A novel lipid-anchored A-kinase anchoring protein facilitates cAMP-responsive membrane events

Compartmentalization of protein kinases with substrates is a mechanism that may promote specificity of intracellular phosphorylation events. We have cloned a low-molecular weight A-kinase Anchoring Protein, called AKAP18, which targets the cAMP-dependent protein kinase (PKA) to the plasma membrane, and permits functional coupling to the L-type calcium channel. Membrane anchoring is mediated by the first 10 amino acids of AKAP18, and involves residues Gly1, Cys4 and Cys5 which are lipid-modified through myristoylation and dual palmitoylation, respectively. Transient transfection of AKAP18 into HEK-293 cells expressing the cardiac L-type Ca²⁺ channel promoted a 34 ± 9% increase in cAMP-responsive Ca²⁺ currents. In contrast, a targeting-deficient mutant of AKAP18 had no effect on Ca²⁺ currents in response to the application of a cAMP analog. Further studies demonstrate that AKAP18 facilitates GLP-1-mediated insulin secretion in a pancreatic β cell line (RINm5F), suggesting that membrane anchoring of the kinase participates in physiologically relevant cAMP-responsive events that may involve ion channel activation.