COMMON STRUCTURE OF THE CATALYTIC SITES OF MAMMALIAN AND BACTERIAL TOXIN ADP-RIBOSYLTRANSFERASES

The amino acid sequences of several bacterial toxin ADP-ribosyltransfe rases, rabbit skeletal muscle transferases, and RT6.2, a rat T-cell NA D glycohydrolase, contain three separate regions of similarity, which can be aligned. Region I contains a critical histidine or arginine res idue, region II, a group of closely spaced aromatic amino acids, and r egion III, an active-site glutamate which is at time sseen as part of an acidic amino acid-rich sequence. In some of the bacterial ADP-ribos yltransferases, the nicotinamide moiety of NAD has been photo-crosslin ked to this glutamate, consistent with its position in the active site . The similarities within these three regions, despite an absence of o verall sequence similarity among the several transferases, are consist ent with a common structure involved in NAD binding and ADP-ribose tra nsfer.