Expression of a tyrosine phosphorylated, DNA binding Stat3β dimer in bacteria

The signal transducer and activator of transcription (STAT) proteins deliver signals from the cell membrane to the nucleus, An N-terminally truncated fragment of murine Stat3 beta, Stat3 beta tc (127-722), was produced in bacteria, STAT proteins must be specifically phosphorylated at a single tyrosine residue for dimerization and DNA binding. Therefore, Stat3 beta tc was coexpressed with the catalytic domain of the Elk receptor tyrosine kinase. Stat3 beta tc,vas quantitatively phosphorylated by this kinase domain, Gel filtration chromatography revealed a Stat3 beta tc dimer, Y705 was identified as the major phosphorylated residue of Stat3 beta tc, This corresponds to the tyrosine residue which is phosphorylated by the Janus kinase in vivo, The phosphorylated Stat3 beta tc specifically bound to DNA binding sites. The described protocol allows the production of large amounts of activated protein for biochemical and pharmaceutical studies, (C) 1998 Federation of European Biochemical Societies.