC transporter (ATPase activity provides energy to cross PM)
- the OM component is a pore-forming protein TolC
- A connecting protein holds them together
17
Q
What do T1SS secrete
A
- Virulence factors & resistance systems
- E.g. α-haemolysin of E. coli
18
Q
Which three of the sec independent systems form
a needle-like structure that extends beyond the OM and
can make contact with other cells
A
Type III, IV and VI
19
Q
Type III Secretion System
A
- Important in virulence in number of bacteria (e.g. E. coli, Salmonella)
- Injects virulence factors directly into animal host cells
- Evolved from flaggellar assembly proteins
20
Q
Examples of virulence factors injected by T3SS
A
- Toxins
- Phagocytosis inhibitors
- Invasins
21
Q
Type IV secretion system
A
- Complex of 12 or more proteins that form a tunnel like pilus that transfers proteins and DNA into host or bacteria cells
- Include conjugation system used to transfer plasmids (e.g. E. coli F plasmid)
- Important for bacterial viruelnce and intracellular survival
- Also present in G+ve but only function is DNA transfer
22
Q
Type VI secretion system T6SS
A
- Evolutionarily related to bacteriophage phage tails
- VrgG forms a conical structure so the spike has a sharp tip
- Delivers toxins such as peptidoglycan hydrolases from one bacterial cell to kill neighbouring cells
- Required for virulence of some species (e.g. V. cholerae)
23
Q
V. cholerae T6SS
A
Helps it to outcompete gut microbiota
24
Q
Firing of effector proteins by T6SS
A
- Type VI system assembles in cytoplasm and loads effectors onto tip of needle.
- In response to a signal indicating contact, the inner tube is ejected, puncturing the target cell and delivering effector proteins into target cell
25
Q
Secretion systems as vaccine and therapeutic targets
A
- Assembly of pilus adhesins on bacterial surface allows bacterial attachment to host cells
- Then delivery of toxins and other effector proteins to the medium or to the host cell via secretion systems occurs
- Blocking these functions might reduce pathogen virulence (antivirulence)
26
Q
A
