Allosteric regulation in STAT3 interdomains is mediated by a rigid core: SH2 domain regulation by CCD in D170A variant

Signal Transducer and Activator of Transcription 3 (STAT3) plays a crucial role in cancer development and thus is a viable target for cancer treatment. STAT3 functions as a dimer mediated by phosphorylation of the SRC-homology 2 (SH2) domain, a key target for therapeutic drugs. While great efforts have been employed towards the development of compounds that directly target the SH2 domain, no compound has yet been approved by the FDA due to a lack of specificity and pharmacologic efficacy. Studies have shown that allosteric regulation of SH2 via the coiled-coil domain (CCD) is an alternative drug design strategy. Several CCD effectors have been shown to modulate SH2 binding and affinity, and at the time of writing at least one drug candidate has entered phase I clinical trials. However, the mechanism for SH2 regulation via CCD is poorly understood. Here, we investigate structural and dynamic features of STAT3 and compare the wild type to the reduced function variant D170A in order to delineate mechanistic differences and propose allosteric pathways. Molecular dynamics simulations were employed to explore conformational space of STAT3 and the variant, followed by structural, conformation, and dynamic analysis. The trajectories explored show distinctive conformational changes in the SH2 domain for the D170A variant, indicating long range allosteric effects. Multiple analyses provide evidence for long range communication pathways between the two STAT3 domains, which seem to be mediated by a rigid core which connects the CCD and SH2 domains via the linker domain (LD) and transmits conformational changes through a network of short-range interactions. The proposed allosteric mechanism provides new insight into the understanding of intramolecular signaling in STAT3 and potential pharmaceutical control of STAT3 specificity and activity. Author summary In all living organisms, the proliferation and survival of cells are regulated by various proteins. Signal Transducers and Activators of Transcription 3 (STAT3) protein is one of these important proteins. However, the abnormal regulation of these proteins will contribute to the proliferation of cancer. The constitutive activation of STAT3 has been linked to several types of solid tumors, leukemia, and lymphomas. Consequently, STAT3 proteins have been a key target for cancer therapy. SH2 (SRC-homology 2) domain is the key interaction site, great efforts have been made to target SH2 domain. However, specificity has been a major challenge in drug discovery. Research showing regulation of SH2 domain via CCD (coiled-coil domain) has opened a new path for drug discovery, but progress is challenged by poor understanding of the allosteric mechanism. Here, we show that CCD regulates SH2 conformation via a rigid backbone. The perturbations in CCD are transmitted through an alpha-helix to the rigid core that orchestrate the movement of CCD and LD (link domain), leading to structural changes in the SH2 domain. The present findings provide an allosteric mechanism with atomistic details underlying the regulation of CCD to SH2 domain in STAT3 protein. A detailed allosteric pathway allows informed drug design targeting CCD for desired downstream effect on SH2 domain and the overall STAT3 function.