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Integration of growth factor signals at the c-fos serum response element
M A Price1, C Hill, R Treisman
1Transcription Laboratory, Imperial Cancer Research Fund, London, U.K.
Abstract:
A transcription factor ternary complex composed of serum response factor (SRF) and a second factor, ternary complex factor (TCF), mediates the response of the c-fos Serum Response Element to growth factors and mitogens. In NIH3T3 fibroblasts, TCF binding is required for transcriptional activation by the SRE in response to activation of the Ras-Raf-ERK pathway. We compared the properties of three members of the TCF family, Elk-1, SAP-1 and SAP-2 (ERP/NET). Although all the proteins contain sequences required for ternary complex formation with SRF, only Elk-1 and SAP-1 appear to interact with the c-fos SRE efficiently in vivo. Each TCF contains a C-terminal activation domain capable of transcriptional activation in response to activation of the Ras-Raf-ERK pathway, and this is dependent on the integrity of S/T-P motifs conserved between all the TCF family members. In contrast, activation of the SRE by whole serum and the mitogenic phospholipid LPA requires SRF binding alone. Constitutively activated members of the Rho subfamily of Ras-like GTPases are also capable of inducing activation of the SRE in the absence of TCF; unlike activated Ras itself, these proteins do not activate the TCFs in NIH3T3 cells. At the SRE, SRF- and TCF-linked signalling pathways act synergistically to potentiate transcription.
Insights
Serum response factor (SRF) and ternary complex factors (TCFs) regulate gene expression. This study shows Elk-1 and SAP-1 TCFs efficiently interact with the c-fos SRE, mediating growth factor responses.
Area of Science:
- Molecular Biology
- Cell Signaling
- Gene Regulation
Background:
- The c-fos Serum Response Element (SRE) is activated by growth factors and mitogens.
- This activation is mediated by a transcription factor ternary complex of serum response factor (SRF) and ternary complex factors (TCFs).
- TCF binding to the SRE is crucial for transcriptional activation via the Ras-Raf-ERK pathway in NIH3T3 fibroblasts.
Purpose of the Study:
- To compare the in vivo interaction efficiencies of TCF family members (Elk-1, SAP-1, SAP-2) with the c-fos SRE.
- To investigate the role of conserved S/T-P motifs in TCF activation domains.
- To elucidate the distinct signaling pathways converging on the SRE.
Main Methods:
- Comparison of TCF family member properties.
- Analysis of ternary complex formation with SRF.
- Assessment of in vivo interaction with the c-fos SRE.
- Investigation of transcriptional activation dependent on Ras-Raf-ERK and Rho GTPase pathways.
Main Results:
- Elk-1 and SAP-1, but not SAP-2, efficiently interact with the c-fos SRE in vivo.
- TCF C-terminal activation domains, dependent on conserved S/T-P motifs, mediate Ras-Raf-ERK pathway activation.
- SRF binding alone activates the SRE in response to serum and LPA, independent of TCF.
- Activated Rho GTPases induce SRE activation without TCF activation.
Conclusions:
- Elk-1 and SAP-1 are key TCFs mediating Ras-Raf-ERK-dependent SRE activation.
- Distinct signaling pathways converge at the SRE, with SRF and TCF pathways acting synergistically.
- SRF-mediated signaling can occur independently of TCFs under certain conditions.