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.
Uemura
Satoshi
S
Division of Glycopathology, Institute of Molecular Biomembrane and Glycobiology, Tohoku Pharmaceutical University, Sendai, Miyagi 981-8558, Japan.
Yoshida
Sayaka
S
Shishido
Fumi
F
Inokuchi
Jin-ichi
J
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