Crystallization and preliminary X-ray study of the edema factor exotoxin adenylyl cyclase domain from Bacillus anthracis in the presence of its activator, calmodulin

Edema factor from Bacillus anthracis is a 92 kDa secreted adenylyl cyclase exotoxin and is activated by the host-resident protein calmodulin. Calmodulin is a ubiquitous intracellular calcium sensor in eukaryotes and activates edema factor nearly 1000-fold upon binding. While calmodulin has many known effectors, including kinases, phosphodiesterases, motor proteins, channels and type 1 adenylyl cyclases, no structures of calmodulin in complex with a functional enzyme have been solved. The crystallization and initial experimental phasing of crystals containing a complex of edema factor adenylyl cyclase domain and calmodulin are reported here. The edema factor-calmodulin complex crystallizes in three different space groups. A native data set in the I222 space group has been collected to 2.7 Å and the self-rotation function solution suggests three edema factor-calmodulin complexes in each asymmetric unit. Initial 4 Å phases were obtained by selenomethionyl MAD in combination with two heavy-atom derivatives. These phases were successfully extended to 2.7 Å using NCS averaging.