Helical structure of phospholamban in membrane bilayers

The regulation of calcium levels across the membrane of the sarcoplasmic re ticulum involves the complex interplay of several membrane proteins. Phosph olamban is a 52 residue integral membrane protein that is involved in rever sibly inhibiting the Ca2+ pump and regulating the flow of Ca ions across th e sarcoplasmic reticulum membrane during muscle contraction and relaxation. The structure of phospholamban is central to its regulatory role. Using ho monuclear rotational resonance NMR methods, we show that the internuclear d istances between [1-C-13]Leu7 and [3-C-13]Ala11 in the cytoplasmic region, between [1-C-13]Pro21 and [3-C-13]Ala24 in the juxtamembrane region and bet ween [1-C-13]Leu42 and [3-C-13]Cys46 in the transmembrane domain of phospho lamban are consistent with alpha -helical secondary structure. Additional h eteronuclear rotational-echo double-resonance NMR measurements confirm that the secondary structure is helical in the region of Pro21 and that there a re no large conformational changes upon phosphorylation. These results supp ort the model of the phospholamban pentamer as a bundle of five long alpha -helices. The long extended helices provide a mechanism by which the cytopl asmic region of phospholamban interacts with residues in the cytoplasmic do main of the Ca2+ pump. (C) 2001 Academic Press.