Plasma membrane Ca2+-ATPase associates with CLP36, α-actinin and actin in human platelets

The plasma membrane Ca2+-ATPase (PMCA) plays an essential role in maintaining low cytosolic Ca2+ in resting platelets. Earlier studies demonstrated that the 4b isoform of PMCA interacts via its C-terminal end with the PDZ domains of membrane-associated guanylate kinase proteins. Activation of saponin-permeabilized platelets in the presence of a peptide composed of the last ten residues of the PMCA4b C-terminus leads to a significant decrease of PMCA associated with the cytoskeleton, suggesting that PDZ domain interactions play a role in tethering the pump to the cytoskeleton. Here we present experiments conducted to evaluate the mechanism of this association. Co-immunoprecipitation assays coupled with liquid chromatography/tandem mass spectrometry analysis and immunoblotting were used to identify proteins that interact with PMCA in the resting platelet. Our results indicate that the only PDZ domain-containing protein associated with PMCA is the LIM family protein, CLP36. Glutathione-S-transferase pull-down from a platelet extract using a fusion protein containing the C-terminal PDZ domain binding motif of PMCA confirmed binding of CLP36 to PMCA. Gel filtration chromatography of detergent-solubilized platelets demonstrated the existence of a 1,000-kDa complex containing PMCA and CLP36, and in addition, a-actinin and actin. Immunoflourescence microscopy confirmed the co-localization of PMCA with CLP36 in resting and activated platelets. Taken together these results suggest that PMCA is localized in non-filamentous actin complexes in resting platelets by means of PDZ domain interactions and then associates with the actin cytoskeleton during cytoskeletal rearrangement upon platelet activation. Thus, in addition to the reversible serine/threonine and tyrosine phosphorylation events previously described in human platelets, PMCA function may be regulated by interactions with anchoring and cytoskeletal proteins.