Homology of the gamma subunit of phosphorylase b kinase with cAMP-dependent protein kinase.

The complete amino acid sequence of the catalytic subunit (gamma subunit) of rabbit skeletal muscle phosphorylase b kinase was determined. The gamma subunit was purified by gel filtration in acidic 8 M urea after reduction and S-carboxymethylation in 7 M guanidine hydrochloride. Cleavage of the gamma subunit at arginyl bonds gave a complete set of nonoverlapping peptides. Overlapping peptides were obtained by cleavage at methionyl, tryptophanyl, or glutamyl bonds and by selected subdigestion of two large peptides obtained by cleavage at methionyl bonds. Sequence analysis established that the protein contains 386 residues corresponding to a molecular weight (Mr) of 44673. Comparison of the gamma subunit with the catalytic subunit of bovine cAMP-dependent protein kinase and with tyrosine-specific kinases of viral origin revealed a significant degree of sequence identity among all of these proteins. These data suggest that calcium-dependent protein kinases may share a common ancestral gene and a common structural basis for catalytic function with a wide variety of other protein kinases which respond to different signals and control quite different processes.