Review Article

Protein Glycosylation in Aspergillus fumigatus Is Essential for Cell Wall Synthesis and Serves as a Promising Model of Multicellular Eukaryotic Development

Figure 4

Comparison of free oligosaccharide catabolism in mammalian and yeast cells. Glycoprotein biosynthesis in mammalian cells is accompanied by the generation of free oligosaccharides (fOS) from both OST-mediated hydrolysis of Glc3Man9GlcNAc2-PP-dolichol in the lumen of the ER and peptide N-glycanase (PNGase-) mediated de-N-glycosylation of newly synthesized glycoproteins, which undergo ER associated protein degradation (ERAD), either in the ER or the cytosol. fOS that are liberated in ER can be transported into the cytosol. In the cytosol, fOS are trimmed by an endo-β-D-N-acetylglucosamine H (endo H-) like enzyme and the α-mannosidase Man2C1p to yield Man5GlcNAc, which can be imported directly into lysosomes to be degraded. In S. cerevisiae, fOS are released from glycoproteins in the cytosol by Png1p, a counterpart of mammalian PNGase. Then the Png1p-generated fOS may be processed in the cytosol by Ams1p, the yeast cytosolic α-mannosidase. It should also be noted that no structural studies have been performed on the products that can be generated from Man8GlcNAc2 by Ams1p, and the ultimate fate of such products remains obscure. On the other hand, two Png1p-independent fOS pools, Man3GlcNAc2 and Man8GlcNAc2, are also seen in S. cerevisiae. The pool comprising small fOS (Man3GlcNAc2) appears to be disposed of by unknown enzymes in the vacuole. The pool containing mainly Man8GlcNAc2 may be generated and disposed of along the secretory pathway.
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