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Interaction between active Pak1 and Raf-1 is necessary for phosphorylation and activation of Raf-1
Mengwei Zang1, Cynthia Hayne, Zhijun Luo
1Diabetes and Metabolism Research Unit, Endocrinology Section, Evans Department of Medicine, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Abstract:
Activation of Raf-1 is a complex process in which phosphorylation of Ser(338)-Tyr(341) is a critical step. Previous studies have shown that Pak1/2 is implicated in both Ras-dependent and -independent activation of Raf-1 by phosphorylating Raf Ser(338). The present study explores the structural basis of Raf-1 phosphorylation by Pak1. We found that Pak directly associates with Raf-1 under both physiological and overexpressed conditions. The association is greatly stimulated by 4beta-12-O-tetradecanoylphorbol-13-acetate and nocodazole and by expression of the active mutants of Rac and Ras. The active forms of Pak generated by mutation of Thr(423) to Glu or truncation of the amino-terminal moiety exhibit a greater binding to Raf than the wild type, whereas the kinase-dead mutant Pak barely binds Raf. The extent of binding to Raf-1 is correlated with the ability of Pak to phosphorylate Raf and induce mitogen-activated protein kinase activation. Furthermore, the Raf-1 binding site is defined to the carboxyl terminus of the Pak catalytic domain. In addition, our results suggest that the amino-terminal regulatory region of Raf inhibits the interaction. Taken together, the results indicate that the interaction depends on the active conformations of Pak and Raf. They also argue that Pak1 is a physiological candidate for phosphorylation of Raf Ser(338) during the course of Raf activation.
Insights
Pak1 directly binds Raf-1, a crucial step in Raf-1 activation. This interaction, dependent on active Pak1 and Raf-1 conformations, suggests Pak1
Area of Science:
- Cellular signaling pathways
- Protein kinase interactions
- Signal transduction mechanisms
Background:
- Raf-1 activation involves critical phosphorylation events at Ser(338)-Tyr(341).
- Pak1/2 kinases are implicated in both Ras-dependent and -independent Raf-1 activation.
- Understanding the structural basis of Pak1-Raf-1 interaction is key to elucidating Raf-1 regulation.
Purpose of the Study:
- To investigate the structural basis of Raf-1 phosphorylation by Pak1.
- To identify the binding sites and regulatory mechanisms governing Pak1-Raf-1 association.
- To confirm Pak1's role as a physiological kinase for Raf-1 Ser(338) phosphorylation.
Main Methods:
- Co-immunoprecipitation assays to demonstrate direct association between Pak1 and Raf-1.
- Analysis of Pak1 mutants (active, kinase-dead) and Raf-1 constructs to map binding domains.
- Stimulation studies using phorbol ester (TPA), nocodazole, and active Rac/Ras mutants.
- Correlation analysis between Pak1 binding, Raf-1 phosphorylation, and downstream MAPK activation.
Main Results:
- Pak1 directly associates with Raf-1 under various conditions, enhanced by TPA, nocodazole, and active Rac/Ras.
- Active Pak1 mutants show increased binding to Raf-1, while kinase-dead mutants exhibit minimal binding.
- Pak1 binding to Raf-1 correlates with its ability to phosphorylate Raf and activate MAPK signaling.
- The carboxyl terminus of the Pak1 catalytic domain is identified as the Raf-1 binding site.
- The amino-terminal regulatory region of Raf-1 inhibits Pak1 interaction.
Conclusions:
- Pak1 directly binds Raf-1, with interaction strength dependent on the active conformations of both proteins.
- Pak1's interaction with Raf-1 is structurally defined and regulated by specific domains within both kinases.
- Pak1 is a strong candidate for the physiological phosphorylation of Raf-1 Ser(338) during Raf activation pathways.