Molecular cloning of a rat testis form of the inhibitor protein of cAMP-dependent protein kinase.

The form of inhibitor protein of the cAMP-dependent protein kinase (PKI) that has been most thoroughly studied is a protein purified from rabbit skeletal muscle. Beale et al. previously isolated a species of PKI from rat testis that appeared from its amino acid composition to be quite distinct from the rabbit skeletal muscle protein [Beale, E. G., Dedman, J. R. & Means, A. R. (1977) J. Biol. Chem. 252, 6322-6327]. The amino acid sequence of a form of rat testis PKI has now been determined both by sequencing overlapping peptide fragments for 95% of the protein and by the isolation of a cDNA clone containing the coding region for the 70-amino acid protein. The sequence of the 70-amino acid testis PKI displays a maximum of only 41% sequence identity with the previously sequenced 75-amino acid rabbit skeletal muscle PKI. However, the two forms have identical potency as inhibitors and the key amino acids of the pseudosubstrate site, shown to be critical for maximal inhibition with the rabbit skeletal muscle PKI, have been conserved in the testis protein. The rabbit skeletal muscle and rat testis PKIs most likely represent distinct isoforms. The nucleotide sequence of the rat testis PKI cDNA suggests that a second form of testis PKI, longer by 8 additional amino-terminal amino acids, might also be produced.