PURIFICATION OF A NOVEL ISOFORM OF THE REGULATORY SUBUNIT OF CAMP-DEPENDENT PROTEIN-KINASE FROM THE BIVALVE MOLLUSK MYTILUS-GALLOPROVINCIALIS

Cytosolic extracts from the posterior adductor muscle of the bivalve m ollusk Mytilus galloprovincialis contain significant amounts of both c GMP-binding and cGMP-stimulated protein kinase activities. However, ph otoaffinity labeling with 8-azido-[P-32]cGMP revealed only a major cGM P-binding protein with an apparent molecular mass of 54 kDa (p54), lac king protein kinase activity itself. Instead, the purified and cGMP-fr ee p54 protein has the ability to inhibit a mussel protein kinase homo logous to the mammalian cAMP-dependent protein kinase (cAPK) catalytic subunit, the inhibition being relieved by cAMP or cGMP, which suggest s that it can act as a regulatory subunit of cAPK. However, p54 failed to be recognized by a specific antibody against the regulatory subuni t (type RII) previously isolated from mussel. Therefore, p54 must be a novel isoform of cAPK regulatory subunit that seems to have high affi nity for both cAMP and cAMP. (C) 1998 Academic Press.