Targeting of type I protein kinase A to lipid rafts is required for platelet inhibition by the 3,5-cyclic adenosine monophosphate-signaling pathway

BackgroundPlatelet adhesion to von Willebrand factor (VWF) is modulated by 3,5-cyclic adenosine monophosphate (cAMP) signaling through protein kinase A (PKA)-mediated phosphorylation of glycoprotein (GP)Ib. A-kinase anchoring proteins (AKAPs) are proposed to control the localization and substrate specificity of individual PKA isoforms. However, the role of PKA isoforms in regulating the phosphorylation of GPIb and platelet response to VWF is unknown. ObjectivesWe wished to determine the role of PKA isoforms in the phosphorylation of GPIb and platelet activation by VWF as a model for exploring the selective partitioning of cAMP signaling in platelets. ResultsThe two isoforms of PKA in platelets, type I (PKA-I) and type II (PKA-II), were differentially localized, with a small pool of PKA-I found in lipid rafts. Using a combination of Far Western blotting, immunoprecipitation, proximity ligation assay and cAMP pull-down we identified moesin as an AKAP that potentially localizes PKA-I to rafts. Introduction of cell-permeable anchoring disruptor peptide, RI anchoring disruptor (RIAD-Arg(11)), to block PKA-I/AKAP interactions, uncoupled PKA-RI from moesin, displaced PKA-RI from rafts and reduced kinase activity in rafts. Examination of GPIb demonstrated that it was phosphorylated in response to low concentrations of PGI(2) in a PKA-dependent manner and occurred primarily in lipid raft fractions. RIAD-Arg(11) caused a significant reduction in raft-localized phosphoGPIb and diminished the ability of PGI(2) to regulate VWF-mediated aggregation and thrombus formation invitro. ConclusionWe propose that PKA-I-specific AKAPs in platelets, including moesin, organize a selective localization of PKA-I required for phosphorylation of GPIb and contribute to inhibition of platelet VWF interactions.