7 - Bacterial secretion systems

Question 1

Where is one third of bacterial proteins found

Answer 1

• PM, OM and periplasm (by translocation)
• Or external environment (by secretion)

Question 2

Secretion in gram positive bacteria

Answer 2

• Proteins must be translocated across PM
• Either pass through porous CW (secreted) or become embedded and attached to CW

Question 2

Secretion in gram negative bacteria

Answer 2

• 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

Question 2

The Sec System (General secretion pathway)

Answer 2

• Major pathway for translocating proteins across the plasma membrane
• Common to G+ and G- bacteria
• Transports proteins in unfolded state

Question 3

Co-translational translocation in the sec system

Answer 3

• 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

Question 4

Post translational translocation in the sec system

Answer 4

• 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

Question 5

Tat pathway

Answer 5

• 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

Question 6

Secretion systems across OM of gram -ve bacteria

Answer 6

• At least nine different mechanisms (type I to IX)
• Sec dependent or sec independent

Question 7

Sec dependent systems

Answer 7

• Depends on general sec (or tat) system for transport from cytoplasm into periplasm
• Eg Type II and type V

Question 8

Sec independent systems

Answer 8

• DO NOT depend on general sec or tat systems for transport
• Eg Type I, III, IV, VI

Question 9

Type II secretion system (T2SS)

Answer 9

• 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

Question 10

What does T2SS secrete

Answer 10

• Numerous enzymes (e.g. proteases, lipases
• Some AB toxins (e.g.
  cholera toxin)

Question 11

Pseudopilus

Answer 11

• 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

Question 12

Type V secretion system (T5SS)

Answer 12

• 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)

Question 13

What does T5SS secrete

Answer 13

Virulence proteins (e.g. IgA protease of N. gonorrhoeae destroys host antibodies

Question 14

2 domains of autotransporter

Answer 14

• 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

Question 15

Type I secretion system T1SS

Answer 15

• 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

Question 16

Three protein complex of T1SS

Answer 16

1. the PM component is an ATP-binding cassette ABC transporter (ATPase activity provides energy to cross PM)
2. the OM component is a pore-forming protein TolC
3. A connecting protein holds them together

Question 17

What do T1SS secrete

Answer 17

• Virulence factors & resistance systems
• E.g. α-haemolysin of E. coli

Question 18

Which three of the sec independent systems form
a needle-like structure that extends beyond the OM and
can make contact with other cells

Answer 18

Type III, IV and VI

Question 19

Type III Secretion System

Answer 19

• 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

Question 20

Examples of virulence factors injected by T3SS

Answer 20

• Toxins
• Phagocytosis inhibitors
• Invasins

Question 21

Type IV secretion system

Answer 21

• 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

Question 22

Type VI secretion system T6SS

Answer 22

• 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)

Question 23

V. cholerae T6SS

Answer 23

Helps it to outcompete gut microbiota

Question 24

Firing of effector proteins by T6SS

Answer 24

• 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

Question 25

Secretion systems as vaccine and therapeutic targets

Answer 25

• 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)