The cytoplasmic tail of GM3 synthase defines its subcellular localization, stability, and in vivo activity.
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.
Division of Glycopathology, Institute of Molecular Biomembrane and Glycobiology, Tohoku Pharmaceutical University, Sendai, Miyagi 981-8558, Japan.
Satoshi
S
Uemura
Sayaka
S
Yoshida
Fumi
F
Shishido
Jin-ichi
J
Inokuchi
D000818:Animals; D001120:Arginine / Q000235:genetics / Q000378:metabolism; D016466:CHO Cells; D045744:Cell Line, Tumor; D006224:Cricetinae; D003412:Cricetulus; D003593:Cytoplasm / Q000201:enzymology; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D004795:Enzyme Stability; D005260:Female; D020869:Gene Expression Profiling; D015971:Gene Expression Regulation, Enzymologic; D006056:Golgi Apparatus / Q000201:enzymology*; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D009154:Mutation; D014176:Protein Biosynthesis; D021381:Protein Transport; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012799:Sialyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D007527:Isoenzymes; D049452:Green Fluorescent Proteins; D001120:Arginine; D012799:Sialyltransferases; C021343:haematoside synthetase