PDK Enhances Ral-GEF Catalytic Activity

Ral is a member of the Ras family of guanosine triphosphatases (GTPases). Ral is also a downstream target for Ras through interactions between Ras and Ral guanine exchange factors (Ral-GEFs). Tian et al. showed that Ral-GEF activity is also stimulated by overexpression of phosphatidylinositol 3-kinase (PI3K) or PI3-dependent kinase 1 (PKD1), but not by overexpression at the kinase Akt. This stimulation of Ral activity was independent of Ras activation. The Ral-GEF, Ral-GDS, has a central catalytic domain flanked by an NH 2 -terminal Ras exchange motif, which inhibits the Ral-GDS, and a COOH-terminal Ras-binding domain. Deletion of the NH 2 -terminal domain resulted in the loss of activation by PDK1, but retained activation by Ras. PDK1 stimulated the intrinsic Ral-GEF catalytic activity by binding to the inhibitory NH 2 -terminal portion of Ral-GDS. Ras stimulates the Ral-GDS by bringing it closer to its substrate without increasing catalytic activity. Stimulation of Ral-GDS by PDK1 did not require kinase activity or the pleckstrin homology (PH) domain, but was dependent on the NH 2 -terminal domain of PDK1. The coprecipitation of PDK1 and Ral-GDS was enhanced after treatment of cells with epidermal growth factor, and this interaction was blocked by the presence of an NH 2 -terminal peptide of Ral-GDS. Thus, activation of Ral is complex and involves stimulation of the catalytic activity of Ral-GEF by PDK1 and recruitment of Ral-GEF to the membrane by Ras. X. Tian, G. Rusanescu, W. Hou, B. Schaffhausen, L. A. Feig, PDK1 mediates growth factor-induced Ral-GEF activation by a kinase-independent mechanism. EMBO J. 21 , 1327-1338 (2002). [Abstract] [Full Text]