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Structural foundation for transcriptional regulation by Ets1 and CBP Kang, Hyun-Seo


Using cell-based assays and biophysical measurements, we have defined the mechanism by which Ras/MAP kinase signaling enhances Ets1 regulated gene expression via phosphorylation-enhanced recruitment of the transcriptional co-activator CBP. As confirmed by ³¹P/¹³C-NMR experiments, the MAP kinase ERK2 phosphorylates Thr38 and Ser41 within the unstructured region of Ets1, immediately N- terminal to the PNT domain. The NMR-derived structure of residues 29-138 of Ets1 revealed that the PNT domain is composed of a core four-helix bundle (H2-H5), also known as the SAM fold, appended with two additional helices (H0-H1). Most importantly, helix H0 is only marginally stable as shown by various NMR methods, including chemical shift, amide hydrogen exchange, and ¹⁵N relaxation analyses. Dual phosphorylation of Ets1 perturbs a "closed-open" conformational equilibrium of the PNT domain, displacing the dynamic helix H0 from the core bundle. These modifications increase the affinity of Ets1 for the TAZ1 (or CH1) domain of CBP by ~30 fold as measured with isothermal titration calorimetry (Kd ~ 60 to 2 μM). NMR-monitored titration experiments mapped the interaction surfaces of the TAZ1 domain and Ets1, the latter encompassing both the phosphoacceptors and PNT domain, also showing sensitivity to ionic strength. Charge complementarity of these surfaces indicates that electrostatic forces act in concert with the conformational equilibrium to mediate phosphorylation effects. We conclude that the dynamic helical elements of Ets1, appended to a conserved structural core, constitute a "phospho-switch" to direct Ras/MAPK signaling to downstream changes in gene expression. This detailed structural and mechanistic information illustrates an evolutionary development within a gene family to increase the capacity for biological regulation. We also discovered that the CBP TAZ1 domain associates intramolecularly with residues 28-82 in its N-terminal nuclear receptor interacting domain (NRID). NMR studies indicated that the NRID undergoes a coil-helix conformational transition upon binding the same interface on TAZ1 as recognized by many transcription factor partners. This led us to hypothesize that CBP is regulated by an auto-inhibitory mechanism. In support of this model, affinity of the hypoxia inducible factor HIF-1α for TAZ1 is reduced competitively by the presence of the NRID.

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