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PDK1 mediates growth factor-induced Ral-GEF activation by a kinase-independent mechanism
Xuejun Tian1, Gabriel Rusanescu, Weimin Hou
1Department of Biochemistry, Tufts University School of Medicine, Boston, MA 02111, USA.
Abstract:
Ras proteins transduce extracellular signals to intracellular signaling pathways by binding to and promoting the activation of at least three classes of downstream signaling molecules: Raf kinases, phosphoinositide-3-kinase (PI3-K) and Ral guanine nucleotide exchange factors (Ral-GEFs). Previous work has demonstrated that epidermal growth factor (EGF) activates Ral-GEFs, at least in part, by a Ras-mediated redistribution of the GEFs to their target, Ral-GTPases, in the plasma membrane. Here we show that Ral-GEF stimulation by EGF involves an additional mechanism, PI3-K-dependent kinase 1 (PDK1)-induced enhancement of Ral-GEF catalytic activity. Remarkably, this PDK1 function is not dependent upon its kinase activity. Instead, the non-catalytic N-terminus of PDK1 mediates the formation of an EGF-induced complex with the N-terminus of the Ral-GEF, Ral-GDS, thereby relieving its auto-inhibitory effect on the catalytic domain of Ral-GDS. These results elucidate a novel function for PDK1 and demonstrate that two Ras effector pathways cooperate to promote Ral-GTPase activation.
Insights
Epidermal growth factor (EGF) activates Ral guanine nucleotide exchange factors (Ral-GEFs) through a novel mechanism involving PI3-K-dependent kinase 1 (PDK1). This non-catalytic PDK1 function enhances Ral-GEF activity, revealing a cooperative Ras effector pathway.
Area of Science:
- Cellular signaling
- Molecular biology
- Signal transduction
Background:
- Ras proteins are key mediators of extracellular signals.
- Epidermal growth factor (EGF) activates Ral guanine nucleotide exchange factors (Ral-GEFs) partly via Ras-mediated redistribution.
- Existing knowledge suggests Ras effectors regulate Ral-GEF activation.
Purpose of the Study:
- To investigate the complete mechanism of Ral-GEF stimulation by EGF.
- To elucidate the role of PI3-K-dependent kinase 1 (PDK1) in EGF-induced Ral-GEF activation.
- To identify novel functions of PDK1 in cellular signaling pathways.
Main Methods:
- Investigated EGF-induced signaling pathways.
- Analyzed the interaction between PDK1 and Ral-GEF (Ral-GDS).
- Assessed the catalytic activity of Ral-GEF and the role of PDK1's N-terminus.
Main Results:
- EGF stimulation of Ral-GEFs involves a PI3-K-dependent kinase 1 (PDK1)-dependent enhancement of catalytic activity.
- PDK1's function in this process is independent of its kinase activity.
- The non-catalytic N-terminus of PDK1 forms a complex with the N-terminus of Ral-GDS, relieving auto-inhibition.
Conclusions:
- Discovered a novel, non-catalytic function for PDK1 in regulating Ral-GEF activity.
- Demonstrated that two Ras effector pathways cooperate to activate Ral-GTPases.
- Elucidated a new mechanism for signal transduction involving PDK1 and Ral-GEFs.