Antibodies against the carboxyl-terminal 5-kDa peptide of the alpha subunit of transducin crossreact with the 40-kDa but not the 39-kDa guanine nucleotide binding protein from brain.

We tested 18 antisera showing reactivity against the alpha subunit of transducin, the guanine nucleotide binding protein from rod outer segment, for crossreactivity against the 40- and 39-kDa guanine nucleotide binding proteins purified from bovine brain. A single antiserum, CW6, showed crossreactivity, and this was predominantly against the 40-kDa protein. Immunoblots of the tryptic fragments of transducin alpha subunit with multiple antisera raised against that subunit showed that only CW6 recognizes a COOH-terminal 5-kDa peptide that includes the site of pertussis toxin ADP-ribosylation. Antibodies against the 5-kDa peptide, affinity-purified from CW6, specifically react with the 40-kDa brain protein on immunoblots. The results show that the 39- and 40-kDa guanine nucleotide binding proteins from brain differ immunochemically and that the COOH-terminal 5-kDa peptide of transducin alpha subunit is homologous to a region in the 40-kDa brain protein. We speculate that this homologous region may be in a domain that confers specificity for receptor interactions of guanine nucleotide binding proteins.