Activation of the protein tyrosine kinase tyk2 by interferon α/β

We previously demonstrated that the gene tyk 2 rescues the phenotype of a human mutant cell line unresponsive to α (IFN) and partially responsive to IFN‐β. Here, we describe functional complementation of the mutant cells with the corresponding cDNA. To characterize the putative non‐receptor protein tyrosine kinase encoded by the gene tyk 2 and begin to understand its functioning, we have raised polyclonal antibodies against a segment of the protein. Using these, we have identified tyk2 as a 134‐kDa protein which is rapidly and transiently phosphorylated on tyrosine in response to IFN‐α/β and possesses an inducible kinase activity when tested in vitro . IFN‐γ has no effect on the phosphorylation state of the protein. In agreement with previous genetic evidence, these results assign a role to tyk2 in the IFN‐α/β signalling pathway and not in the IFN‐γ pathway. Fractionation of cell lysates have helped to localize the bulk of the protein in the cytoplasm, with a minor fraction associated with the cell membrane. Both protein pools undergo activation upon short‐term IFN treatment of intact cells. Through the study of the effect of pervanadate on the phosphorylation level and the activity of tyk2, we conclude that activation of tyk2 by IFN‐α does not require an intermediate regulatory tyrosine phosphatase.