Structures and functions of the sugar chains of human leukocyte common antigen CD45

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CD45 is a leukocvte-specific cell surface glycoprotein with protein tyrosin phosphatase activity, and involved in the antigen-driven T cell growth.Recently B cell surface antigen CD22 has been shown to bind sialic acid on the sugar chains of T cell surface CD45.In order to eludate the function of the sugar chains of CD45, the carbohydrate structures of CD45 sugar chains were determined in the present study.(1) Structures of Asn-linked sugar chains : The sugar chains were released by hydrazinolysis from CD45 purified from human peripheral blood T cells, and reduced with NaB^3H_4.Carbohydrate structures of the radio-active sugar chains were determined by Bio-Gel P-4 column chromato-graphy in combination with exoglycosidase digestion, and by methylation analysis.The results showed that CD45 contains mainly tetra-and tri-antennary complex-type sugar chains with one to two sialic acid residues.(2) Structure of mucin-type sugar chains : The sugar chains were released by incubation of CD45 with alkaline-borohydride/borotritide solution.The redioactive sugar chains were analyzed as described above.The results showed that most sugar chains are neutral, and they are with core 1[Galbeta1-3GalNAc] and core 2 [Galbeta1-4GlcNAcbeta1-6(Galbeta1-3GalNAc)] structures.The sugar chain also contained i and l antigens, although their amounts are extremely small.(3)Binding assay of CD45 sugar chains to a CD22-immobilized column : The radioactive Asn-and mucin-type sugar chains thus prepared were applied to the column.The result showed that no sugar chains bind to the column, while heavily sialylated oligosaccharides prepared from fetuin and orosomucoid interact to.These results indicate that the sugar chains of CD45 from human peripheral blood T cells are not adequately sialylated for binding to CD22.