Question

what are the 3 known contact dependent secretion systems? in detail, describe them and state their role in bacterial pathogenesis and their evolutionary origins

Answer 1

• Type 1
• found in large number of g-ve bacteria
• they transport their substrates across both inner and outer membranes in a one-step process
• evolutionarily close to ATB binding cassette transporters
• Structural components
  • an ABC transporter protein in the inner membrane; this functions to catalyze ATP to provide energy for the transfer and functions in substrate recognition
  • a membrane fusion protein which crosses the inner membrane joining to the outer membrane factor on the outer membrane; functions in substrate selection
  • outer membrane factor generates a pore in the outer membrane

Type II

• conserved in most g-ve bacteria
• transport folded proteins from periplasm to extracellular environment
• substrates transported are generally enzymes like proteases or lipases
• four subassemblies
  • outer membrane complex, composed of secretin
  • inner membrane platform
  • secretion ATPase, located in the cytoplasm and provides energy
  • pseudopilus share evolutionary closeness with pilus

Type III

• T3SSs; found in gram -ve bacterial pathogens and symbionts
• described as injectisomes
• secrete a wide variety of proteinaceous substrates across both inner and outer bacterial membranes
• might also transfer into a target eukaryotic cell in the same step therefore capable of transferring across three membranes
• has a core of 9 proteins; regarded conserved among the known systems; remaining 10-20 proteins play necessary or important roles
• 8 of these proteins share an evolutionarty relationship with flagellin
• three main components
  • base complex: contains cytoplasmic components and expands through inner and outer membrane, consisting of several rings and central rod
  • needle component: is a wide enough hollow core to allow passage of effector substrates; important for sensing contact
  • translocon: are assembled upon contact with the host cells, for passage of effector substrates through host cells

These secretion systems secrete bacterial products which generally seek to disarm host defenses or penetrate the host. They might include enzymes, toxins say for example YopE (effector) or Yersinia or SLO (toxin) of S.pyogenes.