Observation of unphosphorylated STAT3 core protein binding to target dsDNA by PEMSA and X-ray crystallography

► Stoichiometric uSTAT3 interaction with M67 DNA. ► Interaction is weaker compared to pSTAT3 M67 DNA interaction. ► Report of a novel protein electrophoretic mobility shift assay (PEMSA). The STAT3 transcription factor plays a central role in a wide range of cancer types where it is over-expressed. Previously, phosphorylation of this protein was thought to be a prerequisite for direct binding to DNA. However, we have now shown complete binding of a purified unphosphorylated STAT3 (uSTAT3) core directly to M67 DNA, the high affinity STAT3 target DNA sequence, by a protein electrophoretic mobility shift assay (PEMSA). Binding to M67 DNA was inhibited by addition of increasing concentrations of a phosphotyrosyl peptide. X-ray crystallography demonstrates one mode of binding that is similar to that known for the STAT3 core phosphorylated at Y705. pSTAT3βtcandpSTAT3βtcbindbymolecular sieving(View interaction)