Glycoprotein biosynthesis in castor bean endosperm

Excised castor bean ('Ricinus communis' L) endosperm tissue supplied with (14 C) galactose or (14 C) mannose incorporated radioactivity into particulate cell components. The protein constituents of these organelles, when separated electrophoretically, contain radioactive sugars. That these organellar proteins contain bound carbohydrate was confirmed using gas liquid chromatography of the cleaved sugars. Three of the sugars detected, mannose, N-acetyl glucosamine and galactose, were found to be incorporated into glycoprotein via lipid-linked intermediates. Mannose is transferred from GDP mannose to an oligo saccharide-lipid via dolicol monophosphate mannose, this oligo saccharide is composed of both mannose and N-acetyl glucosamine, which is itself donated from the nucleotide-sugar via a lipid disaccharide. Oligo saccharide is then transferred from the lipid carrier to a nascent polypeptide acceptor en bloc. Galactose is added to previously core glycosylated protein via a lipid-linked intermediate in monosaccharide units. Although all enzymes catalysing these steps are exclusively housed in the endoplasmic reticulum (ER), glycoprotein products do not accumulate in the ER, but are exported to other parts of the cell, in particular the glyoxysome. The importance of post-translational glycosylation with respect to protein segregation and membrane assembly is discussed..