IkappaBalpha and IkappaBalpha /NF-kappa B complexes are retained in the cytoplasm through interaction with a novel partner, RasGAP SH3-binding protein 2.

IkappaBalpha inhibits the transcriptional activity of NF-kappaB both in the cytoplasm by preventing the nuclear translocation of NF-kappaB and in the nucleus where it dissociates NF-kappaB from DNA and transports it back to the cytoplasm. Cytoplasmic localization of inactive NF-kappaB/IkappaBalpha complexes is controlled by mutual masking of nuclear import sequences of NF-kappaB p65 and IkappaBalpha and active CRM1-mediated nuclear export. Here, we describe an additional mechanism accounting for the cytoplasmic anchoring of IkappaBalpha or NF-kappaB/IkappaBalpha complexes.

A novel phosphorylation-dependent RNase activity of GAP-SH3 binding protein: a potential link between signal transduction and RNA stability.

A potential p120 GTPase-activating protein (RasGAP) effector, G3BP (RasGAP Src homology 3 [SH3] binding protein), was previously identified based on its ability to bind the SH3 domain of RasGAP. Here we show that G3BP colocalizes and physically interacts with RasGAP at the plasma membrane of serum-stimulated but not quiescent Chinese hamster lung fibroblasts. In quiescent cells, G3BP was hyperphosphorylated on serine residues, and this modification was essential for its activity.

Sam68 is a Ras-GAP-associated protein in mitosis.

Sam68 is the major tyrosine-phosphorylated and Src-associated protein in mitotic cells. Sam68 stimulates G1/S transition and this effect is dependent on the integrity of its KH domain (hnRNPK Homology) which confers nucleic acid binding properties. During mitosis, Sam68 undergoes tyrosine phosphorylation, which negatively regulates its nucleic acid binding properties and mediates the interaction of Sam68 with critical SH2-containing signaling proteins such as Grb2, PLC gamma 1 and Ras-GAP.

A role for Sam68 in cell cycle progression antagonized by a spliced variant within the KH domain.

Sam68 is the main tyrosine-phosphorylated and Src-associated protein in mitotic cells. Sam68 exhibits a conserved functional KH (hnRNPK homology) RNA binding domain and binds single strand nucleic acids. Tyrosine phosphorylation mediates the interaction of Sam68 with many SH3- and SH2-containing proteins and negatively regulates its nucleic acid binding properties.

TGF-beta 1 and cAMP attenuate cyclin A gene transcription via a cAMP responsive element through independent pathways.

Transforming growth factor beta (TGF-beta) is a potent inhibitor of the proliferation of many cell lines. The expression of Cyclin A is down-regulated by TGF-beta 1 in Chinese hamster lung fibroblasts and most of this effect is mediated at the transcriptional level through a cAMP-responsive element (CRE), but does not require a functional cAMP-dependent protein kinase. However, activation of the cAMP pathway in these cells gives rise to a strong inhibition of proliferation, paralleled by a down-regulation of Cyclin A promoter activity.

Nuclear import of serum response factor (SRF) requires a short amino-terminal nuclear localization sequence and is independent of the casein kinase II phosphorylation site.

Serum stimulation of resting cells is mediated at least in part at the transcriptional level by the activation of numerous genes among which c-fos constitutes a model. Serum response factor (SRF) forms a ternary complex at the c-fos serum response element (SRE) with an accessory protein p62TCF/Elk-1. Both proteins are the targets of multiple phosphorylation events and their role is still unknown in the amino terminus of SRF.

Saccharomyces cerevisiae CDC25 (1028-1589) is a guanine nucleotide releasing factor for mammalian ras proteins and is oncogenic in NIH3T3 cells.

The best characterized yeast guanine nucleotide releasing factor is CDC25, which acts on RAS and thereby stimulates cAMP production in Saccharomyces cerevisiae. In order to determine if CDC25 could be a specific GDP-GTP releasing factor for the mammalian proteins Ha-ras, Ki-ras, and N-ras, its functions were studied both in vitro and in NIH3T3 cells. The 561 amino acid composing the C-terminal domain of CDC25 (CDC25 C-domain) released guanine nucleotides (both GDP and GTP) from Ha-, Ki-, and N-ras but not from Rap1A, Rab5, and Rab11.

Loss of the G1-S control of cyclin A expression during tumoral progression of Chinese hamster lung fibroblasts.

The expression of cyclin A, one of the key regulators of cell cycle progression in association with cdc2/cdk2 protein kinases and which undergoes cyclic accumulation during the cell cycle, has been investigated in CCL39 Chinese hamster lung fibroblasts and in two transformed variants, A71 and 39Py. Whereas A71 (selected after tumor induction in nude mice) is subject to growth arrest (less than 5% of labeled nuclei after 24 h of serum starvation), 39Py (obtained after transformation by polyoma virus) is not (more than 50% of labeled nuclei). In both cells, cyclin A expression was correlated with establishment of S phase, with a progressive deregulation of its G1 controls.

The Saccharomyces cerevisiae gene product SDC25 C-domain functions as an oncoprotein in NIH3T3 cells.

Ras proteins in mammalian cells cycle between a GTP-bound 'on' state and a GDP-bound 'off' state. Activation of Ras p21 results from the dissociation of tightly bound GDP and the exchange of bound GDP for GTP. A guanine nucleotide exchange factor is required for this activation.

Implication of GAP in Ras-dependent transactivation of a polyoma enhancer sequence.

Controversy exists as to whether the interaction of a guanosine triphosphatase activating protein (GAP) with Ras proteins functions both to initiate and to terminate Ras-dependent signaling events or only to terminate them. GAP-C, a carboxyl-terminal fragment of GAP that is sufficient to stimulate GTPase activity, inhibited the stimulation of transcription produced by some oncoproteins (v-Src, polyoma middle T, wild-type Ras, and oncogenic Ras) but not that produced by v-Mos. Wild-type GAP did not affect transcription induced by oncogenic Ras but reversed the inhibitory effect of GAP-C on transcription induced by oncogenic Ras.

The COOH-domain of the product of the Saccharomyces cerevisiae SCD25 gene elicits activation of p21-ras proteins in mammalian cells.

The evidence presented here indicates that the domain containing the COOH-terminal part of the Saccharomyces cerevisiae SCD25 gene product (C-domain), which is homologous to the COOH-terminal part of CDC25 protein, can elicit activation of mammalian ras proteins in CHO cells. Transfection of expression vectors carrying the C-domain of SCD25, but not of CDC25, promotes the GTP-bound form of ras proteins as determined by analysis of the guanine nucleotides bound to ras proteins immunoprecipitated by Y13-259 mAb, and enhances transcription of a HIV-LTR-CAT construct. This is the first demonstration of the activation of ras proteins by transfection of a single heterologous gene.