Abrogation of glucosidase I (but not glucosidase II)-mediated deglucosylation is extremely toxic for the cell

GALLO GIOVANNA LUCRECIA; VALKO, AYELEN; ETCHEGARAY, EMILIANA; ARAMBURU, SOFÍA IVANA; PARODI ARMANDO; D'ALESSIO, CECILIA

Partido de San Martín, Buenos Aires

Simposio; 3rd Argentinean Glycobiology Symposium "GlycoAr 2019"; 2019

Universidad Nacional de San Martín

Glucosidase I (GI) is the first N-glycan-remodeling enzyme upon N-glycosylation in the endoplasmic reticulum (ER) as it removesthe outermost glucose from protein-linked Glc3Man9GlcNAc2 (G3M9). Individuals with mutations in GI-encoding-gene bearcongenital disorder of glycosylation CDG-IIb. In an attempt to understand the severe syndrome produced in the patients weconstructed a Schizosaccharomyces pombe mutant lacking GI (Dgls1) as an experimental model system. Cells were extremelysick, with a delayed cell growth rate, high mortality and an altered morphology that included a distorted cell wall and the absenceof underlying ER membranes. We found that an additional mutation in gene alg10+, which results in cells transferringGlc2Man9GlcNAc2 (G2M9) instead of G3M9, was capable to suppress the severe defects. Surprisingly, mutants lackingglucosidase II did not show similar phenotypes suggesting that persistence of three but not two Glc has deleterious effects,which could not be ascribed to disruption of glycoprotein entrance into calnexin-folding cycles or to a precluded elongation of Nglycansin the Golgi. Further studies also disclosed that the severity produced by the lack of GI is neither due to an inhibition ofthe oligosaccharyltransferase by its transfer reaction products nor to a potential reduced degradation of misfolded glycoproteinsduring ERAD. Expression of human Golgi endomannosidase significantly (but not completely) restored normal phenotype in thesick Dgls1 mutants. We propose that accumulation of G3M9-bearing glycoproteins (GI substrates) is deletereous and could beat least partially responsible for the defects observed in CDG-IIb disorder.