Saccharomyces cerevisiae contains twofunctional genesencoding 3-hydroxy-3-methylglutaryl-coenz yme

ABSTRACT Wehave isolated two genes from yeast en-coding 3-hydroxy-3-methylglutaryl-coenzyme A reductase[hydroxymethylglutaryl-coenzyme A reductase (NADPH);HMG-CoAreductase; EC1.1.1.34], the rate-limiting enzyme of sterol biosynthesis. These genes, HMGIandHMG2, were identified byhybridization to a cDNA cloneencodinghamsterHMG-CoAreductase. DNAsequence analysis reveals homol-ogy between the aminoacid sequence ofthe proteins encoded by the two yeast genes andthe carboxyl-terminal halfof thehamsterprotein. Cellscontainingmutant alleles of both HMGIand HMG2 are unable to undergo spore germination andvegetativegrowth.However,cellscontainingamutantalleleof either HMGI or HMG2are viable but are more sensitive tocompactin, a competitive inhibitor of HMG-CoAreductase,than are wild-type cells. Assays of HMG-CoA reductaseactivity in extracts fromhmglP andhmg2- mutants indicatethat HMG1contributes at least 83% ofthe activity found in wild-type cells.3-Hydroxy-3-methylglutaryl-coenzyme A reductase [hy-droxymethylglutaryl-coenzyme A reductase (NADPH);HMG-CoAreductase; EC 1.1.1.34] is the rate-limiting en-zyme of the sterol biosynthetic pathway. This pathwayproduces all isoprene-containing compounds, including ste-rols, ubiquinone, dolichol, and isopentenylated adenosine.Sincethevarious endproductsofthis pathwayhaveroles inmembrane structure, electron transport, glycoprotein bio-synthesis, translation, and DNAreplication, the level ofHMG-CoAreductase activity maycoordinate manycellularprocesses (reviewed in ref. 1).In mammalian cells, HMG-CoAreductase is an integralmembraneglycoproteinoftheendoplasmicreticulum(2) andis encodedby