Molecular mechanism of calcium uptake and release by cardiac sarcoplasmic reticulum

Cardiac sarcoplasmic reticulum (SR) plays a special role in controlling free calcium ions (Ca) in heart muscle cells. Ca stored in the SR is released through the Ca release channels when the sarcolemmal membrane is depolarized, thereby inducing contraction, while Ca is reaccumulated by the Ca pump to induce relaxation. In the latter process, the Ca pump of cardiac SR has a regulatory system by cAMP-dependent phosphorylation of a SR protein, phospholamban. Recently, significant progress has been achieved in understanding the molecular mechanisms of Ca release and uptake and its regulation. The structures of the Ca pump and phospholamban have been defined at molecular levels. A direct interaction between these two proteins was demonstrated. The Ca release channel was identified, and turned out to be the foot structure which in situ connects the SR to the sarcolemma/transverse tubule.