Anthrax protective antigen

Function

Anthrax protective antigen (PA) is the cell-binding part of the Bacillus anthracis anthrax toxin. The anthrax toxin is composed of the PA, lethal factor and edema factor. The PA translocates the lethal (LF) and edema factors (EF) into the cell. The 83kD form of PA (PA83) binds to the anthrax toxin receptor. A 20kD fragment (PA20) is cleaved off the PA83 and the remaining PA63 assembles into ring-shaped oligomer which becomes a membrane pre-channel. The PA pre-channel binds the lethal and edema factors allowing them to penetrate the membrane via pH gradient. For more details see Molecular Playground/Protective Antigen.

Structural highlights

PA contains 4 domains:

• (yellow). It contains the furin protease cleavage site enabling the release of PA20.
  
• Domain 2 is the heptamerization domain of PA63.
  
• Domain 3 forms the heptameric pore with domain 2 and enabling LF and EF to penetrate the cell.
  
• Domain 4 is important for receptor recognition and pore formation.