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 [32P]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.