Expression of A kinase anchor protein 75 depletes type II cAMP-dependent protein kinases from the cytoplasm and sequesters the kinases in a particulate pool

A kinase anchor proteins (AKAPs) have a C-terminal binding site for the regulatory subunit (RIIβ) of cAMP-dependent protein kinase IIβ (PKAIIβ) and an N-terminal domain that mediates the targeting and attachment of the anchor protein to intracellular structures. In vitro biochemical studies and recent in situ immunocytochemical analysis (Glantz, S. B., Amat, J. A., and Rubin, C. S. (1992) Mol. Biol. Cell 3, 1215-1228) suggest that AKAPs anchor PKAIIβ at specific sites in the dendritic cytoskeleton of neurons. This arrangement would place PKAIIβ in proximity with its substrates and create 'target sites' for cAMP actions. The foregoing model predicts that (a) RII subunits are freely accessible to AKAPs, (b) PKAII holoenzymes, as well as RII subunits, are anchored, and (c) changes in the level of AKAP can alter the intracellular distribution of type II PKAs. We have addressed these previously untested propositions by overexpressing bovine AKAP75 in a human cell line (HEK293). Non-transfected cells express a low level of endogenous AKAP79, and ~90% of RIIα and RIIβ subunits are isolated in the cell cytosol. In contrast, stably transfected cells, which express a 10-fold excess of AKAP75, sequester >90% of their RII subunits in a particulate pool. Catalytic subunits are also transferred to this pool. AKAP75 accumulates in a cell compartment with biochemical properties characteristic of cytoskeleton. Thus, AKAPs have access to and avidly bind cytoplasmic type II PKAs. Moreover, an increase in AKAP content can alter the particulate/cytoplasmic distribution of PKAIIβ and PKAIIα.