IN-VITRO ACTIVATION OF STAT3 BY EPIDERMAL GROWTH-FACTOR RECEPTOR KINASE

Stat proteins are SH2 domain-containing transcription factors that are activated in cells by various cytokines and growth factors, In the ca se of cytokines whose receptors lack protein kinase activity, phosphor ylation-activation is mediated by members of the JAK family of tyrosin e protein kinases, In the case of growth factors whose receptors have intrinsic tyrosine protein kinase activity, it is thought that Stat pr oteins can be activated either directly by the receptor or indirectly through JAK proteins, To test the possibility of direct activation, we have used purified Stat3 alpha, Stat3 beta, and epidermal growth fact or receptor kinase produced in recombinant baculovirus-infected Sf9 in sect cells, The Stat proteins formed a stable complex with the recepto r kinase, and they were phosphorylated on tyrosine by the receptor kin ase and activated for binding to DNA, properties shared with Stat prot eins purified from Sf9 cells coexpressing JAK1 or JAK2, Both JAK-phosp horylated Stat3 beta and Stat3 beta phosphorylated irt vitro by the re ceptor kinase were 20-50 times more active on a molar basis for DNA bi nding than phosphorylated Stat3 alpha. We conclude that Stat3 isoforms can be directly phosphorylated and thereby activated in vitro by the epidermal growth factor receptor kinase.