STAT3 interacts directly with Hsp90

Heat shock protein 90 (Hsp90) functionally modulates signal transduction. The signal transducer and activator of transcription 3 (STAT3) mediates interleukin-6 family cytokine signaling. Aberrant activation and mutation of STAT3 is associated with oncogenesis and immune disorders, respectively. Hsp90 and STAT3 have previously been shown to colocalize and coimmunoprecipitate in common complexes. Surface plasmon resonance spectroscopy revealed a direct, high affinity specific interaction between recombinant Hsp90β and STAT3β in the presence and absence of adenosine triphosphate (ATP) in molar excess. Furthermore, comparative analysis using a phosphomimetic mutation at tyrosine 705 showed that the direct interaction appeared to favor neither unactivated nor activated STAT3. Destabilizing mutation of STAT3 at arginine residues 414/417 to alanine in the DNA-binding domain, previously shown to disrupt nuclear translocation in vivo, reduced interaction with a STAT3 DNA binding site oligonucleotide and Hsp90β in vitro, indicating that STAT3 requires a functional DNA-binding domain for full direct interaction with Hsp90. Site-directed mutagenesis of a mammalian STAT3-EGFP-N1 fusion construct at RR414/417 and subsequent transfection into human MCF7 epithelial breast cancer cells showed no impaired nuclear translocation when observed by confocal laser scanning microscopy. However, costaining for Hsp90α/β isoforms and colocalization analysis revealed a defined decrease in pixel-on-pixel colocalization compared with the wild-type confirming the requirement of the DNA-binding domain for high-affinity interaction.