Type III Secretion Systems

Question 1

what are the General Pathways for Secretion of Proteins in Bacteria?

Answer 1

1. General secretory pathway (Sec) across cytoplasmic membrane
2. Twin arginine (Tat) translocation – Across plasma membrane into periplasm or extracellular matrix.
   - Uses signal sequence (S- R-R-x-F-L-K) in N-terminal region. - Translocates folded proteins using proton motive force.

Question 2

What are the functions of bacterial secreted proteins?

Answer 2

Immune modulators
Toxins
Adhesions
Modulation of host membranes
Invasion

Question 3

what are the two types of Sec-Dependent Secretion Pathways?

Answer 3

1. Type II secretion (2-step)

2. Type V (autotransporters)

Question 4

what are the Sec-Independent Secretion Pathways?

Answer 4

1. Type I secretion (ABC transporters)
2. Type III secretion (contact-dependent)
3. Type IV secretion (conjugal transfer)
4. Type VI secretion
5. Type VII secretion

Question 5

how are proteins secreted from bacteria?

Answer 5

1. General secretory pathway (Sec) across cytoplasmic membrane – Gram positive & negative
2. Co-translational translocation of membrane proteins and proteins with very hydrophobic signal sequences.
   - Signal recognition particle targets ribosome to translocase through FtsY receptor.
3. Type II secretion(2-step)–Gram negative Sec-dependent transport of ‘secretins’ that form outer membrane pore for secretion of pilus components, enzymes, toxins (cholera toxin, exotoxin A, elastase), etc.

Question 6

Type I secretion(ABCtransporters)–Gram negative

Answer 6

Complex pore crosses both membranes. One group specific for large proteins and second group for small proteins and peptides.

Question 7

Type IV secretion(conjugaltransfer)–Gram negative

Answer 7

Pilus-like structure for transfer of DNA and proteins to another bacterial cell, to external environment, or to eukaryotic cells

Question 8

Type III secretion

Answer 8

Gram negative needle-like structure transfer effector proteins to another cell

Question 9

give 2 Examples of type III secretion systems

Answer 9

1. Yersinia enterocolitica
   - Yop – blocks phagocytosis, inhibits inflammatory
   response
2. Pseudomonas aeruginosa
   - Psc – cytoxicity, blocks phagoctyosis

Question 10

name 3 Yersinia species and the diseases they cause.

Answer 10

• Y. pestis – bubonic plague (lymph nodes; tick borne), septicaemic plague (blood and organs; tick borne), pneumonic plague (pulmonary; inhalation)
• Y. enterolitica – enteritis with spread to mesenteric lymph nodes (food/water)
• Y. pseudotuberculosis – mesenteric adenitis with granulomas in lymph nodes (food/water)

Question 11

what are the Major Yop Effectors?

Answer 11

1. YopH – 468 aa, major effect on pathogenesis, tyrosine phosphatase, inhibits phagocytosis, lysosome maturation, degranulation of neutrophils. Inhibits signaling via chemokines, possibly also B and T cell responses
2. YopM – ~409 aa, reduces pathogenesis, non-enzymatic protein, reduces proinflammatory cytokine release and causes depletion of NK cells
3. YopE – 219 aa, reduces pathogenesis, GTPase activating protein, inhibits non-opsonic phagocytosis and release of proinflammatory

Question 12

name 3 Minor Yop Effectors

Answer 12

1. YopJ – 288 aa, required for spread of bacteria inside host, cysteine protease inhibits MAPK and NF-kB, causes apoptosis of macrophages and dendritic cells in combination with TLR4 signal, suppresses cytokine release
2. YopO – 730 aa, variable effect on pathogenesis, anti-phagocytic, multifunctional, serine/threonine kinase binds to Rho
3. YopT – 322 aa, no effect on pathogenesis, antiphagocytic, cysteine protease.

Question 13

whats the prevalence of Diarrhoeal diseases?

Answer 13

• ~2 million deaths per year
• mainly children 50% of deaths due to enteropathogenic E. coli (EPEC)
• survivors may suffer irreversible damage to physical and mental development

Question 14

effectors encode by Locus of enterocyte effacement (LEE)

Answer 14

biogenesis of the type III secretion system 
chaperones
effector molecule
regulator
 damages mitochondria
translocated intimin receptor
intimin
filament
translocation pore
needle component
disrupts tight junctions

Question 15

what are the Functions of Pedestals in EPEC system?

Answer 15

1. Improve colonisation efficiency
   - actin-based motility
   - resistance to detachment
2. Improved translocation of effector proteins
   - EspB facilitates disassociation of host cell contacts
   - EspF/Map disrupt tight junctions, enter mitochondria and cause apoptosis
   - EspF inhibitis phagocytosis
   - EspG/EspG2 active the host protease calpain leading to disruption of tight junctions, cell rounding and cell death
3. Inhibition of phagocytosis

Question 16

Type III Secretion Systems Summary

Answer 16

1.  Bacteria (particular Gram -ves) have multiple pathways for secretion of proteins
2.  Type III secretion systems are multi-component complexes enabling ‘injection’ of effector proteins into host cells
3.  Yersinia use Type III secretion systems to prevent phagocytosis, to inhibit innate immunity and to prevent release of proinflammatory cytokines
4.  Enteropathogenic E. coli (EPEC) use a type III secretion system to ‘re-model’ the host cell and mediate intimate attachment and microcolony formation

Question 17

what are Type V (auto transporters)?

Answer 17

• Gram negative,
• large passenger domain (20-400 kDa)
• C-terminal translocator (~30 kDa).
• Include adhesins (YadA, Hia) and other functions (IgA protease of N. gonorrhoeae).
• Very common (500+ proteins)

Question 18

how does Type II secretion (2-step) work?

Answer 18

– Gram negative transport of ‘secretins’ form outermembrane pore for secretion of pilus components, enzymes,

Question 19

how does Type I secretion (ABC transporters) work?

Answer 19

• Gram positive & negative

- Complex pore crosses both membranes

Question 20

what is Type III secretion (contact-dependent)?

Answer 20

• Gram negative
• Channel across both bacterial membranes,
• injection pore through eukaryotic membrane

Question 21

Type IV secretion (conjugal transfer)

Answer 21

• Gram positive & negative
• Pilus-like structure for transfer of DNA and proteins to another bacterial cell, to external environment, or to eukaryotic cells

Question 22

what is Type VI secretion?

Answer 22

• Gram negative
• Phage-tail-spike-like injectisome similar to Type III & IV
• Present in a quarter of proteobacteria and required for virulence in both human and animal pathogens.
• Involved in delivery of effector proteins

Question 23

how does Type VII secretion work?

Answer 23

– Gram positive (mycobacterium)

• Integral inner membrane channel linked to separate channel in mycomembrane.
• Effectors secreted as heterodimers with chaperone

Question 24

how do the Yersinia cause disease?

Answer 24

1. Disseminate in macrophages during early stages
2. becomes resistant to phagocytosis in late stages
3. Virulence factors YadA (an adhesin), an invasin and the Yop T3SS located on plasmid
4. Proinflammatory cytokines (IFNg gamma and TNFa) protect against Yersinia infections