The role of Glucosidase II beta G2B and MRH domains on N-glycan processing in the endoplasmic reticulum

STIGLIANO, ID; AGUILERA, MC; ALCULUMBRE, S.G.; PARODI, AJ; D'ALESSIO, C

San Luis

Congreso; XLVII Reunión Anual- Sociedad Argentina de Investigación en Bioquímica y Biología Molecular; 2011

SAIB

Glucosidase II (GII) removes the two innermost glc 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 glc 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 6-Phosphate Receptor Homologous (MRH) domain. Here we report that as already described in cell-free assays, also in live S.pombe cells a decrease in the number of mannoses in the glycan results in a decreased GII activity. However, lack of mannose residues in the oligosaccharide did not affect in vivo UGGT activity. Thus, N-glycan demannosylation of misfolded/slow folding species 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. Finally we confirm that the abscence of the canonical ER retention signal (VDEL) in GIIb did not prevent GIIa ER localization or activity, suggesting that VDEL is not the only ER localization signal present in GIIb.