Molecular Structure and Function of Phospholamban: The Regulatory Protein of Calcium Pump in Cardiac Sarcoplasmic Reticulum
The excitation-contraction coupling of the myocardium represents a three-part process, involving three kinds of subcellular systems. These are sarcolemma, sarcoplasmic reticulum (SR), and myofibrillar proteins. Information transfer among these systems is exclusively carried out by Ca ions1 in that both membranes of sarcolemma and SR exhibit bi-directional Ca fluxes, and the myofibrillar system contains Ca receptor protein troponin. It is important to note that all of these three subcellular systems provide phosphorylation sites for protein kinases and, in addition, such phosphorylation reactions are thought to accompany profound alterations in Ca-related events in these systems. Among these, phosphorylation of phospholamban, a membrane protein in cardiac SR, and its functional consequences are extensively defined 2,3, in that phospholamban presumably serves to modulate Ca pump ATPase of SR by augmenting the key elementary steps of ATPase. Phospholamban of cardiac SR was purified to near homogeneity and was sequenced by amino acid and cDNA sequencing, demonstrating a unique molecular properties. This paper defines the functional and structural characteristics of the phospholamban-ATPase system and attemps to propose a molecular model for the functional unit of phospholamban, which provides a basic understanding for the regulatory mechanism of ion transport and bioenergetic transduction across biomembrane.
KeywordsSarcoplasmic Reticulum Cyanogen Bromide Fast Atom Bombardment Mass Spectrometry Sarcoplasmic Reticulum Membrane Cardiac Sarcoplasmic Reticulum
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