c-Src activates the DNA binding and transcriptional activity of Stat3 molecules: serine 727 is not required for transcriptional activation under certain circumstances

Stat3 proteins are constitutively activated in cells transformed by v-Src a nd the proteins have been shown to interact directly. Subsequent studies ha ve shown that Stat3 is required for cellular transformation of MH fibroblas ts by v-Src, suggesting a potential role for Stat3 in aberrant cell growth. Stat3 is phosphorylated on a single tyrosine (tyrosine 705) which is requi red for effective dimer formation, An additional phosphorylation event (ser ine 727) is believed to be required for full transcriptional activity of St at1 and Stat3 molecules. Here we report that c-Src activates the DNA bindin g activity of Stat3 alpha, Stat3 beta, and three Stat3 mutants, one in whic h serine 727 was replaced by alanine (Stat3 alpha S727A) and C-terminal tru ncated molecules Delta 48 and Delta 55. Consistent with this finding is a g eneral increase in the tyrosine 705-phosphorylated Stat3 in cells cotransfe cted with c-Src. Furthermore, transcription from an alpha-2 macroglobulin r eporter gene is activated by Stat3 alpha S727A to the same magnitude as com pared to Stat3 alpha and Stat3 beta in the presence of c-Src. These results suggest that serine 727, contained in a consensus MAP kinase recognition s ite and shown to be the only serine in Stat3 phosphorylated in epidermal gr owth factor (EGF) treated cells, is not necessary for transcriptional activ ity comparable to wild-type Stat3 alpha or Stat3 beta when activated by c-S rc in COS-7 cells. (C) 1999 Academic Press.