[The structure of chordin: characteristics of protein and carbohydrate components and their role in antigenicity].

Using immobilized monoclonal antibodies, a tissue-specific antigen, chordin, was isolated from cell extracts of giant sturgeon (beluga) notochord. The antigen was further purified by gel filtration through SP-Sephadex (pH 2.1) and gel chromatography on TSK Toyopearl HW-60. Purified chordin preparations contained 40% of protein and 60% of carbohydrates. The predominant polar amino acids were threonine, serine, glycine, asparagine and glutamine (or aspartic and glutamic amino acids). The carbohydrate moiety comprised mannose, fucose, galactose, galactosamine and glucosamine. Treatment of chordin with three enroglycosidases specifically hydrolyzing the carbohydrate chains of proteoglycans did not affect the antigenic properties of chordin or its behaviour on gel filtration. These findings and the fact that 75% of galactosamine was converted to galactosaminite after treatment with alkaline NaBH4 permitted to relate chordin to glycoproteins carrying O-glycosidic carbohydrate-peptide bonds between the N-acetyl-galactosamine and beta-hydroxyamino acid residues. Besides, chordin seems to contain a N-glycosylamide carbohydrate-peptide bond as can be judged from glucosaminite formation after treatment of the antigen with alkaline LiHB4. The changes in the antigenic properties of chordin after its treatment with neuraminidase, pronase, sodium periodate, alkali, alkaline NaBH4 or LiBH4 suggest that the polypeptide moiety of the chordin molecule and, perhaps, the N-acetylgalactosamine within the composition of the carbohydrate-peptide bond are involved in the construction of its most immunogenic determinants (P-determinants).