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9.C.1 The Endosomal Oligosaccharide Transporter (EOT)

Mammals normally derivitize potential glycoproteins on asparaginyl residues with sugar moieties in the endoplasmic reticulum (ER) and golgi apparatus. Unconjugated polymannose-type oligosaccharides derived from Glu3Man9GluNAc2-PP-dolichol accumulate in the ER, but they are removed, being transported first to the cytosol where the N terminal N-acetyl glucosamine is cleaved by a chitobiase, and mannose residues are cleaved off by a mannosidase. The product, Man5GlcNAc, is then transported into the lysosomes where the remaining mannosyl residues are hydrolyzed by β-mannosidases. The oligosaccharide transport reactions appear to require ATP, but it is not clear if this requirement is direct (in which case ABC-type transporters may be involved) or indirected (in which case, secondary carriers may be involved).