The conundrum of UDP-Glc entrance into the yeast ER lumen

UDP-Glc entrance into the endoplasmic reticulum (ER) of eukaryotic cells is a key step in the quality control of glycoprotein folding, a mechanism requiring transfer of a Glc residue from the nucleotide sugar (NS) to glycoprotein folding intermediates by the UDP-Glc:glycoprotein glucosyltransferase (UGGT). According to a bioinformatics search there are only eight genes in the Schizosaccharomyces pombe genome belonging to the three Pfam families to which all known nucleotide-sugar transporters (NSTs) of the secretory pathway belong. The protein products of two of them (hut1+ and yea4+) localize to the ER, those of genes gms1+, vrg4+, pet1+, pet2+ and pet3+ to the Golgi, whereas that of gms2+ has an unknown location. Here we demonstrate that (1) hut1 and gpt1 (UGGT null) mutants share several phenotypic features; (2) hut1 mutants show a 50% reduction in UDP-Glc transport into ER-derived membranes; (3) in vivo UDP-Glc ER entrance occurred in hut1yea4gms2 mutants and in cells in which hut1 disruption was combined with that of each of four of the genes encoding Golgi-located proteins. Therefore, disruption of all genes whose products localize to the ER or have an unknown location did not obliterate UDP-Glc ER entrance. We conclude that the hut1+ gene product is involved in UDP-Glc entrance into the ER, but that at least another as yet unknown NST displaying an unconventional sequence operates in the yeast secretory pathway. This conclusion agrees with our previous results showing...

Source: Glycobiology - December 27, 2016 Category: Biology Authors: Bredeston, L. M., Marino-Buslje, C., Mattera, V. S., Buzzi, L. I., Parodi, A. J., D'Alessio, C. Tags: Cell Biology Source Type: research