3-Hydroxy-3-methylglutaryl-CoA reductase: a transmembrane glycoprotein of the endoplasmic reticulum with N-linked "high-mannose" oligosaccharides.

3-Hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34) is an abundant protein of the crystalloid endoplasmic reticulum of UT-1 cells, a line of cultured hamster cells that over-produces the reductase as a result of gene amplification. In the current studies, we show that reductase in UT-1 cells is a glycoprotein. The solubilized enzyme (Mr = 97,000) from UT-1 cells, Chinese hamster ovary cells, and rat liver was adsorbed quantitatively and specifically to concanavalin A-Sepharose. UT-1 cells incorporated [1,6-3H]glucosamine into the reductase; after release with endo-N-acetylglucosaminidase H most of the radioactivity was found in N-linked "high-mannose" chains, including Man6(GlcNAc)2, Man7(GlcNAc)2, and Man8(GlcNAc)2. The carbohydrate of the reductase was localized to a 30- to 35-kilodalton fragment that was separable proteolytically from a cytoplasmic 53-kilodalton fragment that contained the active site of the enzyme. We conclude that 3-hydroxy-3-methylglutaryl-CoA reductase is a transmembrane glycoprotein with an active site facing the cytoplasm and a carbohydrate-bearing site oriented toward the lumen of the endoplasmic reticulum.