IDENTIFICATION OF A HIGH-AFFINITY BINDING-PROTEIN FOR THE REGULATORY SUBUNIT RII-BETA OF CAMP-DEPENDENT PROTEIN-KINASE IN GOLGI ENRICHED MEMBRANES OF HUMAN LYMPHOBLASTS

Immunocytochemical evidence of an association between the regulatory subunit RII of the cAMP-dependent protein kinase (cAMP-PK) and the Golgi apparatus in several cell types has been reported. In order to identify endogenous Golgi proteins binding RII, a fraction enriched in Golgi vesicles was isolated from human lymphoblasts. Only the RII-beta isoform was detected in the Golgi-rich fraction, although RII-alpha has also been found to be present in these cells. A 85 kDa RII-binding protein was identified in Golgi vesicles using a [P-32]RII overlay of Western blots. The existence of an endogenous RII-beta-p85 complex in isolated Golgi vesicles was demonstrated by two independent means: (i) co-immunoprecipitation of both proteins under non-denaturing conditions with an antibody against RII-beta and (ii) co-purification of RII-beta-p85 complexes on a cAMP-analogue affinity column. p85 was phosphorylated by both endogenous and purified catalytic subunits of cAMP-pKII. Extraction experiments and protease protection experiments indicated that p85 is an integral membrane protein although it partitioned atypically during Triton X-114 phase separation. We propose that p85 anchors RII-beta to the Golgi apparatus of human lymphoblasts and thereby defines the Golgi substrate targets most accessible to phosphorylation by C subunit. This mechanism may be relevant to the regulation of processes involving the Golgi apparatus itself, such as membrane traffic and secretion, but also relevant to nearby nuclear events dependent on C subunit.