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 ma jor glycoforms. Plasminogen 1 contains an N-linked oligosaccharide at Asn-289 and an O-linked oligosaccharide at Thr-345. Plasminogen 2 is k nown to contain only an O-linked oligosaccharide at Thr-345. However, plasminogen 2 displays a further well documented microheterogeneity de pendent on the N-acetylneuraminic acid content, which has functional c onsequences with regard to activation of plasminogen. The proposed str ucture and number of known oligosaccharide linkages in plasminogen 2 i s insufficient to account for this microheterogeneity. In the present study, a combination of trypsin digestion, lectin affinity chromatogra phy, Edman degradation amino acid sequence analysis, carbohydrate comp osition analysis, and mass spectrometry revealed the existence of a no vel site for O-linked glycosylation on plasminogen 2 at Ser-248. Direc t evidence for the structure of the carbohydrate was obtained from a c ombination of lectin affinity chromatography, desialylation experiment s, and mass spectrometry analysis, These findings provide a structural basis for some of the observed microheterogeneity, and have implicati ons with regard to the known functional consequences of the extent of sialylation of plasminogen.