Correlation of Structure and Function in the Scallop Sarcoplasmic Reticulum

许多细胞过程，包括运动，是由细胞内的钙离子浓度控制的。在肌肉细胞中，用于调节收缩状态的钙离子储存在一个膜储存库，即肌浆网中。从SR释放钙会导致细胞收缩，而SR内钙的重新蓄积会导致松弛。钙的摄取是由于钙ATPase将钙离子泵过细胞膜。在泵送过程中，酶通过两种中间形式，这两种形式涉及蛋白质的磷酸化。该实验室对钙泵的功能很感兴趣，并一直采用扇贝肌肉制备SR的模型系统，该系统具有钙ATPase独特的准晶排列，这使得电子显微镜衍射分析和其他生物物理方法成为可能。该实验室最近发现，在扇贝肌肉SR中，这两种中间形式在SR膜上显示出不同的晶体排列。这一发现的一个含义是，在泵送周期中，酶分子可能会在膜中旋转。实验室还发现，扇贝SR的脂质成分与研究钙泵的常用生物材料兔肌肉SR有明显不同。将进一步开发这一独特的模式系统。钙的跨膜运输在所有细胞中都具有普遍的生理重要性，这项工作的结果将与细胞生物学和生物生理学的许多方面相关。

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.