Glucosidase II regulates the exit of misfolded glycoproteins from the Endoplasmic Reticulum

STIGLIANO, I.D.; ALCULUMBRE, S.G.; PARODI, A.J.; D'ALESSIO, C.

Puerto Madryn, Chubut, Argentina

Congreso; XLVI Reunión Anual de la Sociedad Argentina de Investigación en Bioquímica y Biología Molecular (SAIB); 2010

Sociedad Argentina de Investigación en Bioquímica y Biología Molecular

Glucosidase II (GII) and UDP-Glc:glycoprotein glucosyltransferase (GT) are key players in glycoprotein biogenesis in the endoplasmic reticulum (ER). GII is a heterodimer composed of GIIa and GIIb subunits that catalyzes the sequential removal of the two innermost Glc residues from the Glc3Man9GlcNAc2 glycan transferred to nascent proteins. GII also removes the single Glc unit added to folding intermediates by GT. In vitro studies have shown that both GII and GT recognize fully mannosylated N-glycans and that de- and reglucosylation activities decrease together with the N-glycan Man content. Using a series of fission yeast mutants transferring truncated N-glycans (G2M9, G2M7, G2M6 or G2M5) to nascent proteins we found that also in vivo GII activity decreases with the Man content and that glycan recognition is mediated by the GIIb subunit man6P receptor homologous domain. On the other hand, we observed in a set of S. pombe mutants that transfer deglucosylated N-glycans M9, M7, M6 or M5 that in vivo GT activity does not depend on the N-glycan Man content. Mannosidase I is an ER enzyme that trims more Man residues from glycoproteins that stay longer in the ER (misfolded species). The difference between GII and GT activities toward demmanosylated glycoproteins may be a key regulating checkpoint for the exit of glycoproteins from the folding quality control calreticulin/calnexin cycle.