Synthesis and processing of molecules bearing thymus leukemia antigen.

Abstract

Thymus-leukemia (TL) antigens are expressed in murine lymphocytes under strict developmental regulation. To elucidate the molecular basis of TL expression the molecular species were identified that react with TL antiserum. At least 3 species could 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 was a homogeneous species with an apparent MW of 45,000 daltons. This molecule, 45K TL, included high-mannose-type carbohydrate attached to a 45,000 dalton glycosidase-resistant backbone. In this form, 45K, it was never exposed on the cell surface. If pulse-labeled cells were further incubated with nonradioactive methionine before lysis, radioactivity disappeared from the 45K TL species and appeared in the slower migrating species 46K and 48K TL. Thus, 46K and 48K appeared to represent products generated from the 45K TL precursor by posttranslational modification. These TL forms were displayed on the cell surface; they lacked high-mannose carbohydrate but evidently included acidic complex-type carbohydrate. Normal thymocytes from Qa:Tla-negative mice lacked not only the surface forms of TL but also the intracellular 45K TL form. Peripheral lymphoid cells of Qa:Tla-positive mice synthesized none of these TL species. But the TL antiserum, which contains Qa antibody, recognized 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, had an apparent MW of 44,000 daltons and consisted of a glycosidase-resistant polypeptide core of only 35,000 daltons linked to more high mannose carbohydrate than 45K TL.