SUGAMURA, K., TAKESHITA, T., ASAO, H., KUMAKI, S., OHBO, K., OHTANI, K. and NAKAMURA, M. The IL-2/IL-2 Receptor System: Involvement of a Novel Receptor Subunit, γ Chain, in Growth Signal Transduction. Tohoku J. Exp. Med., 1992, 168 (2), 231-237-We previously demonstrated the existence of a third component, p64, of IL-2 receptor (IL-2R), tentatively named the γ chain of IL-2R. Our recent studies provided evidence suggesting that the γ chain endows the β chain of IL-2R with IL-2 binding ability. The γ chain was detected in lymphoid transfectants of IL-2Rβ cDNA, which showed the intermediate- affinity IL-2R, but not in nonlymphoid transfectants of IL-2Rβ cDNA, which showed no IL-2 binding activity. The comparative study between two subclones of lymphoid MOLT4 transfectant of IL-2Rβ cDNA demonstrated that the amount of the γ chain coprecipitated with IL-2Rβ was proportional to numbers of the IL-2 binding sites. These results suggest the possibility that the γ chain associates with IL-2Rβ and has an important role in formation of the intermediate-affinity IL-2R complex. On the other hand, we have also demonstrated the association of IL-2Rβ with a certain tyrosine kinase, of which activation by IL-2 could be indispensable process at the initial pathway of signal transduction. - γ-chain; IL-2; IL-2 receptor; signal transduction; tyrosine kinase