Glucosidase II and N-glycan mannose content regulate the half lives of monoglucosylated species in vivo

IVÁN D. STIGLIANO, SOLANA G. ALCULUMBRE, CARLOS A. LABRIOLA, ARMANDO J. PARODI AND CECILIA D´ALESSIO

MOLECULAR BIOLOGY OF THE CELL

AMER SOC CELL BIOLOGY

Año: 2011 vol. 22 p. 1810 - 1823

1059-1524

Glucosidase II (GII) is responsible for the sequential removal of 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:glycoproteinglucosyltransferase (UGGT). GII is a heterodimer whose alpha subunit (GII-alpha) bears the active site whereas its b subunit (GII-beta) modulates GII-alpha activity through its C-terminus mannose receptor homologous (MRH) domain. Here we report that, as already described in cell-free assays, also in liveSchizosaccharomycespombecells, a decrease in the number of mannoses in the glycan results in a decreased GII activity but that, contrary to what has been also reported from cell-free experiments, no such effect was observed in vivo for UGGT. As a consequence, ER alpha-mannosidase-mediated N-glycan demannosylation of misfolded/slow folding glycoproteins favours their interaction with the lectin/chaperones by prolonging the existence of monoglucosylatedglycans. Moreover, we show that even N-glycans bearing only five mannoses may interact in vivo with the GII-beta MRH domain and that the G2B domain present at GII-beta N-terminus is involved in GII-alpha-GII-beta interaction. Finally, we report that protists transferring to proteins glycans with low mannose content and in whose ER there is no demannosylation have nevertheless conserved the possibility of displaying relatively long-lived monoglucosylatedglycans by expressing GII-beta MRH domains with higher specificity for glycans with high mannose content.