Type II regulatory subunits of cAMP-dependent protein kinase and their binding proteins in the nervous system of Aplysia californica

Two type II regulatory (R) subunits of cAMP-dependent protein kinase (PKA) of 50 and 47 kDa have been identified in Aplysia neurons by several criteria which include phosphorylation by the catalytic subunit of PKA and nanomolar affinity for a peptide fragment of the human thyroid protein Ht 31, properties that in mammals distinguish type II from type I R subunits. The neuronal type II R subunits are differentially localized within cells. For example, the 50-kDa polypeptide is enriched in taxol-stabilized microtubules. In addition, at least seven high molecular mass neuronal RII-binding proteins ranging in mass from 110 to 420 kDa have been demonstrated by a blot overlay technique, which uses ³²P-labeled bovine RIIα as a probe. The RII-binding proteins also exhibit discrete patterns of subcellular localization. For example, the 420 kDa species is enriched in taxol-stabilized microtubules and therefore may serve to anchor the 50-kDa RII subunit. The localization of PKA through the association of RII subunits with the binding proteins may anchor the multifunctional kinase close to key substrates and thereby contribute to the spatial organization required to mediate the orderly phosphorylation events that underly neuronal modulation.