Abstract
Phospholemman (PLM), the principal sarcolemmal substrate for protein kinases A and C in the heart, regulates the cardiac sodium pump. We investigated post-translational modifications of PLM additional to phosphorylation in adult rat ventricular myocytes (ARVM). LC-MS/MS of tryptically digested PLM immunoprecipitated from ARVM identified cysteine 40 as palmitoylated in some peptides, but no information was obtained regarding the palmitoylation status of cysteine 42. PLM palmitoylation was confirmed by immunoprecipitating PLM from ARVM loaded with [(3)H]palmitic acid and immunoblotting following streptavidin affinity purification from ARVM lysates subjected to fatty acyl biotin exchange. Mutagenesis identified both Cys-40 and Cys-42 of PLM as palmitoylated. Phosphorylation of PLM at serine 68 by PKA in ARVM or transiently transfected HEK cells increased its palmitoylation, but PKA activation did not increase the palmitoylation of S68A PLM-YFP in HEK cells. Wild type and unpalmitoylatable PLM-YFP were all correctly targeted to the cell surface membrane, but the half-life of unpalmitoylatable PLM was reduced compared with wild type. In cells stably expressing inducible PLM, PLM expression inhibited the sodium pump, but PLM did not inhibit the sodium pump when palmitoylation was inhibited. Hence, palmitoylation of PLM controls its turnover, and palmitoylated PLM inhibits the sodium pump. Surprisingly, phosphorylation of PLM enhances its palmitoylation, probably through the enhanced mobility of the phosphorylated intracellular domain increasing the accessibility of cysteines for the palmitoylating enzyme, with interesting theoretical implications. All FXYD proteins have conserved intracellular cysteines, so FXYD protein palmitoylation may be a universal means to regulate the sodium pump.