112 The Ral guanine nucleotide exchange factors are direct targets of Ras, providing a mechanism for Ral activation by extracellular signals. In addition, Ral can be activated by a Ras-independent pathway. Ral guanine nucleotide exchange factors contribute to cellular transformation induced by oncogenic Ras through an Erk-independent mechanism which may involve activation of transcription. Addresses Laboratory for Physiological Chemistry and Center for Biomedical Genetics, Stratenum, Universiteitsweg 100, 3584 CG Utrecht, The Netherlands *e-mail: j.l.bos@med.uu.nl Current Opinion in Genetics & Development 1999, 9:112–117 http://biomednet.com/elecref/0959437X00900112 © Elsevier Science Ltd ISSN 0959-437X Abbreviations GAP GTPase activating protein GEF guanine nucleotide exchange factor PI3K phosphatidylinositol-3 lipid kinase PLC phospholipase C PLD phospholipase D RBD Ras binding domain SRE serum response element Introduction Activation of the guanine-nucleotide-binding Ras pro- teins is a crucial component in the transduction of extracellular signals that stimulate proliferation and differ- entiation [1]. Wild-type Ras is transiently activated by the exchange of bound GDP for GTP, a process which is catal- ysed by guanine nucleotide exchange factors (GEFs), whereas oncogenic Ras proteins are stalled in their active, GTP-bound form [2]. Active Ras associates with a number of downstream targets, or effectors, to exert its biological effects [3,4]. Although there are indications that the GTP-bound form of Ras has many binding partners [5], three classes of Ras effectors are now established as being functional in vivo: the protein kinases of the Raf family, the catalytic subunit of a phosphatidylinositol-3 lipid kinase (PI3K), and the Ral guanine nucleotide exchange factors (RalGEFs). Raf-kinases are the best-described Ras effectors and con- trol the activation of the MAP kinases Erk1 and Erk2, which are involved in protein phosphorylation events in mitogenesis and differentiation (e.g. see [6]). PI3K pro- tects Ras-transformed cells from going into apoptosis and links Ras activation to rearrangements of the actin cytoskeleton [7,8]. RalGEFs promote the activation of the ubiquitously expressed Ras family member Ral [9]. The roles of the RalGEFs in transmitting signals from Ras and the putative cellular functions of the RalGEF and Ral pro- teins have started to emerge more recently and will be discussed in our review. Activation of RalGEFs by Ras RalGDS, identified on the basis of its homology with yeast RasGEFs, was shown to exhibit specific in vitro guanine nucleotide exchange activity for the highly related RalA and RalB [10]. To date, three additional types of Ral-spe- cific GEFs have been cloned: Rgl, Rlf and Rgr [9]. Rgr was identified as the oncogenic region of a fusion protein des- ignated Rsc ([11]; Figure 1). The concept that RalGEFs are Ras effectors originates from the identification of RalGDS, Rgl and Rlf as binding partners of active Ras and Ras caught in another affair: the exchange factors for Ral Rob MF Wolthuis and Johannes L Bos* Figure 1 Linear representation of the different types of RalGEFs. Shown are the mouse RalGDS, Rgl and Rlf and the rabbit Rsc. Rsc is an oncoprotein that results from a fusion of a homolog of the yeast Rad23 protein (indicated with hatching) and the carboxy-terminal region of a RalGEF, designated Rgr. The Rgr moiety is responsible for the tumorigenic properties of Rsc [11]. The black boxes represent regions homologous to CDC25-like guanine nucleotide exchange factors, and the grey boxes represent the RBDs. Rsc and Rgr do not contain a carboxy-terminal RBD. Rgl and Rlf proteins are ~50% and 34% identical to the mouse RalGDS protein, respectively [32,56]. The Rgr moiety of Rsc is 40% identical to RalGDS [11]. The mRNAs for RalGDS and Rlf are expressed ubiquitously [10,32], the Rgl messenger is mainly expressed in brain, heart, lung, kidney and testis (A Kikuchi, personal communication) whereas Rgr is present predominantly in lung and skin [11]. The in vitro dissociation constants (Kdiss.) of the interaction between the RalGEF-RBD and H-RasGTP are indicated on the right [15,57]. For a comparison, the in vitro dissociation constant of the interaction between Raf1-RBD and H-Ras-GTP is 20 nM [58]. RalGDS Rgl Rlf Rsc 1000 3500 90 Rgr Kdiss. H-RasGTP (nM) Current Opinion in Genetics & Development