Crystal structure of the ADP-ribosylating component of BEC, the binary enterotoxin of Clostridium perfringens

Binary enterotoxin of Clostridium perfringens (BEC), consisting of the components BECa and BECb, was recently identified as a novel enterotoxin produced by C. perfringens that causes acute gastroenteritis in humans. Although the detailed mechanism of cell intoxication by BEC remains to be defined, BECa shows both NAD⁺-glycohydrolase and actin ADP-ribosyltransferase activities in the presence of NAD⁺. In this study, we determined the first crystal structure of BECa in its apo-state and in complex with NADH. The structure of BECa shows striking resemblance with other binary actin ADP-ribosylating toxins (ADPRTs), especially in terms of its overall protein fold and mechanisms of substrate recognition. We present a detailed picture of interactions between BECa and NADH, including bound water molecules located near the C1'-N glycosidic bond of NADH and the catalytically important ADP-ribosylating turn-turn (ARTT) loop. We observed that the conformational rearrangement of the ARTT loop, possibly triggered by a conformational change involving a conserved tyrosine residue coupled with substrate binding, plays a crucial role in catalysis by properly positioning a catalytic glutamate residue in the E-X-E motif of the ARTT loop in contact with the nucleophile. Our results for BECa provide insight into the common catalytic mechanism of the family of binary actin ADPRTs.