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

Abstract

3-Hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34) [3-HMG-CoA] is an abundant protein of the crystalloid endoplasmic reticulum of UT-1 cells, a line of cultured hamster cells that overproduces the reductase as a result of gene amplification. Reductase in UT-1 cells is shown to be a glycoprotein. The solubilized enzyme (MW = 97,000) from UT-1 cells, Chineses 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 located to a 30- to 35-kilodalton [kDa] fragment that was separable proteolytically from a cytoplasmic 53-kDa fragment that contained the active site of the enzyme. 3-HMG-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.