Dual site phospholamban phosphorylation and its physiological relevance in the heart

Phospholamban (PLB) plays a primary role in regulating cardiac sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity. Dephosphorylated PLB suppresses the SR Ca(2+) pump activity, whereas phosphorylation of PLB leads to deinhibition. A widely accepted sequential model of dual site PLB phosphorylation states that PKA-dependent phosphorylation of Ser(16) is obligatory to phosphorylation of Thr(17) by Ca(2+)/calmodulin-dependent kinase II, and mainly accounts for beta-adrenergic receptor mediated cardiac relaxation. However, emerging evidence supports independent phosphorylation of Ser(16) and Thr(17) and their independent contributions to cardiac relaxation. Furthermore, concurrent activation of PKA and CaMKII signaling pathways exhibits a robust synergistic effect on phosphorylation of Thr(17), but not of Ser(16). Thus, the synergistic interaction may masquerade as a sequential phosphorylation of Ser(16) and Thr(17) under certain circumstances. Further studies are required to determine the exact process of dual site PLB phosphorylation and its functional roles in healthy and diseased hearts.