Chemotaxis: new role for Ras revealed

A recent study of chemotaxis revealed a new role for the proto-oncogene Ras in the social ameba Dictyostelium discoideum. Chemotaxis, the directional movement of cells toward chemokines and other chemoattractants, plays critical roles in diverse physiological processes, such as mobilization of immune cells to fight invading microorganisms, targeting of metastatic cancer cells to specific tissues, and guidance of sperm cells to ova during fertilization. This work, published in the July 26 issue of The Journal of Cell Biology, was conducted in Dr. Devreotes’ lab at John Hopkins University and Dr. Parent’s lab at National Cancer Institute. This research team demonstrated that RasC functions as an upstream regulator of TORC2 and thereby governs the effects of TORC2-PKB signaling on the cytoskeleton and cell migration. Many of the core components of the underlying chemotaxis signaling network have been elucidated in D. discoideum. Chemoattractants are sensed by G-protein-coupled receptors (GPCRs), which leads to the activation heterotrimeric Gproteins, small Ras-like G-proteins, and phosphoinsositide 3kinase (PI3K), resulting in the generation of phosphatidylinositol-(3,4,5)-trisphosphate (PIP3). This phospholipid, in turn, prompts the membrane translocation of proteins containing pleckstrin homology (PH) domains, such as cytosolic regulator of adenylyl cyclase (CRAC) and protein kinase B (PKB), which regulate the cytoskeleton rearrangements during chemotaxis. Importantly, many of these components and their activation are highly localized to the leading edge of cells undergoing chemotaxis, assuring that cytoskeletal changes needed for directional movement are spatially restricted (reviewed in Jin et al., 2009). Although it has been well established that the PIP3 pathway plays an important role in the regulation of chemotaxis, additional pathways that act in parallel with the PIP3 pathway have recently been revealed. For instance, phospholipase A2 (PLA2) was reported to mediate chemotaxis in parallel with PIP3 pathway (Chen et al., 2007). In addition, a PIP3independent pathway in which PKB is activated by TORC2 (target of rapamycin complex 2) was found to regulate chemotaxis (Lee et al., 2005; Kamimura et al., 2008). However, the mechanism by which TORC2 is regulated in chemotaxis was poorly understood prior to the publication of the recent report by Cai et al. (2010). D. discoideum possesses two PKB homologs, namely PKBA, which contains a PH domain and is dynamically recruited to the plasma membrane by PIP3, and PKBR1, which is tethered to the plasma membrane via N-terminal myristoylation. In their previous work, the authors discovered that both PKBA and PKBR1 are activated by TORC2mediated phosphorylation of their hydrophobic motifs (HMs) (Kamimura et al., 2008) and phosphoinositide-dependent kinase (PDK)-mediated phosphorylation of their activation loops (ALs) (Kamimura and Devreotes, 2010). In the present study, Cai and colleagues investigated whether Ras family proteins activate TORC2 and, if so, what are the effects of this Ras-TORC2 pathway on chemotactic responsiveness. In order to examine whether Ras proteins are required for TORC2-mediated activation of PKB, the scientists first determined the PKB activity in different Ras knock-out cells. They found that phosphorylation of the HM of PKBR1, the ALs of both PKBR1 and PKBA, and many PKB substrates were significantly reduced in rasC but not rasG cells relative to wild-type cells. To further explore rasC's role in PKB activation, the authors examined the consequences of expressing activated RasC (RasC) and found that it dramatically prolonged the phosphorylation kinetics of the PKBR1 and multiple PKB substrates, suggesting that RasC is indeed involved in regulating the PKB pathway. In addition, RasC expression also prolonged actin polymerization and impaired chemotaxis in wild-type cells, effects that were suppressed in cells lacking Pianissimo (piaA), an essential component of TORC2. Taken together these findings