“In vivo assessment of glucosidase II as a regulator of glycoprotein permanence in calnexin/calreticulin cycles”

STIGLIANO, ID; ALCULUMBRE, S.G.; LABRIOLA, C.A.; PARODI, AJ; D'ALESSIO, C.

Lucca

Conferencia; Glycobiology Gordon Research Conference.; 2011

Gordon Research Conferences

Glucosidase II (GII) sequentially removes the two innermost glucose residues from the glycan (Glc3Man9GlcNAc2) transferred to proteins. GII also participates in cycles involving the lectin/chaperones calnexin and calreticulin as it removes the single glucose unit added to folding intermediates and misfolded glycoproteins by the UDP-Glc:glycoprotein glucosyltransferase (UGGT). GII is a heterodimer whose alpha subunit (GIIa) bears the active site whereas its beta subunit (GIIb) modulates GIIa activity through its C-terminus Mannose Receptor Homologous (MRH) domain. Here we report that as already described in cell-free assays, also in live Schizosaccharomyces pombe cells a decrease in the number of mannoses in the glycan results in a decreased GII activity. However, contrary to what has been also reported from cell-free experiments, lack of mannose residues in the oligosaccharide did not affect in vivo UGGT activity. Therefore, ER alpha-mannosidase-mediated N-glycan demannosylation of misfolded/slow folding glycoproteins favors their interaction with the lectin/chaperones by prolonging the existence of monoglucosylated glycans. Moreover, we show that even N-glycans bearing five mannoses may interact in vivo with the GIIb MRH domain and that the N-terminus GIIb G2B domain is involved in GIIa-GIIb interaction