From sugars to protein folding in the secretory pathway: the role of Glucosidase II beta subunit

ORSI, R.; STIGLIANO, I.D.; OLSON, LJ; PETERSON, F.C; ALCULUMBRE, S.G.; DAHMS, NM; PARODI, A.J.; D'ALESSIO, C

Congreso; Panamerican Association for Biochemistry and Molecular Biology congress; 2013

Glucosidase II (GII) is a key regulator of the quality control of glycoprotein folding mechanism in the endoplasmic reticulum (ER). GII is responsible for sequentially removing the two innermost glucoses from the Glc3Man9GlcNAc2 glycan transferred upon protein N-glycosylation and of the glucose residue added to folding intermediates by the ER glycoprotein conformational sensor, the UDP-Glc:glycoprotein glucosyltransferase. GII is a heterodimer composed of a catalytic GIIα subunit and a regulatory GIIβ subunit. We demonstrated that GIIβ is involved in GIIα?s ER localization and mediates in vivo enhancement of N-glycan trimming by GII through its C-terminal mannose 6-phosphate receptor homology (MRH) domain. Moreover, GII regulates the permanence of slow folding proteins in the ER as it shows a decreased activity toward demannosylated N-glycans present in such species, thus prolonging the half-lives of monoglucosylated glycans able to interact with ER lectin/chaperones calreticulin and calnexin. We determined the  structure of a functional GIIβ MRH domain by NMR spectroscopy. We identified a conserved residue (W409) present only in GIIβ MRH domains that influences GII glucose trimming activity in vivo. A model of the GIIβ MRH domain bound to Man9 provides insight into how GIIβ enhances the catalytic activity of GII and gives an explanation for the reduced GII activity toward slow-folding demannosylated glycoproteins. Grants: ANPCyT, CONICET, Mizutani, NIH, HHMI