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J. Cell Biol.,
Volume 143, Number 3, November 2, 1998 625-635
Instituto de Investigaciones Bioquímicas Fundación Campomar, 1405 Buenos Aires, Argentina
Interaction of monoglucosylated oligosaccharides with ER lectins (calnexin and/or calreticulin)
facilitates glycoprotein folding but this interaction is
not essential for cell viability under normal conditions.
We obtained two distinct single Schizosaccharomyces pombe mutants deficient in either one of the two pathways leading to the formation of monoglucosylated
oligosaccharides. The alg6 mutant does not glucosy-
late lipid-linked oligosaccharides and transfers
Man9GlcNAc2 to nascent polypeptide chains and the
gpt1 mutant lacks UDP-Glc:glycoprotein glucosyltransferase (GT). Both single mutants grew normally at
28°C. On the other hand, gpt1/alg6 double-mutant cells
grew very slowly and with a rounded morphology at
28°C and did not grow at 37°C. The wild-type phenotype was restored by transfection of the double mutant
with a GT-encoding expression vector or by addition of
1 M sorbitol to the medium, indicating that the double
mutant is affected in cell wall formation. It is suggested
that facilitation of glycoprotein folding mediated by the
interaction of monoglucosylated oligosaccharides with
calnexin is essential for cell viability under conditions
of extreme ER stress such as underglycosylation of proteins caused by the alg6 mutation and high temperature. In contrast, gls2/alg6 double-mutant cells that
transfer Man9GlcNAc2 and that are unable to remove the glucose units added by GT as they lack glucosidase
II (GII), grew at 37°C and had, when grown at 28°C, a
phenotype of growth and morphology almost identical
to that of wild-type cells. These results indicate that
facilitation of glycoprotein folding mediated by the
interaction of calnexin and monoglucosylated oligosaccharides does not necessarily require cycles of reglucosylation-deglucosylation catalyzed by GT and GII.
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