Characterization of a type I regulatory subunit of cAMP-dependent protein kinase from the bivalve mollusk Mytilus galloprovincialis

Two isoforms of the regulatory subunit (R) of cAMP-dependent protein kinase (PKA), named Rmyt1 and Rmyt2, had been purified in our laboratory from two different tissues of the sea mussel Mytilus galloprovincialis. In this paper, we report the sequences of several peptides obtained from tryptic digestion of Rmyt1. As a whole, these sequences showed high homology with regions of type I R subunits from invertebrate and also from mammalian sources, but homology with those of fungal and type II R subunits was much lower, which indicates that Rmyt1 can be considered as a type I R isoform. This conclusion is also supported by the following biochemical properties: (1) Rmyt1 was proved to have interchain disulfide bonds stabilizing its dimeric structure; (2) it failed to be phosphorylated by the catalytic (C) subunit purified from mussel; (3) it has a higher pI value than that of the Rmyt2 isoform; and (4) it showed cross-reactivity with mammalian anti-RIβ antibody.