Man2C1, an α-mannosidase, is involved in the trimming of free oligosaccharides in the cytosol

Abstract

The endoplasmic-reticulum-associated degradation of misfolded (glyco)proteins ensures that only functional, correctly folded proteins exit from the endoplasmic reticulum and that misfolded ones are degraded by the ubiquitin-proteasome system. During the degradation of misfolded glycoproteins, they are deglycosylated by the PNGase (peptide:N-glycanase). The free oligosaccharides released by PNGase are known to be further catabolized by a cytosolic alpha-mannosidase, although the gene encoding this enzyme has not been identified unequivocally. The findings in the present study demonstrate that an alpha-mannosidase, Man2C1, is involved in the processing of free oligosaccharides that are formed in the cytosol. When the human Man2C1 orthologue was expressed in HEK-293 cells, most of the enzyme was localized in the cytosol. Its activity was enhanced by Co2+, typical of other known cytosolic alpha-mannosidases so far characterized from animal cells. The down-regulation of Man2C1 activity by a small interfering RNA drastically changed the amount and structure of oligosaccharides accumulating in the cytosol, demonstrating that Man2C1 indeed is involved in free oligosaccharide processing in the cytosol. The oligosaccharide processing in the cytosol by PNGase, endo-beta-N-acetylglucosaminidase and alpha-mannosidase may represent the common 'non-lysosomal' catabolic pathway for N-glycans in animal cells, although the molecular mechanism as well as the functional importance of such processes remains to be determined.

The sugar-binding ability of ERGIC-53 is enhanced by its interaction with MCFD2

“…17 However, it has recently been shown that this enzyme is localized to the cytosol and not the ER, and is involved in free oligosaccharide processing in the cytosol to produce M 5 oligosaccharides. 31,32 Appenzeller et al reported that preincubation of Lec1 cells with DMJ prevents mannose-trimming from M 9 to M 8B on cathepsin Z but has no effect on the cross-linking of cathepsin Z and ERGIC-53. 33 Cathepsin Z has both high mannose-type glycan and ␤-hairpin peptide motifs that are essential for efficient ERGIC-53-mediated ER export.…”

Introduction to “Glycometabolome”

“…With respect to the nonlysosomal degradation of N-glycans, for instance, while PNGase is conserved throughout eukaryotes, ENGase does not exist in some yeast, and Man2C1 is only conserved in vertebrate (19)(20)(21). These facts clearly tell us that the scheme presented in Figs.…”

Generation and Metabolism of Cytosolic Free Oligosaccharides in Caenorhabditis elegans

“…Later, it was revealed that Man2C1 had the same properties as those of the cytosolic a-mannosidase (61). Subsequently, it was also found that knockdown of Man2C1 by RNA interference (RNAi) blocked FOS processing (62). Thisˆnding demonstrated that Man2C1 was the cytosolic a-mannosidase that is involved in the catabolism of FOSs.…”