Phospholamban is the major membrane protein of the heart phosphorylated in response to beta-adrenergic stimulation. In cell-free systems, cAMP-dependent protein kinase catalyzes exclusive phosphorylation of serine 16 of phospholamban, whereas Ca2+/calmodulin-dependent protein kinase gives exclusive phosphorylation of threonine 17 (Simmerman, H. K. B., Collins, J. H., Theibert, J. L., Wegener, A. D., and Jones, L. R. (1986) J. Biol. Chem. 261, 13333-13341).