Delivery of Virulence Factors Flashcards
What is Sec translocase?
• Transport across inner membrane
1. Signal sequence binds secB chaperone
2. Complex recognised by Sec machinery
3. SecB takes protein through Sec translocation pathway and targets protein to the translocase by binding SecA
4. Pre-protein bears SecA, binds to membrane at high affinity SecA binding site
5. Hetero-timeric complex forms (SecY, E, G) and makes up channel for protein
• Not for folded proteins
• Can have cotranslational coupled secretion
o SRP, pause translation, no SecB chaperone
What is the Tat pathway?
- Transport across inner membrane
- TatBC: docking complex
- TatA: pore forming complex
- Twin arginine secretion motif in N terminus
- Protein can refold before secretion (often necessary for cofactors to bind)
- Substrate transported through TatABC translocon (folded state)
- Simple substrates without cofacotrs can be exported unfolded
What is an autotransporter?
• Sec dependent
• E.g. IcsA in shigella
• Protein has the info needed to be transported (domains)
• BAM complex
• Three domains
o Extended N terminal signal sequence, translocation across cytoplasmic membrane via Sec translocase
o Functional domain (alpha) – acts as protease, toxin etc.
o Carboxy-terminal beta domain for B barrel
What is the role of the BAM complex?
o Helps fold B barrels into outer membrane
What is a two partner secretion (TPS) system?
- Sec dependent
- Similar strategy to autotransporters
- Need additional protein than autotransporter (separate B barrel)
What are the features of the chaperone/usher pathway?
- Sec dependent
- Cross cytoplasmic membrane via sec translocase
- Secretion across outer membrane via periplasmic chaperone and outer membrane protein (usher)
- Type I/Pap pili
What are the features of a type I secretion system?
• Sec Independent
• Tocins, proteases, lipases
• Secrete from cytoplasm right across outer membrane without periplasmic intermediate
• Secreted proteins lack typical signal peptide, have C-terminal secretion signal
• Three proteins
o ATP binding cascade (ABC) in cytoplasmic membrane
o Membrane fusion protein (MFP) anchored to cytoplasmic membrane, spans periplasm
o Outer membrane protein (B barrel)
What are the features of a typeII secretion system?
• Sec dependent
• Related to what’s needed for biogenesis of type 4 pili (similar genes)
• Secretion across OM more complex (12-16 proteins)
• Outer membrane pore:
o GspD (forms big complex, 12-14 subunits)
o Secretin family
o Highly stable ring
• Accessory proteins collectively called secreton
How do T2SS and Type IV pili systems compare?
• Type 4 pilus system secretes pilin
o Synthesise pre-pilin (PilA) with unusual signal peptide not recognised by normal peptidase
o Specific signal peptidase removes signal peptide (PilD) on cytoplasmic side (not in periplasm like in Type II secretion system)
What are the features of a T3SS?
- Gram negative pathogens
- Direct secretion, directly translocate bacterial proteins
- Original = Yersinia system (Yops)
- Complex array of proteins spanning IM, Periplasm, OM
- Chaperones for effector proteins
What are the features of T3SS effector proteins?
• Sec independent
• No periplasmic stage
• Triggered by contact with eukaryotic cell
• Secreted proteins have N terminal sequence recognised by specific chaperone
o Specific chaperone (Syc) for specific protein
o Guide to secretion apparatus
o Sequence NOT cleaved off (unlike sec pathway)
What are the features of a T4SS?
- Related to bacterial conjugation systems
- Complex spans periplasm
- Agrobacterium: VirB11, BirB4 (cytoplasmic ATPases for secretion)
- DNA transfer, uptake/competence, virulence
- Stepwise
Pilus
• Major subunit is VirB2
• Cell to cell contact, channel
What are some examples of bacteria with a T4SS?
- Helicobacter
- Bordetella
- Brucella
- Legionella
- Agrobacterium
What are some examples of how different substrates are trafficked by T4SS?
- E.g. Pertussis toxin. Made as pre-protein, typical signal peptide, cross cytoplasmic membrane via Sec, in periplasm, cross OM via Type IV machinery
- E.g DNA and CagA protein. Move in single step from cytoplasm to outside cell, doesn’t enter periplasm
- E.g. Legionella unknown mechanism
What are the features of a T6SS?
- Transport across bacterial envelope, translocate effecters into host cells
- Not much known (Frankenstein)
- Shared features with bacteriophage cell puncturing protein
- Hcp protein like phage tail (extends out of OM)
- IcmF protein like legionella dot/ICM
