Substrate for botulinum ADP-ribosyltransferase, Gb, has an amino acid sequence homologous to a putative rho gene product.

Gb (b for botulinum) is an Mr 21,000-22,000 guanine nucleotide binding protein which is specifically ADP-ribosylated by an ADP-ribosyltransferase produced by types C1 and D Clostridium botulinum (Morii, N., Sekine, A., Ohashi, Y., Nakao, K., Imura, H., Fujiwara, M., and Narumiya, S. (1988) J. Biol. Chem. 263, 12420-12426). In this study, we have identified the partial amino acid sequence of this protein. Purified Gb was digested with various proteases, and the proteolytic fragments were separated by high performance liquid chromatography. Isolated peptides were subjected to protein sequencing by an automated pulse-liquid phase protein sequenator. Eight partial amino acid sequences consisting of a total of 105 amino acid residues were obtained from 16 peptides. Computerized homology analysis revealed that they are homologous to the sequence deduced from cDNA of the rho genes. About 88% of the amino acid residues was identical between the sequenced Gb fragments and the corresponding regions of the Aplysia rho gene product. The result strongly suggests that Gb is a putative rho gene product.