Ras Signaling Requires Dynamic Properties of Ets1 for Phosphorylation-Enhanced Binding to Coactivator CBP
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Ras signaling requires dynamic properties of Ets1 for phosphorylation-enhanced binding to coactivator CBP Mary L. Nelsona,1, Hyun-Seo Kangb,1, Gregory M. Leeb, Adam G. Blaszczaka, Desmond K. W. Laub, Lawrence P. McIntoshb,2, and Barbara J. Gravesa,2 aDepartment of Oncological Sciences, University of Utah School of Medicine, Huntsman Cancer Institute, University of Utah, Salt Lake City, UT, 84112; and bDepartment of Biochemistry and Molecular Biology, Department of Chemistry, and the Michael Smith Laboratories, University of British Columbia, Vancouver, BC, Canada V6T 1Z3 Edited by Peter E. Wright, The Scripps Research Institute, La Jolla, CA, and approved April 8, 2010 (received for review January 4, 2010) Ras/MAPK signaling is often aberrantly activated in human can- Here we describe the coupled roles of the PNT domain and an cers. The downstream effectors are transcription factors, including adjacent disordered region bearing both Thr38 and a previously those encoded by the ETS gene family. Using cell-based assays and unrecognized phosphoacceptor, Ser41, in the ERK2-enhanced biophysical measurements, we have determined the mechanism by binding of the CBP TAZ1 domain by Ets1. Phosphorylation plays which Ras/MAPK signaling affects the function of Ets1 via phos- a dual role by altering a conformational equilibrium of a dynamic phorylation of Thr38 and Ser41. These ERK2 phosphoacceptors helix H0 that tethers the flexible phosphorylated region of Ets1 to lie within the unstructured N-terminal region of Ets1, immediately the core PNT domain and by augmenting electrostatic forces that adjacent to the PNT domain. NMR spectroscopic analyses demon- drive TAZ1 recognition. In this integrated model, structured ele- strated that the PNT domain is a four-helix bundle (H2–H5), resem- ments with intrinsic flexibility provide a scaffold for responsive- bling the SAM domain, appended with two additional helices ness to a signaling pathway. The overall mechanism represents an (H0–H1). Phosphorylation shifted a conformational equilibrium, evolutionary development within a gene family, whereby dynamic displacing the dynamic helix H0 from the core bundle. The affinity elements are appended to conserved core folds to increase the of Ets1 for the TAZ1 (or CH1) domain of the coactivator CBP was capacity for biological regulation. enhanced 34-fold by phosphorylation, and this binding was sensi- tive to ionic strength. NMR-monitored titration experiments Results mapped the interaction surfaces of the TAZ1 domain and Ets1, ERK2 Phosphoacceptors Thr38 and Ser41 Contribute to Ets1 Transac- the latter encompassing both the phosphoacceptors and PNT tivation. To investigate the mechanism by which phosphorylation domain. Charge complementarity of these surfaces indicate that affects the function of Ets1, we developed an in vitro system using electrostatic forces act in concert with a conformational equili- ERK2 to covalently modify purified Ets1. Phosphorylation of brium to mediate phosphorylation effects. We conclude that the both the known ERK2 acceptor site, Thr38, and a previously dynamic helical elements of Ets1, appended to a conserved unrecognized nonconsensus site, Ser41, was detected by mass 31 structural core, constitute a phospho-switch that directs Ras/MAPK spectrometry and P-NMR experiments (13). The in vivo rele- signaling to downstream changes in gene expression. This detailed vance of phosphorylation was tested in a cell-based assay for structural and mechanistic information will guide strategies for Ras/MAPK enhancement of Ets1 activity by the coexpression targeting ETS proteins in human disease. of a constitutively active form of MEK1 (11). Mutation of both phosphoacceptor sites was necessary to reduce relative luciferase MAP kinase ∣ protein structure/dynamics ∣ transcriptional regulation ∣ activity (RLA) to vector-only control levels (Fig. 1). To provide protein-protein interaction ∣ Ets2 better quantification through replica experiments, we measured ð Þ∕ð Þ superactivation as defined by RLAWT or mutant RLAempty vector he Ras/MAP kinase signaling pathway regulates cell prolifera- in the presence of MEK1. Wild-type Ets1 caused two- to three- Ttion. A large number of human tumors have genetic altera- fold (2.4 Æ 0.1) superactivation. Mutation of Thr38 or Ser41 tions in the Ras signaling pathway that drive constitutive to alanine partially reduced the effect (1.2 Æ 0.08 and 1.4 Æ 0.09, signaling. In spite of this importance to human disease, no ther- respectively), whereas Ets1 with a double phosphoacceptor apeutics currently disrupt the nuclear effectors of this pathway, site mutation displayed no superactivation (1.1 Æ 0.06). Substitu- which offer the potential for both tissue and disease specificity. tion of glutamic acid residues at positions 38 and 41 could Several known effectors belong to the ETS transcription factor only partially reconstitute the effects of phosphorylation (1.4Æ family (1), but even in this case only a few specific target genes 0.2). Phosphorylation of Thr38 in vivo was confirmed for wild- have been identified. Examples include the serum-response-ele- type and the S41A mutant with phospho-specific antibodies ment (SRE) of the c-Fos promoter, at which the ETS protein Elk1 plays a central role (2). Other Ras-responsive elements that Author contributions: M.L.N., H.-S.K., G.M.L., A.G.B., L.P.M., and B.J.G. designed research; have been characterized include tandem ETS or ETS/AP1 M.L.N., H.-S.K., G.M.L., A.G.B., and D.K.W.L. performed research; M.L.N., H.-S.K., G.M.L., composite sites associated with Ets1 and Ets2 (3–5). In Drosophi- A.G.B., D.K.W.L., L.P.M., and B.J.G. analyzed data; and M.L.N., H.-S.K., L.P.M., and B.J.G. la, a genetic link between the ETS family and Ras/MAPK signal- wrote the paper. ing implicates Pnt-P2, the ortholog of vertebrate Ets1 and Ets2, The authors declare no conflict of interest. and Yan, the apparent ortholog of vertebrate Tel (6). Phosphor- This article is a PNAS Direct Submission. ylation affects nuclear export (Yan) (7) and CBP (CREB-binding Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, protein)/p300 recruitment (Elk1, Ets1, and Ets2) (2, 8, 9). Ets1 www.pdb.org (PDB ID codes 2jv3 and 2kmd). The NMR chemical shifts and restraints have been deposited in the BioMagResBank, www.bmrb.wisc.edu (accession nos. 4205 bears one characterized MAP kinase ERK2 phosphoacceptor, and 16426). Thr38, found in an unstructured region N-terminal to its struc- 1M.L.N. and H.-S.K. contributed equally to this work. tured PNT domain (10–12). Although both these regions are 2To whom correspondence may be addressed. E-mail: [email protected] or barbara. critical for Ras/MAPK-dependent enhanced transcriptional [email protected]. regulation (8), the molecular mechanism of this signaling path- This article contains supporting information online at www.pnas.org/lookup/suppl/ way has not been elucidated. doi:10.1073/pnas.0915137107/-/DCSupplemental. 10026–10031 ∣ PNAS ∣ June 1, 2010 ∣ vol. 107 ∣ no. 22 www.pnas.org/cgi/doi/10.1073/pnas.0915137107 Downloaded by guest on September 25, 2021 (Fig. 1). Thus, the full effect of the signaling required the two and Phe56 with Trp80, His128, and Ile131. More importantly, the phosphoacceptors. refined model demonstrated an Ets1 PNT domain-specific helix, designated H0 (residues 42–52), which is also present in Ets2 Phosphorylation Increases the Affinity of the Ets1 PNT Domain for the (Fig. S2). This amphipathic helix packs against helix H5 and CBP TAZ1 Domain. The CBP interaction domain(s) for binding to the end of helix H2 via hydrophobic interactions involving Ets1 and Ets2 was mapped by pull-down assays with GST-tagged Met44, Met45, and Leu49 with Phe88, Phe120, and Ile124, as well CBP fragments. An ∼50 kDa N-terminal fragment of CBP, which as potential salt bridges between Lys42-Asp123 and Lys50- contains three transcription factor interaction domains, displayed Glu127. Helix H0 is preceded by an unstructured region (residues binding and was subjected to further analysis (Fig. S1). The TAZ1 29–41) bearing the phosphoacceptors Thr38 and Ser41. domain-bearing fragments bound full length Ets1, as previously Although clearly defined in the structural ensemble of shown in cell extracts (14, 15). Furthermore, the deletion mutants Ets129-138, helix H0 is marginally stable and conformationally Ets11-138 and the analogous Ets21-172, which contain PNT dynamic by several criteria. Using the algorithm SSP (secondary domains, also bound the TAZ1 domain. In contrast, the isolated structure propensity) to predict secondary structure from main N-terminal domain of CBP, which binds nuclear receptors, and chain chemical shifts (17), H0 exhibits helical scores less than the KIX domain, best known for CREB binding, did not measur- those of the core PNT domain (Fig S3A). 15N relaxation measure- ably associate with Ets1. The phosphorylation effect on the inter- ments confirmed that the backbone of helix H0 is mobile on the action of Ets11-138 with the isolated TAZ1 domain was quantified sub-nsec time scale (Fig. S3 B–E). In addition, residues in this by isothermal titration calorimetry (ITC). The affinity of unmo- helix undergo facile amide hydrogen exchange (HX), exhibiting 1 138 dified Ets1 - for TAZ1 (Kd ¼ 58 Æ 12 μM) was enhanced protection factors of only ∼2 relative to a corresponding random 34 Æ 1-fold upon phosphorylation. coil polypeptide (Fig. S3F). This behavior, indicative of extensive local unfolding, was supported by partial proteolysis measure- Ets1 PNT Domain Structure Couples Dynamic Phosphoacceptors and ments. Cleavage at Lys42, Lys50, and Arg62 within helices Helix H0 to a Core Helical Bundle. To develop a mechanistic model H0 and H1 revealed at least transient accessibility of these for Ets1/CBP binding, we undertook additional biochemical and residues to trypsin, whereas lysines/arginines within the core PNT biophysical approaches.