Antibodies to synthetic peptide from the residue 33 to 42 domain of c-Ha-ras p21 block reconstitution of the protein with different effectors

Response to Bile Salts in Clinical Strains of Acinetobacter baumannii Lacking the AdeABC Efflux Pump: Virulence Associated with Quorum Sensing

Introduction:Acinetobacter baumannii is an opportunistic nosocomial pathogen associated with multiple infections. This pathogen usually colonizes (first stage of microbial infection) host tissues that are in contact with the external environment. As one of the sites of entry in human hosts is the gastrointestinal tract, the pathogen must be capable of tolerating bile salts. However, studies analyzing the molecular characteristics involved in the response to bile salts in clinical strains of A. baumannii are scarce. Material and Methods: Microbiological and transcriptional studies (arrays and RT-PCR) in the response to bile salts were carried out in isogenic (A. baumanni ΔadeB ATCC 17978 and A. baumannii ΔadeL ATCC 17978) and clinical strains from clone ST79/PFGE-HUI-1 which is characterized by lacking the AdeABC efflux pump and by overexpression the AdeFGH efflux pump. Results and Discussion: In presence of bile salts, in addition to the glutamate/aspartate transporter were found overexpressed in A. baumannii ΔadeB ATCC 17978, the virulence factors (surface motility, biofilm, and Type VI Secretion System) which are associated with activation of the Quorum Sensing system. Overexpression of these factors was confirmed in clinical strains of clone ST79/PFGE-HUI-1. Conclusions: This the first study about the adaptive response to bile salts investigating the molecular and microbiological characteristics in response to bile salts of an isogenic model of A. baumannii ATCC 17978 and clinical isolates of A. baumannii (clinical strains of ST79/PFGE-HUI-1) lacking the main RND efflux pump (AdeABC). Clinical isolates of A. baumannii lacking the AdeABC efflux pump (clone ST79/PFGE-HUI-1) displayed a new clinical profile (increased invasiveness) possibly associated with the response to stress conditions (such as the presence of bile salts).

Ras-GAP controls Rho-mediated cytoskeletal reorganization through its SH3 domain

Proteins of the Ras superfamily, Ras, Rac, Rho, and Cdc42, control the remodelling of the cortical actin cytoskeleton following growth factor stimulation. A major regulator of Ras, Ras-GAP, contains several structural motifs, including an SH3 domain and two SH2 domains, and there is evidence that they harbor a signalling function. We have previously described a monoclonal antibody to the SH3 domain of Ras-GAP which blocks Ras signalling in Xenopus oocytes. We now show that microinjection of this antibody into Swiss 3T3 cells prevents the formation of actin stress fibers stimulated by growth factors or activated Ras, but not membrane ruffling. This inhibition is bypassed by coinjection of activated Rho, suggesting that the Ras-GAP SH3 domain is necessary for endogenous Rho activation. In agreement, the antibody blocks lysophosphatidic acid-induced neurite retraction in differentiated PC12 cells. Furthermore, we demonstrate that microinjection of full-length Ras-GAP triggers stress fiber polymerization in fibroblasts in an SH3-dependent manner, strongly suggesting an effector function besides its role as a Ras downregulator. These results support the idea that Ras-GAP connects the Ras and Rho pathways and, therefore, regulates the actin cytoskeleton through a mechanism which probably does not involve p190 Rho-GAP.

The Ras-GTPase-activating protein SH3 domain is required for Cdc2 activation and mos induction by oncogenic Ras in Xenopus oocytes independently of mitogen-activated protein kinase activation

The Ras-GTPase-activating protein (RasGAP) is an important modulator of p21ras - dependent signal transduction in Xenopus oocytes and in mammalian cells. We investigated the role of the RasGAP SH3 domain in signal transduction with a monoclonal antibody against the SH3 domain of RasGaP. This antibody prevented the activation of the maturation-promoting factor complex (cyclin B-p34cdc2) by oncogenic Ras. The antibody appears to be specific because as little as 5 ng injected per oocyte reduced the level of Cdc2 activation by 50% whereas 100 ng of nonspecific immunoglobulin G did not affect Cdc2 activation. The antibody blocked the Cdc2 activation induced by oncogenic Ras but not that induced by progesterone, which acts independently of Ras. A peptide corresponding to positions 317 to 326 of a sequence in the SH3 domain of human RasGAP blocked Cdc2 activation, whereas a peptide corresponding to positions 273 to 305 of a sequence in the N-terminal moiety of the SH3 domain of RasGAP had no effect. The antibody did not block the mitogen-activated protein (MAP) kinase cascade (activation of MAPK/ERK kinase [MEK], MAP kinase, and S6 kinase p90rsk). Surprisingly, injection of the negative MAP kinase mutant protein ERK2 K52R (containing a K-to-R mutation at position 52) blocked the Cdc2 activation induced by oncogenic Ras as well as blocking the activation of MAP kinase. Thus, MAP kinase is also implicated in the regulation of Cdc2 activity. In this study, we further investigated the regulation of the synthesis of the c-mos oncogene product, which is necessary for the activation of Cdc2. We report that the synthesis of the c-mos oncogene product, which is necessary for the activation antibody to the SH3 domain of RasGAP and by injecting the negative MAP kinase mutant protein ERK2 K52R. These results suggest that oncogenic Ras activates two signaling mechanisms: the MAP kinase cascade and a signaling pathway implicating the SH3 domain of RasGAP. These mechanisms might control Mos protein expression implicated in Cdc2 activation.

Complex formation between RAS and RAF and other protein kinases

We used a Saccharomyces cerevisiae genetic system to detect the physical interaction of RAS and RAF oncoproteins. We also observed interaction between RAS and byr2, a protein kinase implicated as a mediator of the Schizosaccharomyces pombe ras1 protein. Interaction with RAS required only the N-terminal domains of RAF or byr2 and was disrupted by mutations in either the guanine nucleotide-binding or effector-loop domains of RAS. We observed interaction between MEK (a kinase that phosphorylates mitogen-activated protein kinases) and the catalytic domain of RAF. RAS and MEK also interacted but only when RAF was overexpressed.

The Saccharomyces cerevisiae SDC25 C-domain gene product overcomes the dominant inhibitory activity of Ha-Ras Asn-17

The carboxy-terminal part of the Saccharomyces cerevisiae SDC25 gene product (SDC25 C domain) can elicit activation of mammalian Ras proteins. Specifically, SDC25 C domain functions as an exchange factor for cellular Ras proteins in CHO cells. In this study, we used the dominant inhibitory Ha-Ras Asn-17 mutant and SDC25 C domain to further investigate the interaction between cellular Ras proteins and their putative endogenous guanine nucleotide-releasing factors. Transcription from the polyomavirus thymidine kinase gene (Py tk) promoter is strongly inhibited by the expression of Ha-Ras Asn-17 in NIH 3T3 cells. Coexpression of SDC25 C domain overcomes the negative effect of the Ras mutant on the Py tk promoter. On the other hand, transactivation of the Ras-responsive element of the Py tk promoter induced by SDC25 C domain is lost upon coexpression of increasing amounts of Ha-Ras Asn-17. In addition, coexpression of SDC25 C domain overcomes the inhibition of proliferation of NIH 3T3 cells caused by Ha-Ras Asn-17. These results are consistent with the idea that the Ha-Ras Asn-17 mutant functions by titrating an upstream activator of cellular Ras proteins.

The Saccharomyces cerevisiae SDC25 C-domain gene product overcomes the dominant inhibitory activity of Ha-Ras Asn-17

The carboxy-terminal part of the Saccharomyces cerevisiae SDC25 gene product (SDC25 C domain) can elicit activation of mammalian Ras proteins. Specifically, SDC25 C domain functions as an exchange factor for cellular Ras proteins in CHO cells. In this study, we used the dominant inhibitory Ha-Ras Asn-17 mutant and SDC25 C domain to further investigate the interaction between cellular Ras proteins and their putative endogenous guanine nucleotide-releasing factors. Transcription from the polyomavirus thymidine kinase gene (Py tk) promoter is strongly inhibited by the expression of Ha-Ras Asn-17 in NIH 3T3 cells. Coexpression of SDC25 C domain overcomes the negative effect of the Ras mutant on the Py tk promoter. On the other hand, transactivation of the Ras-responsive element of the Py tk promoter induced by SDC25 C domain is lost upon coexpression of increasing amounts of Ha-Ras Asn-17. In addition, coexpression of SDC25 C domain overcomes the inhibition of proliferation of NIH 3T3 cells caused by Ha-Ras Asn-17. These results are consistent with the idea that the Ha-Ras Asn-17 mutant functions by titrating an upstream activator of cellular Ras proteins.