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A growth factor-induced kinase phosphorylates the serum response factor at a site that regulates its DNA-binding
V M Rivera1, C K Miranti, R P Misra
1Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115.
Abstract:
A signaling pathway by which growth factors may induce transcription of the c-fos proto-oncogene has been characterized. Growth factor stimulation of quiescent fibroblasts activates a protein kinase cascade that leads to the rapid and transient phosphorylation of the serum response factor (SRF), a regulator of c-fos transcription. The in vivo kinetics of SRF phosphorylation and dephosphorylation parallel the activation and subsequent repression of c-fos transcription, suggesting that this phosphorylation event plays a critical role in the control of c-fos expression. The ribosomal S6 kinase pp90rsk, a growth factor-inducible kinase, phosphorylates SRF in vitro at serine 103, the site that becomes newly phosphorylated upon growth factor stimulation in vivo. Phosphorylation of serine 103 significantly enhances the affinity and rate with which SRF associates with its binding site, the serum response element, within the c-fos promoter. These results suggest a model in which the growth factor-induced phosphorylation of SRF at serine 103 contributes to the activation of c-fos transcription by facilitating the formation of an active transcription complex at the serum response element.
Insights
Growth factors activate a signaling pathway to control c-fos gene transcription. This involves phosphorylating the serum response factor (SRF), which enhances its binding to the c-fos promoter.
Area of Science:
- Molecular Biology
- Cell Signaling
- Gene Regulation
Background:
- Growth factors play a crucial role in cellular processes, including gene expression.
- The c-fos proto-oncogene is rapidly induced by growth factors, but the underlying signaling mechanisms are not fully understood.
- The serum response factor (SRF) is a known regulator of c-fos transcription.
Purpose of the Study:
- To characterize the signaling pathway by which growth factors induce c-fos transcription.
- To investigate the role of protein phosphorylation in the regulation of c-fos expression.
- To identify the specific kinase and phosphorylation site involved in growth factor-mediated c-fos induction.
Main Methods:
- Utilized quiescent fibroblasts and growth factor stimulation.
- Analyzed protein kinase cascades and phosphorylation events.
- Investigated the kinetics of serum response factor (SRF) phosphorylation and dephosphorylation in vivo.
- Performed in vitro phosphorylation assays using ribosomal S6 kinase pp90rsk.
- Assessed the effect of SRF phosphorylation on its binding affinity to the serum response element.
Main Results:
- Growth factor stimulation activates a protein kinase cascade leading to rapid and transient phosphorylation of SRF.
- The kinetics of SRF phosphorylation and dephosphorylation mirrored c-fos transcription changes.
- The growth factor-inducible kinase, pp90rsk, phosphorylates SRF at serine 103 in vitro.
- Phosphorylation of serine 103 enhanced SRF's affinity and rate of association with the serum response element.
- This phosphorylation event is critical for the control of c-fos expression.
Conclusions:
- Growth factor-induced phosphorylation of SRF at serine 103 is a key mechanism for activating c-fos transcription.
- This phosphorylation facilitates the formation of an active transcription complex at the c-fos promoter.
- The characterized pathway provides insight into how growth factors regulate proto-oncogene expression.