Ras-independent activation of Ral by a Ca(2+)-dependent pathway.
The RalA and RalB proteins comprise a distinct family of small GTPases [1]. Ral-specific guanine-nucleotide exchange factors such as RalGDS, Rlf and RGL interact with activated Ras and cooperate with Ras in the transformation of murine fibroblasts [2-5]. Thus, the interaction of RalGDS with Ras and the subsequent activation of Ral are thought to constitute a distinct Ras-dependent signaling pathway. The function of Ral is largely unknown. There is circumstantial evidence that Ral may have a function in regulating the cytoskeleton through its interaction with RIP1 (also known as RLIP or RalBP1), a GTPase-activating protein specific for the small GTPases Cdc42 and Rac [6-8]. Ral also binds to phospholipase D (PLD) and thus may play a role in signaling through phospholipids [9]. We have examined endogenous levels of activated, GTP-bound Ral (Ral-GTP) in Rat-2 fibroblasts stimulated with various mitogens. Lysophosphatidic acid (LPA) and epidermal growth factor (EGF), which activate both Ras-dependent and Ras-independent signaling pathways [10,11], rapidly activated Ral. Inhibition of Ras activation by dominant-negative Ras (RasS17N) or pertussis toxin had little effect on Ral-GTP levels, however. Ral was activated by the Ca2+ ionophore ionomycin, and activation by LPA or EGF could be blocked by a phospholipase C (PLC) inhibitor. The results presented here demonstrate a Ca(2+)-dependent mechanism for the activation of Ral.
Hofer
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F
Department of Molecular and Cell Biology, University of California at Berkeley 94720-3204, USA.
Berdeaux
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R
Martin
G S
GS
D000818:Animals; D019556:COS Cells; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000494:pharmacology; D005347:Fibroblasts; D019204:GTP-Binding Proteins / Q000378:metabolism*; D015759:Ionomycin / Q000494:pharmacology; D008246:Lysophospholipids / Q000494:pharmacology; D016297:Mutagenesis, Site-Directed; D037342:Pertussis Toxin; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D014162:Transfection; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D020725:ral GTP-Binding Proteins; D018631:ras Proteins / Q000737:chemistry / Q000378:metabolism*
D004791:Enzyme Inhibitors; D008246:Lysophospholipids; C082190:Ralb protein, rat; D011994:Recombinant Proteins; D010566:Virulence Factors, Bordetella; D015759:Ionomycin; D004815:Epidermal Growth Factor; D037342:Pertussis Toxin; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D020725:ral GTP-Binding Proteins; D018631:ras Proteins; D002118:Calcium