Many cellular processes, including motility, are controlled by the calcium ion concentration within the cell. In muscle cells, the calcium ions used to regulate the state of contraction are stored within a membranous reservoir, the sarcoplasmic reticulum. Release of calcium from the SR results in cellular contraction; resequestration of the calcium within the SR results in relaxation. The uptake of calcium is due to a calcium ATPase which pumps calcium ions across the membrane. During pumping, the enzyme passes through two intermediate forms which involve phosphorylation of the protein. This laboratory is interested in how this calcium pump functions, and has been employing as a model system a preparation of SR from scallop muscle, which has a uniquely paracrystalline arrangement of the calcium ATPase which makes possible electron microscopic diffraction analysis and other biophysical approaches. This laboratory has recently found that, in scallop muscle SR, the two intermediate forms show a different crystalline arrangement in the SR membrane. One of the implications of this finding is that the enzyme molecules may rotate in the membrane during the pumping cycle. The laboratory has also found that the lipid composition of the scallop SR is strikingly different from rabbit muscle SR, which is the common biological material for studies of the calcium pump. Further exploitation of this unique model system will be pursued. Transport of calcium across membranes is of universal physiological importance in all cells, and results from this work will have relevance to many aspects of cell biology and organismal physiology.