Evidence for a novel O-linked sialylated trisaccharide on Ser-248 of human plasminogen 2

Human plasminogen, the inactive precursor of plasmin, exists in two major glycoforms. Plasminogen 1 contains an N-linked oligosaccharide at Asn-289 and an O-linked oligosaccharide at Thr-345. Plasminogen 2 is known to contain only an O-linked oligosaccharide at Thr-345. However, plasminogen 2 displays a further well documented microheterogeneity dependent on the N-acetylneuraminic acid content, which has functional consequences with regard to activation of plasminogen. The proposed structure and number of known oligosaccharide linkages in plasminogen 2 is insufficient to account for this microheterogeneity. In the present study, a combination of trypsin digestion, lectin affinity chromatography, Edman degradation amino acid sequence analysis, carbohydrate composition analysis, and mass spectrometry revealed the existence of a novel site for O-linked glycosylation on plasminogen 2 at Ser-248. Direct evidence for the structure of the carbohydrate was obtained from a combination of lectin affinity chromatography, desialylation experiments, and mass spectrometry analysis. These findings provide a structural basis for some of the observed microheterogeneity, and have implications with regard to the known functional consequences of the extent of sialylation of plasminogen.