Phosphorylation and activation of the Jak-3 Janus kinase in response to interleukin-2

INTERLEUKIN-2 is an autocrine growth factor for T cells1,2 which also activates other cells including B cells3 and natural killer cells4. The subunits of the interleukin-2 receptor (IL-2R) lack intrinsic enzymatic activity, but protein tyrosine phosphorylation is a critical event following ligand binding and src family kinases, such as Lck, are known to be activated by IL-2 (refs 5–9). However, IL- 2 signalling can occur in the absence of receptor interaction with Lck, suggesting that other protein tyrosine kinases might be important10. Here we report that a new member of the Janus family of kinases (Jak-3) is coupled to the IL-2R in human peripheral blood T cells and natural killer cells.