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

VALKO, A.; PARODI, A.J.; ETCHEGARAY, E.; DALESSIO, C. ; GALLO, G.L.; ARAMBURU, S.I.

Buenos Aires

Simposio; 3rd Argentinian Symposium on Glycobiology; 2019

Glucosidase I (GI) is the first N-glycan-remodeling enzyme upon N-glycosylation in the endoplasmic reticulum (ER) as it removes the outermost glucose from protein-linked Glc3Man9GlcNAc2 (G3M9). Individuals with mutations in GI-encoding-gene bear congenital disorder of glycosylation CDG-IIb. In an attempt to understand the severe syndrome produced in the patients we constructed a Schizosaccharomyces pombe mutant lacking GI (gls1) as an experimental model system. Cells were extremely sick, with a delayed cell growth rate, high mortality and an altered morphology that included a distorted cell wall and the absence of underlying ER membranes. We found that an additional mutation in gene alg10+, which results in cells transferring Glc2Man9GlcNAc2 (G2M9) instead of G3M9, was capable to suppress the severe defects. Surprisingly, mutants lacking glucosidase 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 N-glycans in the Golgi. Further studies also disclosed that the severity produced by the lack of GI is neither due to an inhibition of the oligosaccharyltransferase by its transfer reaction products nor to a potential reduced degradation of misfolded glycoproteins during ERAD. Expression of human Golgi endomannosidase significantly (but not completely) restored normal phenotype in the sick gls1 mutants. We propose that accumulation of G3M9-bearing glycoproteins (GI substrates) is deletereous and could be at least partially responsible for the defects observed in CDG-IIb disorder.