This domain is found in Oedema factor (EF), the exotoxin with adenylyl cyclase activity produced by Bacillus anthracis to increase the concentration of cyclic AMP in infected host cells and disrupts intracellular signalling pathways. EF consists of four domains: a PA-binding domain (PABD), two adenylate cyclase domains (ACD) and a helical domain (HD, this entry). HD moves and forms a large clamp that almost completely encircles EF's activator calmodulin (CaM), naturally present in host cells, stabilizing a disordered loop and leading to enzyme activation [1,4].

1: Structural basis for the activation of anthrax adenylyl cyclase exotoxin by calmodulin. Drum CL, Yan SZ, Bard J, Shen YQ, Lu D, Soelaiman S, Grabarek Z, Bohm A, Tang WJ; Nature. 2002;415:396-402. PMID:11807546

2: Selective inhibition of anthrax edema factor by adefovir, a drug for chronic hepatitis B virus infection. Shen Y, Zhukovskaya NL, Zimmer MI, Soelaiman S, Bergson P, Wang CR, Gibbs CS, Tang WJ; Proc Natl Acad Sci U S A. 2004;101:3242-3247. PMID:14978283

3: Calcium-independent calmodulin binding and two-metal-ion catalytic mechanism of anthrax edema factor. Shen Y, Zhukovskaya NL, Guo Q, Florian J, Tang WJ; EMBO J. 2005;24:929-941. PMID:15719022

4: Atomic structures of anthrax toxin protective antigen channels bound to partially unfolded lethal and edema factors. Hardenbrook NJ, Liu S, Zhou K, Ghosal K, Hong Zhou Z, Krantz BA; Nat Commun. 2020;11:840. PMID:32047164