Differential substrate recognition capabilities of Janus family protein tyrosine kinases within the interleukin 2 receptor (Il2R) system: Jak3 as a potential molecular target for treatment of leukemias with a hyperactive Jak-STAT signaling machinery

Substrate recognition by Janus family protein tyrosine kinases was examined utilizing recombinant baculovirus produced components of the interleukin 2 receptor (IL2R) system i.e. Jak1, Transducers and Activators of Transcript ion (STAT). Wild type Jak3 was able to tyrosine phosphorylate a kinase-dead Jak1 (Jak1E(908)). In contrast wild type Jak1 was unable to tyrosine phosp horylate kinase dead Jak3 (Jak3E(851)). This unilateral transphosphorylatio n between Jak3 and Jak1 prompts the hypothesis that in the IL2R system the activation of Jak3 precedes Jak1 activation. Both the IL2R beta and IL2R ga mma(c), subunits underwent tyrosine phosphorylation when co-expressed with wild-type Jak3. By comparison only IL2R beta was recognized and tyrosine ph osphorylated by wild-type Jak1. These results are consistent with the notio n that Jak1 is pre-associated with IL2R beta and Jak3 is pre-associated wit h IL2R gamma(c). STAT1, STAT3, and STAT5 underwent tyrosine phosphorylation when co-expressed with Jak1 and therefore are substrates for the respectiv e Jak kinases. In contrast, Jak3 co-expression resulted in tyrosine phospho rylation of STAT3 and STAT5 but not STAT1. Notably a polypeptide representi ng the kinase domain of Jak 3(Jak3-JH1) gained the ability to tyrosine phos phorylate STAT1, suggesting that the changes in substrate recognition may b e influenced by domains outside the kinase domain. These findings provide e vidence that Jak1 and Jak3 differentially recognize specific substrates, th ereby having the ability to contribute specific signals, and the substrate specificity may be influenced by multiple domains of these tyrosine kinases .