The Coiled-Coil Domain of Stat3 Is Essential for Its SH2 Domain-Mediated Receptor Binding and Subsequent Activation Induced by Epidermal Growth Factor and Interleukin-6

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Abstract
STAT proteins are a family of latent transcription factors that mediate the response to various cytokines and growth factors. Upon stimulation by cytokines, STAT proteins are recruited to the receptors via their SH2 domains, phosphorylated on a specific tyrosine, dimerized, and translocated into the nucleus, where they bind specific DNA sequences and activate the target gene transcription. STATs share highly conserved structures, including an N-domain, a coiled-coil domain, a DNA-binding domain, a linker domain, and an SH2 domain. To investigate the role of the coiled-coil domain, we performed a systematic deletion analysis of the N-domain and each of the α-helices and mutagenesis of conserved residues in the coiled-coil region of Stat3. Our results indicate that the coiled-coil domain is essential for Stat3 recruitment to the receptor and the subsequent tyrosine phosphorylation and tyrosine phosphorylation-dependent activities, such as dimer formation, nuclear translocation, and DNA binding, stimulated by epidermal growth factor (EGF) or interleukin-6 (IL-6). Single mutation of Asp170 or, to a lesser extent, Lys177 in α-helix 1 diminishes both receptor binding and tyrosine phosphorylation. Furthermore, the Asp170 mutant retains its ability to bind to DNA when phosphorylated on Tyr705 by Src kinase in vitro, implying a functional SH2 domain. Finally, we demonstrate a direct binding of Stat3 to the receptor. Taken together, our data reveal a novel role for the coiled-coil domain that regulates the early events in Stat3 activation and function.