Ruiz-Canada Catalina, Kelleher Daniel J, Gilmore Reid
Department of Biochemistry and Molecular Pharmacology University of Massachusetts Medical School, Worcester, MA 01605, USA.
Cell. 2009 Jan 23;136(2):272-83. doi: 10.1016/j.cell.2008.11.047.
Asparagine-linked glycosylation of polypeptides in the lumen of the endoplasmic reticulum is catalyzed by the hetero-oligomeric oligosaccharyltransferase (OST). OST isoforms with different catalytic subunits (STT3A versus STT3B) and distinct enzymatic properties are coexpressed in mammalian cells. Using siRNA to achieve isoform-specific knockdowns, we show that the OST isoforms cooperate and act sequentially to mediate protein N-glycosylation. The STT3A OST isoform is primarily responsible for cotranslational glycosylation of the nascent polypeptide as it enters the lumen of the endoplasmic reticulum. The STT3B isoform is required for efficient cotranslational glycosylation of an acceptor site adjacent to the N-terminal signal sequence of a secreted protein. Unlike STT3A, STT3B efficiently mediates posttranslational glycosylation of a carboxyl-terminal glycosylation site in an unfolded protein. These distinct and complementary roles for the OST isoforms allow sequential scanning of polypeptides for acceptor sites to insure the maximal efficiency of N-glycosylation.
内质网腔中多肽的天冬酰胺连接糖基化由异源寡聚寡糖基转移酶（OST）催化。具有不同催化亚基（STT3A与STT3B）及不同酶特性的OST同工型在哺乳动物细胞中共表达。利用小干扰RNA实现同工型特异性敲低，我们发现OST同工型协同作用并依次发挥功能来介导蛋白质N-糖基化。STT3A OST同工型主要负责新生多肽进入内质网腔时的共翻译糖基化。STT3B同工型对于分泌蛋白N端信号序列附近的受体位点进行有效的共翻译糖基化是必需的。与STT3A不同，STT3B能有效地介导未折叠蛋白中羧基末端糖基化位点的翻译后糖基化。OST同工型的这些独特且互补的作用使得能够对多肽的受体位点进行顺序扫描，以确保N-糖基化的最大效率。
