Uemura Satoshi, Yoshida Sayaka, Shishido Fumi, Inokuchi Jin-ichi
Division of Glycopathology, Institute of Molecular Biomembrane and Glycobiology, Tohoku Pharmaceutical University, Sendai, Miyagi 981-8558, Japan.
Mol Biol Cell. 2009 Jul;20(13):3088-100. doi: 10.1091/mbc.e08-12-1219. Epub 2009 May 6.
GM3 synthase (SAT-I) is the primary glycosyltransferase responsible for the biosynthesis of ganglio-series gangliosides. In this study, we identify three isoforms of mouse SAT-I proteins, named M1-SAT-I, M2-SAT-I, and M3-SAT-I, which possess distinct lengths in their NH(2)-terminal cytoplasmic tails. These isoforms are produced by leaky scanning from mRNA variants of mSAT-Ia and mSAT-Ib. M2-SAT-I and M3-SAT-I were found to be localized in the Golgi apparatus, as expected, whereas M1-SAT-I was exclusively found in the endoplasmic reticulum (ER). Specific multiple arginines (R) arranged in an R-based motif, RRXXXXR necessary for ER targeting, were found in the cytoplasmic tail of M1-SAT-I, and in vivo GM3 biosynthesis by M1-SAT-I was very low because of restricted transport to the Golgi apparatus. In addition, M1-SAT-I and M3-SAT-I had a long half-life relative to M2-SAT-I. This is the first report demonstrating the presence of an ER-targeting R-based motif in the cytoplasmic tail of a protein in the mammalian glycosyltransferase family of enzymes. The system, which produces SAT-I isoforms having distinct characteristics, is likely to be of critical importance for the regulation of GM3 biosynthesis under various pathological and physiological conditions.
GM3合酶（SAT-I）是负责神经节系列神经节苷脂生物合成的主要糖基转移酶。在本研究中，我们鉴定出小鼠SAT-I蛋白的三种同工型，命名为M1-SAT-I、M2-SAT-I和M3-SAT-I，它们在NH(2)端胞质尾巴的长度不同。这些同工型是由mSAT-Ia和mSAT-Ib的mRNA变体通过漏扫描产生的。正如预期的那样，发现M2-SAT-I和M3-SAT-I定位于高尔基体，而M1-SAT-I仅在内质网（ER）中发现。在M1-SAT-I的胞质尾巴中发现了在内质网靶向所必需的基于R的基序RRXXXXR中排列的特定多个精氨酸（R），并且由于向高尔基体的转运受限，M1-SAT-I在体内的GM3生物合成非常低。此外，相对于M2-SAT-I，M1-SAT-I和M3-SAT-I具有较长的半衰期。这是第一份证明在哺乳动物糖基转移酶家族的一种酶的蛋白质胞质尾巴中存在基于内质网靶向的R基序的报告。产生具有不同特征的SAT-I同工型的系统，对于在各种病理和生理条件下调节GM3生物合成可能至关重要。
