Anthrax protective antigen (PA) is one of the three proteins forming the anthrax toxin. It is involved in the first step of toxin entry into host cells, enabling lethal factor (LF) and oedema factor (EF) to cross to the cytosol. PA undergoes cleavage by Furin to generate PA20 and PA63. The latter, which adopts a ring-shaped heptameric assembly, shows four distinct domains. The fourth domain, a carboxy- terminal receptor-binding domain, shows an initial hairpin and helix, followed by a beta-sandwich with an immunoglobulin-like fold (this entry) [1-5].

1: Crystal structure of the anthrax toxin protective antigen. Petosa C, Collier RJ, Klimpel KR, Leppla SH, Liddington RC; Nature. 1997;385:833-838. PMID:9039918

2: Crystal structure of a complex between anthrax toxin and its host cell receptor. Santelli E, Bankston LA, Leppla SH, Liddington RC; Nature. 2004;430:905-908. PMID:15243628

3: Structure of heptameric protective antigen bound to an anthrax toxin receptor: a role for receptor in pH-dependent pore formation. Lacy DB, Wigelsworth DJ, Melnyk RA, Harrison SC, Collier RJ; Proc Natl Acad Sci U S A. 2004;101:13147-13151. PMID:15326297

4: Crystal structure of the engineered neutralizing antibody M18 complexed to domain 4 of the anthrax protective antigen. Leysath CE, Monzingo AF, Maynard JA, Barnett J, Georgiou G, Iverson BL, Robertus JD; J Mol Biol. 2009;387:680-693. PMID:19361425

5: The protective antigen component of anthrax toxin forms functional octameric complexes. Kintzer AF, Thoren KL, Sterling HJ, Dong KC, Feld GK, Tang II, Zhang TT, Williams ER, Berger JM, Krantz BA; J Mol Biol. 2009;392:614-629. PMID:19627991

6: Domain 4 of the anthrax protective antigen maintains structure and binding to the host receptor CMG2 at low pH. Williams AS, Lovell S, Anbanandam A, El-Chami R, Bann JG; Protein Sci. 2009;18:2277-2286. PMID:19722284