Terminal glycosylation may be a mechanism to control the function of specific biologically active glycoproteins. The biosynthesis of terminal sialyl and fucosyl residues on certain glycoproteins has been linked to the expression of the respective glycosyltransferase. In contrast, a human erythroleukemia cell line, HEL, contained a highly active GDP‐l‐Fuc: Gal(β1‐4)GlcNAc‐R (Fuc to GlcNAc) α‐1,3‐fucosyltransferase (α‐1,3‐fucosyltransferase) but no detectable α‐1,3‐linked fucosyl residues on the glycoproteins. The α‐1,3‐fucosyltransferase gave apparent Km values for Fuc(α1‐2)Gal(β1‐4)GlcNAc β‐O ‐benzyl, Gal(β1‐4)GlcNAc and GDP‐fucose of 0.04, 0.68 and 0.12 mM, respectively. The lack of detectable fucosyl residues in α‐1,3‐linkage to GlcNAc on the [3H]fucose‐labeled glycoproteins was shown with the use of almond α‐1,3/4‐fucosidase and internal controls to verify that the enzyme was active. Using Western‐blot analysis, HEL cell glycoproteins reacted with blood group H type‐2 antibody, confirming the presence of Fuc(α1‐2)Gal(β1‐4)GlcNAc as reported by others and the presence of the preferred substrate for the enzyme. It is proposed that controls for terminal glycosylation in addition to glycosyltransferase expression are operative in HEL cells and that they are part of a multi‐regulated process controlling terminal modifications of glycoproteins.
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