title: Glucosidase II and N-glycan mannose content regulate the half-lives of monoglucosylated species in vivo.
abstract: Glucosidase II (GII) sequentially removes the two innermost glucose residues from the glycan (Glc(3)Man(9)GlcNAc(2)) transferred to proteins. GII also participates in cycles involving the lectin/chaperones calnexin (CNX) and calreticulin (CRT) 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 in which the α subunit (GIIα) bears the active site, and the β subunit (GIIβ) modulates GIIα activity through its C-terminal mannose 6-phosphate receptor homologous (MRH) domain. Here we report that, as already described in cell-free assays, in live Schizosaccharomyces pombe cells a decrease in the number of mannoses in the glycan results in decreased GII activity. Contrary to previously reported cell-free experiments, however, no such effect was observed in vivo for UGGT. We propose that endoplasmic reticulum α-mannosidase-mediated N-glycan demannosylation of misfolded/slow-folding glycoproteins may favor their interaction with the lectin/chaperone CNX present in S. pombe by prolonging the half-lives of the monoglucosylated glycans (S. pombe lacks CRT). Moreover, we show that even N-glycans bearing five mannoses may interact in vivo with the GIIβ MRH domain and that the N-terminal GIIβ G2B domain is involved in the GIIα-GIIβ interaction. Finally, we report that protists that transfer glycans with low mannose content to proteins have nevertheless conserved the possibility of displaying relatively long-lived monoglucosylated glycans by expressing GIIβ MRH domains with a higher specificity for glycans with high mannose content.
authors.affiliation: Laboratory of Glycobiology, Fundación Instituto Leloir and Instituto de Investigaciones Bioquímicas de Buenos Aires-CONICET, C1405BWE, Buenos Aires, Argentina.
authors.forename: Ivan D
authors.initials: ID
authors.lastname: Stigliano
authors.forename: Solana G
authors.initials: SG
authors.lastname: Alculumbre
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Labriola
authors.forename: Armando J
authors.initials: AJ
authors.lastname: Parodi
authors.forename: Cecilia
authors.initials: C
authors.lastname: D'Alessio
mesh_terms: D002240:Carbohydrate Sequence; D004721:Endoplasmic Reticulum / Q000378:metabolism; D055786:Gene Knockout Techniques; D005964:Glucosyltransferases / Q000235:genetics / Q000378:metabolism; D006023:Glycoproteins / Q000378:metabolism*; D006207:Half-Life; D006602:Hexosyltransferases / Q000378:metabolism; D008358:Mannose / Q000378:metabolism*; D008969:Molecular Sequence Data; D011134:Polysaccharides / Q000737:chemistry / Q000378:metabolism; D017510:Protein Folding; D054730:Protein Interaction Domains and Motifs; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D012568:Schizosaccharomyces / Q000201:enzymology* / Q000235:genetics; D000520:alpha-Glucosidases / Q000235:genetics / Q000378:metabolism*; D043323:alpha-Mannosidase
chemical_list: D006023:Glycoproteins; D011134:Polysaccharides; D005964:Glucosyltransferases; D006602:Hexosyltransferases; C020823:glycoprotein glycosyltransferase; C034251:4-nitrophenyl-alpha-glucosidase; D000520:alpha-Glucosidases; D043323:alpha-Mannosidase; D008358:Mannose