CONICET | Buscador de Institutos y Recursos Humanos

GLUCOSIDASE II BETA SUBUNIT MODULATES N-GLYCAN TRIMMING IN FISSION YEASTS AND MAMMALS Ivan D. Stigliano*, Julio J. Caramelo, Carlos A. Labriola*, Armando J. Parodi*, and Cecilia DAlessio* 1 Fundación Instituto Leloir and IIBBA, CONICET; and 2 Facultad de Ciencias Exactas y Naturales, UBA Glucosidase II (GII) is a key player in glycoprotein biogenesis in the endoplasmic reticulum (ER). It catalizes the sequential removal of the two innermost Glc residues from the Glc3Man9GlcNAc2 glycan transferred to Asn residues in proteins. GII is involved in the calnexin/calreticulin cycle as it removes the single Glc unit added to folding intermediates and misfolded glycoproteins by the UDP-Glc:glycoprotein glucosyltransferase. GII is an ER heterodimer whose a subunit (GIIa) holds the glycosyl hydrolase active site whereas its b subunit (GIIb) role is controversial and has been suggested to be responsible for GIIa ER retention and folding. Here we report that in the absence of GIIb, the catalytic subunit GIIa of the fission yeast Schizosaccharomyces pombe (an organism displaying a glycoprotein folding quality control mechanism similar to that occurring in mammalian cells) has an active conformation able to hydrolyze p-nitrophenyl a-D-glucopiranoside. However, the heterodimer is required to efficiently deglucosylate the physiological substrates Glc2Man9GlcNAc2 (G2M9) and Glc1Man9GlcNAc2 (G1M9). The interaction of the mannose 6-phosphate receptor homologous domain (MRH domain) present in GIIb and mannoses in the B and/or C arms of the oligosaccharide mediates glycan hydrolysis enhancement. We also present evidence that in mammalian cells GIIb modulates G2M9 and G1M9 trimming.