NOVEL T-CELL ANTIGEN 4-1BB ASSOCIATES WITH THE PROTEIN-TYROSINE KINASE P56(LCK1)

4-1BB is a 30-kDa inducible T cell Ag, and is expressed predominantly as a 55-kDa dimer on both CD4+ and CD8+ T lymphocytes. The cytoplasmic tail of 4-1BB contains the sequence Cys-Arg-Cys-Pro, which is similar to the sequence Cys-X-Cys-Pro, which mediates the binding of the CD4 and CD8 molecules to the protein tyrosine kinase p56lck . An anti-4-1B B mAb, 53A2, was used to determine whether 4-1BB may associate with p5 6lck. The 53A2 mAb specifically recognized 4-1BB on a CD8+ T cell line , CTLL-2, and coimmunoprecipitated a 56-kDa protein along with 4-1 BB. Peptide mapping indicated that the 56-kDa phosphoprotein was identica l to p56lck. The coimmunoprecipitation of p56lck with 4-1BB also occur red in nonlymphoid cells such as insect (Sf-21) and HeLa cells when th e two recombinant proteins were coexpressed. Analysis of mutant p56lck recombinant proteins showed that two cysteine residues critical for p 56lck-D4 (or -CD8) complex formation are also required for the p56lck- 4-1BB interaction. These studies establish that 4-1BB physically assoc iates with p56lck. 151: 1255.