Choose your own path: specificity in Ras GTPase signaling.
ABSTRACT The Ras superfamily of small G proteins contributes importantly to numerous cellular and physiological processes (M. F. Olsen and R. Marais, Semin. Immunol., 2000, 12, 63). This family comprises a large class of proteins (more than 150) which all share a common enzymatic function: hydrolysis of the gamma-phosphate of guanosine triphosphate (GTP) to create the products guanosine diphosphate (GDP) and inorganic phosphate (Y. Takai, T. Sasaki and T. Matozaki, Physiol. Rev., 2001, 81, 153). For this reason Ras family proteins, which include the Ras, Rho, Arf/Sara, Ran and Rab subfamilies, are classified as GTPases (G. W. Reuther and C. J. Der, Curr. Opin. Cell Biol., 2000, 12, 157). Guanine nucleotide coupling is a key regulator of enzymatic function; thus, Ras family GTPases participate in signal transduction. Ras signaling depends on binding to effectors. Many of the known effectors can bind to multiple Ras isotypes, often leading to common cellular outcomes, but each Ras isotype also engages specific effector pathways to mediate unique functions. Further, each Ras isotype can propagate multiple signaling pathways, indicating the presence of cellular determinants which allow for promiscuity in Ras-effector interactions while also maintaining specificity. Small distinctions in sequence, structure, and/or cellular regulation contribute to these differences in Ras-effector binding and subsequent cellular effects. A major focus of investigation in the Ras signaling field is identifying the determinants of these individualized functions. This review will attempt to summarize the current state of understanding of this question (with a particular focus on the Ras subfamily) and the approaches being taken to address it, and will discuss prospective areas for future investigation.
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ABSTRACT: Regulator of G-protein signaling (RGS) proteins have been well-described as accelerators of Galpha-mediated GTP hydrolysis ("GTPase-accelerating proteins" or GAPs). However, RGS proteins with complex domain architectures are now known to regulate much more than Galpha GTPase activity. RGS14 contains tandem Ras-binding domains that have been reported to bind to Rap- but not Ras GTPases in vitro, leading to the suggestion that RGS14 is a Rap-specific effector. However, more recent data from mammals and Drosophila imply that, in vivo, RGS14 may instead be an effector of Ras. Full-length and truncated forms of purified RGS14 protein were found to bind indiscriminately in vitro to both Rap- and Ras-family GTPases, consistent with prior literature reports. In stark contrast, however, we found that in a cellular context RGS14 selectively binds to activated H-Ras and not to Rap isoforms. Co-transfection / co-immunoprecipitation experiments demonstrated the ability of full-length RGS14 to assemble a multiprotein complex with components of the ERK MAPK pathway in a manner dependent on activated H-Ras. Small interfering RNA-mediated knockdown of RGS14 inhibited both nerve growth factor- and basic fibrobast growth factor-mediated neuronal differentiation of PC12 cells, a process which is known to be dependent on Ras-ERK signaling. In cells, RGS14 facilitates the formation of a selective Ras.GTP-Raf-MEK-ERK multiprotein complex to promote sustained ERK activation and regulate H-Ras-dependent neuritogenesis. This cellular function for RGS14 is similar but distinct from that recently described for its closely-related paralogue, RGS12, which shares the tandem Ras-binding domain architecture with RGS14.PLoS ONE 02/2009; 4(3):e4884. · 4.09 Impact Factor
Article: RTKN2 Induces NF-KappaB Dependent Resistance to Intrinsic Apoptosis in HEK Cells and Regulates BCL-2 Genes in Human CD4+ Lymphocytes[show abstract] [hide abstract]
ABSTRACT: The gene for Rhotekin 2 (RTKN2) was originally identified in a promyelocytic cell line resistant to oxysterol-induced apoptosis. It is differentially expressed in freshly isolated CD4+ T-cells compared with other hematopoietic cells and is down-regulated following activation of the T-cell receptor. However, very little is known about the function of RTKN2 other than its homology to Rho-GTPase effector, rhotekin, and the possibility that they may have similar roles. Here we show that stable expression of RTKN2 in HEK cells enhanced survival in response to intrinsic apoptotic agents; 25-hydroxy cholesterol and camptothecin, but not the extrinsic agent, TNFα. Inhibitors of NF-KappaB, but not MAPK, reversed the resistance and mitochondrial pro-apoptotic genes, Bax and Bim, were down regulated. In these cells, there was no evidence of RTKN2 binding to the GTPases, RhoA or Rac2. Consistent with the role of RTKN2 in HEK over-expressing cells, suppression of RTKN2 in primary human CD4+ T-cells reduced viability and increased sensitivity to 25-OHC. The expression of the pro-apoptotic genes, Bax and Bim were increased while BCL-2 was decreased. In both cell models RTKN2 played a role in the process of intrinsic apoptosis and this was dependent on either NF-KappaB signaling or expression of downstream BCL-2 genes. As RTKN2 is a highly expressed in CD4+ T-cells it may play a role as a key signaling switch for regulation of genes involved in T-cell survival.Journal of Cell Death. 01/2009;