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14-3-3 zeta negatively regulates raf-1 activity by interactions with the Raf-1 cysteine-rich domain
G J Clark1, J K Drugan, K L Rossman
1Department of Pharmacology, Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, North Carolina 27599, USA.
Abstract:
Although Raf-1 is a critical effector of Ras signaling and transformation, the mechanism by which Ras promotes Raf-1 activation is complex and remains poorly understood. We recently reported that Ras interaction with the Raf-1 cysteine-rich domain (Raf-CRD, residues 139-184) may be required for Raf-1 activation. The Raf-CRD is located in the NH2-terminal negative regulatory domain of Raf-1 and is highly homologous to cysteine-rich domains found in protein kinase C family members. Recent studies indicate that the structural integrity of the Raf-CRD is also critical for Raf-1 interaction with 14-3-3 proteins. However, whether 14-3-3 proteins interact directly with the Raf-CRD and how this interaction may mediate Raf-1 function has not been determined. In the present study, we demonstrate that 14-3-3 zeta binds directly to the isolated Raf-CRD. Moreover, mutation of Raf-1 residues 143-145 impairs binding of 14-3-3, but not Ras, to the Raf-CRD. Introduction of mutations that impair 14-3-3 binding resulted in full-length Raf-1 mutants with enhanced transforming activity. Thus, 14-3-3 interaction with the Raf-CRD may serve in negative regulation of Raf-1 function by facilitating dissociation of 14-3-3 from the NH2 terminus of Raf-1 to promote subsequent events necessary for full activation of Raf-1.
Insights
Ras signaling activates Raf-1 kinase through its cysteine-rich domain (Raf-CRD). 14-3-3 proteins bind Raf-CRD, inhibiting Raf-1 activity and promoting cell transformation.
Area of Science:
- Molecular Biology
- Cell Signaling
- Oncogenesis
Background:
- Ras signaling is crucial for cell transformation, with Raf-1 as a key effector.
- The precise mechanism of Ras-mediated Raf-1 activation remains incompletely understood.
- Ras interaction with the Raf-1 cysteine-rich domain (Raf-CRD) is implicated in Raf-1 activation.
Purpose of the Study:
- To investigate the direct interaction between 14-3-3 proteins and the Raf-CRD.
- To determine how 14-3-3 binding to Raf-CRD influences Raf-1 function and activation.
- To elucidate the role of Raf-CRD in negative regulation of Raf-1.
Main Methods:
- Biochemical assays to demonstrate direct binding of 14-3-3 zeta to isolated Raf-CRD.
- Site-directed mutagenesis of Raf-1 residues (143-145) within the Raf-CRD.
- Assessment of Ras and 14-3-3 binding to mutated Raf-CRD.
- Evaluation of the transforming activity of full-length Raf-1 mutants.
Main Results:
- 14-3-3 zeta directly binds to the isolated Raf-CRD.
- Mutations at residues 143-145 of Raf-1 disrupt 14-3-3 binding but not Ras binding to the Raf-CRD.
- Mutants with impaired 14-3-3 binding exhibit enhanced transforming activity.
- 14-3-3 interaction with Raf-CRD negatively regulates Raf-1 function.
Conclusions:
- 14-3-3 proteins directly bind to the Raf-CRD, mediating negative regulation of Raf-1.
- This interaction is distinct from Ras binding and is critical for controlling Raf-1 activity.
- Disruption of 14-3-3 binding to Raf-CRD potentiates Raf-1-driven cell transformation, highlighting a novel regulatory mechanism.