Colocalization of Ras and Ral on the membrane is required for Ras-dependent Ral activation through Ral GDP dissociation stimulator

All in the family? New insights and questions regarding interconnectivity of Ras, Rap1 and Ral

Ras, Rap1 and Ral are related small GTPases. While the function of Ras in signal transduction is well established, it has been recognized only recently that Rap1 and Ral also are activated rapidly in response to a large variety of extracellular signals. Between the three GTPase an intriguing interconnectivity exists, in that guanine nucleotide exchange factors for Ral associate with the GTP-bound form of both Ras and Rap1. Furthermore, Rap1 is considered to function as an antagonist of Ras signalling by trapping Ras effectors in an inactive complex. Here, I summarize the recent developments in understanding the functional relationship between these three GTPase and argue that Rap1 functions in a signalling pathway distinct from Ras, while using similar or identical effectors.

Extracellular signal-regulated activation of Rap1 fails to interfere in Ras effector signalling

The small GTPase Rap1 has been implicated in both negative and positive control of Ras-mediated signalling events. We have investigated which extracellular signals can activate Rap1 and whether this activation leads to a modulation of Ras effector signalling, i.e. the activation of ERK and the small GTPase Ral. We found that Rap1 is rapidly activated following stimulation of a large variety of growth factor receptors. These receptors include receptor tyrosine kinases for platelet-derived growth factor (PDGF) and epithelial growth factor (EGF), and G protein-coupled receptors for lysophosphatidic acid (LPA), thrombin and endothelin. At least three distinct pathways may transduce a signal towards Rap1 activation: increase in intracellular calcium, release of diacylglycerol and cAMP synthesis. Surprisingly, activation of endogenous Rap1 fails to affect Ras-dependent ERK activation. In addition, we found that although overexpression of active Rap1 is able to activate the Ral pathway, activation of endogenous Rap1 in fibroblasts does not result in Ral activation. Rap1 also does not negatively influence Ras-mediated Ral activation. We conclude that activation of Rap1 is a common event upon growth factor treatment and that the physiological function of Rap1 is likely to be different from modulation of Ras effector signalling.

Activation of the small GTPase Ral in platelets

Ral is a ubiquitously expressed Ras-like small GTPase which is abundantly present in human platelets. The biological function of Ral and the signaling pathway in which Ral is involved are largely unknown. Here we describe a novel method to measure Ral activation utilizing the Ral binding domain of the putative Ral effector RLIP76 as an activation-specific probe. With this assay we investigated the signaling pathway that leads to Ral activation in human platelets. We found that Ral is rapidly activated after stimulation with various platelet agonists, including alpha-thrombin. In contrast, the platelet antagonist prostaglandin I2 inhibited alpha-thrombin-induced Ral activation. Activation of Ral by alpha-thrombin could be inhibited by depletion of intracellular Ca2+, whereas the induction of intracellular Ca2+ resulted in the activation of Ral. Our results show that Ral can be activated by extracellular stimuli. Furthermore, we show that increased levels of intracellular Ca2+ are sufficient for Ral activation in platelets. This activation mechanism correlates with the activation mechanism of the small GTPase Rap1, a putative upstream regulator of Ral guanine nucleotide exchange factors.

Interactions of the amino acid residue at position 31 of the c-Ha-Ras protein with Raf-1 and RalGDS

The Ras and Rap1A proteins can bind to the Raf and RalGDS families. Ras and Rap1A have Glu and Lys, respectively, at position 31. In the present study, we analyzed the effects of mutating the Glu at position 31 of the c-Ha-Ras protein to Asp, Ala, Arg, and Lys on the interactions with Raf-1 and RalGDS. The Ras-binding domain (RBD) of Raf-1 binds the E31R and E31K Ras mutants less tightly than the wild-type, E31A, and E31D Ras proteins; the introduction of the positively charged Lys or Arg residue at position 31 specifically impairs the binding of Ras with the Raf-1 RBD. On the other hand, the ability of the oncogenic RasG12V protein to activate Raf-1 in HEK293 cells was only partially reduced by the E31R mutation but was drastically impaired by the E31K mutation. Correspondingly, RasG12V(E31K) as well as Rap1A, but not RasG12V(E31R), exhibited abnormally tight binding with the cysteine-rich domain of Raf-1. On the other hand, the E31A, E31R, and E31K mutations, but not the E31D mutation, enhanced the RalGDS RBD-binding activity of Ras, indicating that the negative charge at position 31 of Ras is particularly unfavorable to the interaction with the RalGDS RBD. RasG12V(E31K), RasG12V(E31A), and Rap1A stimulate the RalGDS action more efficiently than the wild-type Ras in the liposome reconstitution assay. All of these results clearly show that the sharp contrast between the characteristics of Ras and Rap1A, with respect to the interactions with Raf-1 and RalGDS, depends on their residues at position 31.

Identification and characterization of a novel protein interacting with Ral-binding protein 1, a putative effector protein of Ral

Ral-binding protein 1 (RalBP1) is a putative effector protein of Ral and exhibits a GTPase activating activity for Rac and CDC42. To clarify the function of RalBP1, we isolated a novel protein that interacts with RalBP1 by yeast two-hybrid screening and designated it POB1 (partner of RalBP1). POB1 consists of 521 amino acids, shares a homology with Eps15, which has been identified as an epidermal growth factor (EGF) receptor substrate, and has two proline-rich motifs. The POB1 mRNA was expressed in cerebrum, cerebellum, lung, kidney, and testis. POB1 interacted with RalBP1 in COS cells and the C-terminal region of POB1 was responsible for this interaction. The binding domain of RalBP1 to POB1 was distinct from its binding domain to Ral. Ral and POB1 simultaneously interacted with RalBP1 in COS cells. The binding of POB1 to RalBP1 did not affect the GTPase activating activity of RalBP1. Furthermore, POB1 bound to Grb2 but not to Nck or Crk. POB1 was tyrosine-phosphorylated in COS cells upon stimulation with EGF and made a complex with EGF receptor. These results suggest that RalBP1 makes a complex with POB1 and that this complex may provide a link between tyrosine kinase, Src homology 3 (SH3)-containing protein, and Ral.