Enhanced antiviral and antiproliferative properties of a STAT1 mutant unable to interact with the protein kinase PKR

We have previously reported a physical association between STAT1 and the pr otein kinase double-stranded RNA-activated protein kinase (PKR). PKR inhibi ted STAT1 function in a manner independent of PKR kinase activity. In this report, we have further characterized the properties of both molecules by m apping the sites of their interaction. A STAT1 mutant unable to interact wi th PKR displays enhanced interferon gamma (IFN-gamma)-induced transactivati on capacity compared with STAT1. This effect appears to be mediated by the higher capacity of STAT1 mutant to heterodimerize with STAT3. Furthermore, expression of STAT1 mutant in STAT1(-/-) cells enhances both the antiviral and antiproliferative effects of IFNs as opposed to STAT1. We also provide evidence that STAT1 functions as an inhibitor of PKR in vitro and in vivo. That is, phosphorylation of eIF-2 alpha is enhanced in STAT1(-/-) than STAT 1(+/+) cells in vivo, and this correlates with higher activation capacity o f PKR in STAT1(-/-) cells. Genetic experiments in yeast demonstrate the inh ibition of PKR activation and eIF-2 alpha phosphorylation by STAT1 but not by STAT1 mutant. These data substantiate our previous findings on the inhib itory effects of PKR on STAT1 and implicate STAT1 in translational control through the modulation of PKR activation and eIF-2 alpha phosphorylation.