Synthesis and processing of molecules bearing thymus leukemia antigen

Thymus-leukemia (TL) antigens are expressed in murine lymphocytes under strict developmental regulation. To elucidate the molecular basis of TL expression, we have identified the molecular species that react with TL antiserum. At least three species can be resolved by metabolic radiolabeling of thymocytes and ASL1 leukemia cells, lysis, immune precipitation, and sodium dodecyl sulfate-polyacrylamide. After a brief incubation with [35S]methionine, the only radioactive molecule recognized by TL antiserum is a homogeneous species with an apparent Mr of 45,000 daltons. This molecule, 45K TL, includes high-mannose-type carbohydrate attached to a 45,000 dalton glycosidase-resistant backbone. In this form, 45K, it is never exposed on the cell surface. If pulse-labeled cells are further incubated with nonradioactive methionine before lysis, however, radioactivity disappears from the 45K TL species and appears in the slower migrating species 46K and 48K TL. Thus, 46K and 48K appear to represent products generated from the 45K TL precursor by posttranslational modification. These TL forms are displayed on the cell surface; they lack high-mannose carbohydrate but evidently include acidic complex-type carbohydrate. Normal thymocytes from Qa:Tla-negative mice lack not only the surface forms of TL but also the intracellular 45K TL form. Peripheral lymphoid cells of Qa:Tla-positive mice synthesize none of these TL species. But the TL antiserum, which contains Qa antibody, recognizes a distinct gene product in spleen and thymus of Qa-Tla-positive mice. In its pulse-labeled form, this molecule, which may represent Qa-1, has an apparent Mr of 44,000 daltons, and consists of a glycosidase-resistant polypeptide core of only 35,000 daltons linked to more high mannose carbohydrate than 45K TL.