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.

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Abstract
Antisera (18) showing reactivity against the .alpha. subunit of transducin, the guanine nucleotide binding protein from rod outer segment, were tested for crossreactivity against the 40- and 39-kDa [kilodalton] 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 39- and 40-kDa guanine nucleotide binding proteins from brain differ immnochemically and that the COOH-terminal 5-kDa peptide of transducin .alpha. subunit is homologous to a region in the 40-kDa brain protein. This homologous region may be in a domain that confers specificity for receptor interactions of guanine nucleotide binding proteins.