Phospholamban of cardiac sarcoplasmic reticulum consists of two functionally distinct proteolipids

Phosphorylation of phospholamban by either a cAMP-dependent or a calmodulin-dependent kinase stimulates the Ca²⁺ transporting activity of cardiac sarcoplasmic reticulum membranes. It has now been found that phospholamban consists of 2 distinct proteins; one is the specific substrate for the cAMP-dependent phosphorylation, and the other for the calmodulin-dependent kinase. In spite of functional diversity, the 2 polypeptides share a number of properties. Among them, the proteolipid character, M_r, resistance to trypsinization, and subunit composition.