McGee T P, Cheng H H, Kumagai H, Omura S, Simoni R D
Department of Biological Sciences, Stanford University, Stanford, California 94305-5020, USA.
J Biol Chem. 1996 Oct 11;271(41):25630-8. doi: 10.1074/jbc.271.41.25630.
The endoplasmic reticulum (ER) membrane protein 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is subject to regulated degradation when cells are presented with an excess of sterols or mevalonate. In this report, we demonstrate the degradation of HMG-CoA reductase in ER membranes prepared from cells which have been pretreated with mevalonate or sterols prior to membrane purification. Degradation of HMG-CoA reductase in membranes prepared from pretreated cells is more rapid than in membranes prepared from cells which have received no regulatory molecules. In vitro degradation is blocked by protease inhibitors previously shown to inhibit reductase degradation in vivo and is specific for intact HMG-CoA reductase. The lumenal contents of the ER membranes are dispensible for the regulated proteolysis and the proteases responsible for reductase degradation are stably associated with the ER membrane. Regulated proteolysis of HMG-CoA reductase is inhibited by lactacystin, a newly defined inhibitor of the multicatalytic protease, the proteasome, and in vitro degradation of reductase correlates with the presence of proteasome subunits in purified ER membranes. The ubiquitin system for protein degradation, which has recently been shown to be required for the degradation of several ER membrane proteins, is not required for the degradation of HMG-CoA reductase. Finally, we conclude that the regulated proteolysis of HMG-CoA reductase in response to regulatory molecules such as mevalonate or sterols is mediated by increased susceptibility of the reductase to ER proteases, rather than the induction of a new proteolytic activity.
当细胞中存在过量的固醇或甲羟戊酸时，内质网（ER）膜蛋白3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶会发生受调控的降解。在本报告中，我们展示了从经甲羟戊酸或固醇预处理后再进行膜纯化的细胞中制备的内质网膜中HMG-CoA还原酶的降解情况。与从未接受调控分子处理的细胞制备的膜相比，经预处理的细胞制备的膜中HMG-CoA还原酶的降解速度更快。体外降解被先前证明能在体内抑制还原酶降解的蛋白酶抑制剂所阻断，且对完整的HMG-CoA还原酶具有特异性。内质网膜的腔内容物对于这种受调控的蛋白水解是可有可无的，负责还原酶降解的蛋白酶与内质网膜稳定结合。HMG-CoA还原酶的受调控蛋白水解受到乳胞素的抑制，乳胞素是一种新定义的多催化蛋白酶即蛋白酶体的抑制剂，还原酶的体外降解与纯化的内质网膜中蛋白酶体亚基的存在相关。最近已证明蛋白质降解的泛素系统是几种内质网 膜蛋白降解所必需的，但它不是HMG-CoA还原酶降解所必需的。最后，我们得出结论，HMG-CoA还原酶对甲羟戊酸或固醇等调控分子的受调控蛋白水解是由还原酶对内质网蛋白酶敏感性增加介导的，而不是诱导新的蛋白水解活性。
