14-863 Sigma-AldrichRaf-1 RBD Protein, GST, 300 µg

Endothelin-1 (ET-1) induces cell proliferation and differentiation through multiple G-protein-linked signaling systems, including p21ras activation. Whereas p21ras activation and desensitization by receptor tyrosine kinases have been extensively investigated, the kinetics of p21ras activation induced by engagement of G-protein-coupled receptors remains to be fully elucidated. In the present study we show that ET-1 induces a biphasic activation of p21ras in rat glomerular mesangial cells. The first peak of activation of p21ras, at 2-5 min, is mediated by immediate association of phosphorylated Shc with the guanosine exchange factor Sos1 via the adaptor protein Grb2. This initial activation of p21ras results in activation of the extracellular signal-regulated kinase (ERK) cascade. We demonstrate that ET-1 signaling elicits a negative feedback mechanism, modulating p21ras activity through ERK-dependent Sos1 phosphorylation, findings which were confirmed using an adenovirus MEK construct. Subsequent to p21ras and ERK deactivation, Sos1 reverts to the non-phosphorylated condition, enabling it to bind again to the Grb2/Shc complex, which is stabilized by persistent Shc phosphorylation. However, the resulting secondary activation of p21ras at 30 min does not lead to ERK activation, correlating with intensive, ET-1-induced expression of MAP kinase phosphatase-1, but does result in increased p21ras-associated phosphatidylinositol 3-kinase activity. Our data provide evidence that ET-1-induced biphasic p21ras activation causes sequential stimulation of divergent downstream signaling pathways.

Ras is a small GTPase that cycles between an inactive GDP-bound and an active GTP-bound form. A large variety of ligands that stimulate cell surface receptors induce the activation of Ras. Thus far, this activation could only be measured by the increase of GTP bound to Ras, which was precipitated from radio-labelled cell extract. We have used the minimal Ras-binding domain (RBD) of Raf1 (aa 51-131) to identify in vivo activated Ras. This novel method is based on the observation that RBD binds RasGTP in vitro with a Kd of 20 nM whereas the affinity between RBD and RasGDP is three orders of magnitude lower. Here we show that the Gst-RBD fusion protein precipitates transfected RasL61 (RasGTP) but not RasN17 (RasGDP) from cell lysates. In addition, we demonstrate for two different cell lines that the increase in RasGTP is reflected by an increase in Ras bound to Gst-RBD. From these results we conclude that the minimal Ras-binding domain of Raf1 is an excellent activation specific-probe for Ras.