Phospholamban: Proteins and Enzymes

Phospholamban (PLB) is a 52 amino acid phosphoprotein which regulates the calcium pump of cardiac sarcoplasmic reticulum (SR). PLB is an oligomer of five identical subunits each having a cytoplasmic and transmembrane domain. The cytoplasmic domain (residues 1-25) contains the phosphorylation sites and is highly basic and readily cleaved by proteases; whereas the transmembrane domain (residues 25-52) is mostly hydrophobic, protease resistant and stabilizes the pentamer. Following adrenergic stimulation of cardiac muscle, PLB is phosphorylated at Ser16 and at Thr17 which is correlated with stimulation of calcium transport activity across the SR membrane and relaxation of cardiac fibers.