Proliferation of T lymphocytes is triggered by the interaction of IL-2 with its specific receptor following T lymphocyte activation. The receptor for IL-2 consists of at least three distinct subunits, the a chain (IL-2Ra), the fJ chain (IL-2RfJ), and the y chain (IL-2Ry). Although the role of IL-2Ry in IL-2 signalling remains unclear, IL-2RfJ is the subunit critical for receptor-mediated signalling. Because IL-2RfJlacks any apparent catalytic motifs, IL-2RfJ may be physically or functionally coupled to other sigн nalling molecules.

Structure-function studies of IL-2RfJ have revealed that at least two distinct cytoplasmic regions ofIL-2RfJ are involved in IL-2-induced celluн lar signalling. The" serine-rich" region of IL-2RfJ was identified as a region critical for IL-2-induced mitotic signalling from experiments in which IL-2RfJ mutant cDNAs lacking a particular cytoplasmic region or regions were expressed in an IL-3-dependent mouse pro-B cell line (BAFн B03). Meanwhile, another cytoplasmic region of IL-2RfJ, the" acidic" region, is responsible for its physical association with an src-family protein tyrosine kinase (PTK), p56lck and is critical for activating the p56lck PTK following IL-2 stimulation.