3-Hydroxy-3-methylglutaryl Coenzyme A Reductase Is Sterol-Dependently Cleaved by Cathepsin L-Type Cysteine Protease in the Isolated Endoplasmic Reticulum

We have recently shown that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, an endoplasmic reticulum (ER) membrane protein, is degraded in ER membranes prepared from sterol pretreated cells and that such degradation is catalyzed by a cysteine protease within the reductase membrane domain. The use of various protease inhibitors suggested that degradation of HMG-CoA reductase in vitro is catalyzed by a cathepsin L-type cysteine protease. Purified ER contains E-64-sensitive cathepsin L activity whose inhibitor sensitivity was well matched to that of HMG-CoA reductase degradation in vitro. CLIK-148 (cathepsin L inhibitor) inhibited degradation of HMG-CoA reductase in vitro. Purified cathepsin L also efficiently cleaved HMG-CoA reductase in isolated ER preparations. To determine whether a cathepsin L-type cysteine protease is involved in sterol-regulated degradation of HMG-CoA reductase in vivo, we examined the effect of E-64d, a membrane-permeable cysteine protease inhibitor, in living cells. While lactacystin, a proteasome-specific inhibitor, inhibited sterol-dependent degradation of HMG-CoA reductase, E-64d failed to do so. In contrast, degradation of HMG-CoA reductase in sonicated cells was inhibited by E-64d, CLIK-148, and leupeptin but not by lactacystin. Our results indicate that HMG-CoA reductase is degraded by the proteasome under normal conditions in living cells and that it is cleaved by cathepsin L leaked from lysosomes during preparation of the ER, thus clarifying the apparently paradoxical in vivo and in vitro results. Cathepsin L-dependent proteolysis was observed to occur preferentially in sterol-pretreated cells, suggesting that sterol treatment results in conformational changes in HMG-CoA reductase that make it more susceptible to such cleavage.