Membrane-bound domain of HMG CoA reductase is required for sterol-enhanced degradation of the enzyme.
3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) is a single polypeptide chain with two contiguous domains: a soluble domain (548 amino acids) that catalyzes the rate-controlling step in cholesterol synthesis and a membrane-bound domain (339 amino acids) that anchors the protein to the endoplasmic reticulum (ER). HMG CoA reductase is degraded at least 10-fold more rapidly than other ER proteins; degradation is accelerated in the presence of cholesterol. To understand this controlled degradation, we transfected reductase-deficient Chinese hamster ovary (CHO) cells with a plasmid expression vector containing a reductase cDNA that lacks the segment encoding the membrane domain. The plasmid produced a truncated reductase (37 kd smaller than normal) that was enzymatically active with normal kinetics; most of the truncated enzyme was found in the cytosol. The truncated enzyme was degraded one-fifth as fast as the holoenzyme; degradation was no longer accelerated by sterols. We conclude that the membrane-bound domain of reductase plays a crucial role in the rapid and regulated degradation of this ER protein.
D000818:Animals; D002460:Cell Line; D002784:Cholesterol / Q000494:pharmacology*; D006224:Cricetinae; D003600:Cytosol / Q000201:enzymology; D004274:DNA, Recombinant; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D005260:Female; D006888:Hydroxycholesterols / Q000494:pharmacology*; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics / Q000378:metabolism*; D007425:Intracellular Membranes / Q000201:enzymology*; D007700:Kinetics; D008148:Lovastatin / Q000031:analogs & derivatives*; D009281:Naphthalenes / Q000494:pharmacology; D010053:Ovary; D010957:Plasmids; D014162:Transfection
D004274:DNA, Recombinant; D006888:Hydroxycholesterols; D009281:Naphthalenes; C012258:mevastatin; C007997:25-hydroxycholesterol; D002784:Cholesterol; D008148:Lovastatin; D006903:Hydroxymethylglutaryl CoA Reductases