Characterization of a posttranslational fucosylation in the growth factor domain of urinary plasminogen activator.

A posttranslational modification site in natural and recombinant urinary-type plasminogen activators (urokinases; EC 3.4.21.31) has been localized to Thr-18, in the growth factor domain of the molecule. This is the region of urinary plasminogen activator responsible for its specific receptor binding. An unusual carbohydrate-protein linkage, a single monosaccharide, fucose, covalently attached directly to threonine in the peptide, is described here. The glycan moiety and the site of modification have been identified with mass spectrometry and confirmed by carbohydrate composition analysis, Edman degradation, and one- and two-dimensional NMR studies. This type of modification is normally not detected without mass spectrometry because the fucose-threonine bond is hydrolyzed under standard acidic conditions of the amino acid analysis and Edman sequencing. This modification may be widely found in other proteins.