A single site (Ser(16)) phosphorylation in phospholamban is sufficient in mediating its maximal cardiac responses to beta-agonists

Phospholamban (PLB) can be phosphorylated at Ser(16) by cyclic AMP-dependen t protein kinase and at Thr(17) by Ca2+-calmodulin-dependent protein kinase during beta -agonist stimulation. A previous study indicated that mutation of S16A in PLB resulted in lack of Thr(17) phosphorylation and attenuation of the beta -agonist stimulatory effects in perfused mouse hearts. To furt her delineate the functional interplay between dual-site PLB phosphorylatio n, we generated transgenic mice expressing the T17A mutant PLB in the cardi ac compartment of the null background. Lines expressing similar levels of T 17A mutant, S16A mutant, or wild-type PLB in the null background mere chara cterized in parallel. Cardiac myocyte basal mechanics and Ca2+ kinetics wer e similar among the three groups. Isoproterenol stimulation was associated with phosphorylation of both Ser(16) and Thr(17) in wild-type PLB and Ser(1 6) phosphorylation in T17A mutant PLB, whereas there was no detectable phos phorylation of S16A mutant PLB. Phosphorylation of Ser(16) alone in T17A mu tant PLB resulted in responses of the mechanical and Ca2+ kinetic parameter s to isoproterenol similar to those in wild-type myocytes, which exhibited dual-site PLB phosphorylation. However, those parameters were significantly attenuated in the S16A mutant myocytes. Thus, Ser(16) in PLB can be phosph orylated independently of Thr(17) in vivo, and phosphorylation of Ser(16) i s sufficient for mediating the maximal cardiac responses to beta -adrenergi c stimulation.