Further biochemical characterization of the human thymocyte differentiation antigen T6

Charge heterogeneity of the human thymocyte antigen T6 was studied by isoelectric focusing and two-dimensional gel electrophoresis. The larger subunit of T6 (a 49,000 m.w. glycoprotein) contained several oligosaccharide side chains bearing up to 12 terminal sialic acids. When T6 antigens from 19 individual thymuses were analyzed, no differences in the isoelectric focusing patterns of the larger subunit could be detected. The larger subunit of the T6 antigen from MOLT-4 cells (52,000 m.w.) contained an extra oligosaccharide if compared with the T6 antigen from thymus or three other T leukemic cell lines. Two types of small subunits of T6 were found. In addition to a protein of m.w. 12,000, pI 5.5 identified as beta 2-microglobulin, a (m.w. 12,000, pI 7.0) nonglycosylated protein, was detected on two dimensional gels. This protein does not cross-react with beta 2-microglobulin, and its amount varied in different T6 preparations. The tissue distribution, the m.w. the association with beta 2-microglobulin, and the limited structural heterogeneity of T6 support the idea that T6 is the human homologue of the murine thymus leukemia antigen (TL).