KINASE-NEGATIVE MUTANTS OF JAK1 CAN SUSTAIN INTERFERON-GAMMA-INDUCIBLE GENE-EXPRESSION BUT NOT AN ANTIVIRAL STATE

The receptor-associated protein tyrosine kinases JAK1 and JAK2 are bot h required for the interferon (IFN)-gamma response. The effects of exp ressing kinase-negative JAK mutant proteins on signal transduction in response to IFN-lr in wild-type cells and in mutant cells lacking eith er JAK1 or JAK2 have been analysed. In cells lacking endogenous JAK1 t he expression of a transfected kinase-negative JAK1 can sustain substa ntial IFN-gamma-inducible gene expression, consistent with a structura l as well as an enzymic role for JAK1. Kinase-negative JAK2, expressed in cells lacking endogenous JAK2, cannot sustain IFN-gamma-inducible gene expression, despite low level activation of STAT1 DNA binding act ivity. When expressed in wild-type cells, kinase-negative JAK2 acts as a dominant-negative inhibitor of the IFN-gamma response. Further anal ysis of the JAW STAT pathway suggests a model for the IFN-gamma respon se in which the initial phosphorylation of JAK1 and JAK2 is mediated b y JAK2, whereas phosphorylation of the IFN-gamma receptor is normally carried out by JAK1. The efficient phosphorylation of STAT 1 in the re ceptor-JAK complex may again depend on JAK2. Interestingly, a JAK1-dep endent signal, in addition to STAT1 activation, appears to be required for the expression of the antiviral state.