The PDZ protein TIP-1 interacts with the Rho effector rhotekin and is involved in Rho signaling to the serum response element

The human T-cell lymphotrophic virus, type 1 Tax protein can interact via its C terminus with various proteins including a PDZ domain. In this work, one of them, TIP-1, is characterized as a cytoplasmic 14-kDa protein mainly corresponding to one PDZ domain. A two-hybrid screen performed with TIP-1 as bait showed that it interacts with the human homologue of rhotekin that was previously identified in mice as a Rho effector. Both human and mouse rhotekins exhibit at their C termini the sequence QSPV-COOH that matches the X(S/T)XV-COOH consensus known for proteins recognizing PDZ domains. Mutation of the serine and valine residues to alanine impairs interaction of rhotekin with TIP-1. Transient expression experiments with a reporter construct including the c-Fos serum response element (SRE) showed that coexpression of TIP-1 with the constitutively active RhoA.V14 mutant and human rhotekin caused a strong activation of the SRE. A negative mutant of Rho, RhoA.N19, was unable to cooperate with TIP-1 and rhotekin. The positive effect of TIP-1 was also lost when the C terminus of rhotekin was mutated. These data show that the complex of active Rho with its effector rhotekin bound to TIP-1 produces in the cytoplasm a signal that triggers strong activation of the SRE.     

Critical role of the pleckstrin homology domain in Dbs signaling and growth regulation

Dbl family proteins act as guanine nucleotide exchange factors and positive regulators of Rho GTPase function by stimulating formation of the active, GTP-bound state. All Dbl family Rho guanine nucleotide exchange factors possess an invariant tandem domain structure consisting of a Dbl homology (DH) catalytic domain followed by a pleckstrin homology (PH) regulatory domain. We determined previously that the PH domain of Dbs was critical for the intrinsic catalytic activity of the DH domain in vitro and for Dbs transformation in vivo. In this study, we evaluated the role of phosphoinositide binding to the PH domain in regulating the DH domain function of Dbs in vitro and in vivo. We determined that mutation of basic amino acids located within the beta1-beta2 and beta3-beta4 loops of the PH domain resulted in impaired phospholipid binding in vitro, yet full guanine nucleotide exchange activity in vitro was retained for RhoA and Cdc42. Surprisingly, these mutants were compromised in their ability to activate Rho GTPases in vivo and to cause transformation of NIH 3T3 cells. However, Dbs subcellular localization was impaired by these PH domain mutations, supporting a role for phospholipid interactions in facilitating membrane association. Despite the importance of phospholipid binding for Dbs function in vivo, we found that Dbs signaling and transforming activity was not stimulated by phosphatidylinositol 3-kinase activation. We suggest that the PH domain of Dbs facilitates two distinct roles in the regulation of DH domain function, one critical for GTPase association and activation in vitro and one critical for phosphoinositide binding and GTPase interaction in vivo, that together promote Dbs association with membranes.     

3BP-1, an SH3 domain binding protein, has GAP activity for Rac and inhibits growth factor-induced membrane ruffling in fibroblasts.

The SH3 binding protein, 3BP-1, was originally cloned as a partial cDNA from an expression library using the Abl SH3 domain as a probe. In addition to an SH3 binding domain, 3BP-1 displayed homology to a class of GTPase activating proteins (GAPs) active against Rac and Rho proteins. We report here a full length cDNA of 3BP-1 which extends the homology to GAP proteins previously noted. 3BP-1 functions in vitro as a GAP with a specificity for Rac-related G proteins. Microinjection of the 3BP-1 protein into serum-starved fibroblasts produces an inhibition of platelet-derived growth factor (PDGF)-induced membrane ruffling mediated by Rac. Co-injection of 3BP-1 with an activated Rac mutant that is unresponsive to GAPs, counter-acts this inhibition. 3BP-1 does not show in vitro activity towards Rho and, in agreement with this finding, microinjection of 3BP-1 into fibroblasts has no effect on lysophosphatidic acid (LPA)-induced stress fiber assembly mediated by Rho. Thus 3BP-1 is a new and specific Rac GAP that can act in cells to counter Rac-mediated membrane ruffling. How its SH3 binding site interacts with its GAP activity remains to be understood.     

Scaffold proteins dictate Rho GTPase-signaling specificity

Given the numerous mechanisms that regulate the activity of Rho GTPases and the multiple effectors for Rho proteins, how is specificity achieved when transducing signals via Rho GTPase-regulated molecular networks? The finding that the scaffold protein hCNK1 links Rho guanine-nucleotide-exchange factors and Rho to JNK (c-Jun N-terminal kinase), while limiting stress-fiber formation and serum-response-factor activation, suggests that scaffold proteins govern the selection of signal outputs, thus helping to solve the Rho GTPase-signaling puzzle.     

The human orthologue of CdGAP is a phosphoprotein and a GTPase-activating protein for Cdc42 and Rac1 but not RhoA

BACKGROUND INFORMATION: Rho GTPases regulate a wide range of cellular functions affecting both cell proliferation and cytoskeletal dynamics. They cycle between inactive GDP- and active GTP-bound states. This cycle is tightly regulated by GEFs (guanine nucleotide-exchange factors) and GAPs (GTPase-activating proteins). Mouse CdGAP (mCdc42 GTPase-activating protein) has been previously identified and characterized as a specific GAP for Rac1 and Cdc42, but not for RhoA. It consists of an N-terminal RhoGAP domain and a C-terminal proline-rich region. In addition, CdGAP-related genes are present in both vertebrates and invertebrates. We have recently reported that two predominant isoforms of CdGAP (250 and 90 kDa) exist in specific mouse tissues. RESULTS: In the present study, we have identified and characterized human CdGAP (KIAA1204) which shares 76% sequence identity to the long isoform of mCdGAP (mCdGAP-l). Similar to mCdGAP, it is active in vitro and in vivo on both Cdc42 and Rac1, but not RhoA, and is phosphorylated in vivo on serine and threonine residues. In contrast with mCdGAP-l, human CdGAP interacts with ERK1/2 (extracellular-signal-regulated kinase 1/2) through a region that does not involve a DEF (docking site for ERK Phe-Xaa-Phe-Pro) domain. Also, the tissue distribution of CdGAP proteins appears to be different between human and mouse species. Interestingly, we found that CdGAP proteins cause membrane blebbing in COS-7 cells. CONCLUSIONS: Our results suggest that CdGAP properties are well conserved between human and mouse species, and that CdGAP may play an unexpected role in apoptosis.     

Physical and functional interactions of Galphaq with Rho and its exchange factors

Recent reports have shown that several heterotrimeric protein-coupled receptors that signal through Galpha(q) can induce Rho-dependent responses, but the pathways that mediate the interaction between Galpha(q) and Rho have not yet been identified. In this report we present evidence that Galpha(q) expressed in COS-7 cells coprecipitates with the Rho guanine nucleotide exchange factor (GEF) Lbc. Furthermore, Galpha(q) expression enhances Rho-dependent responses. Coexpressed Galpha(q) and Lbc have a synergistic effect on the Rho-dependent rounding of 1321N1 astrocytoma cells. In addition, serum response factor-dependent gene expression, as assessed by the SRE.L reporter gene, is synergistically activated by Galpha(q) and Rho GEFs. The synergistic effect of Galpha(q) on this response is inhibited by C3 exoenzyme and requires phospholipase C activation. Surprisingly, expression of Galpha(q), in contrast to that of Galpha(12) and Galpha(13), does not increase the amount of activated Rho. We also observe that Galpha(q) enhances SRE.L stimulation by activated Rho, indicating that the effect of Galpha(q) occurs downstream of Rho activation. Thus, Galpha(q) interacts physically and/or functionally with Rho GEFs; however this does not appear to lead to or result from increased activation of Rho. We suggest that Galpha(q)-generated signals enhance responses downstream of Rho activation.     

Rich, a rho GTPase-activating protein domain-containing protein involved in signaling by Cdc42 and Rac1.

A previously unidentified Rho GTPase-activating protein (GAP) domain-containing protein was found in a yeast two-hybrid screen for cDNAs encoding proteins binding to the Src homology 3 domain of Cdc42-interacting protein 4 (CIP4). The protein was named RICH-1 (RhoGAP interacting with CIP4 homologues), and, in addition to the RhoGAP domain, it contained an N-terminal domain with endophilin homology and a C-terminal proline-rich domain. Transient transfections of RICH-1 indicated that it bound to CIP4 in vivo, as shown by co-immunoprecipitation experiments, as well as co-localization assays. In vitro assays demonstrated that the RhoGAP domain of RICH-1 catalyzed GTP hydrolysis on Cdc42 and Rac1, but not on RhoA. Ectopic expression of the RhoGAP domain as well as the full-length protein interfered with platelet-derived growth factor BB-induced membrane ruffling, but not with serum-induced stress fiber formation, further emphasizing the notion that, in vivo, RICH-1 is a GAP for Cdc42 and Rac1.     

The pleckstrin homology domain of phospholipase C-beta2 as an effector site for Rac

Increasing evidence links the activation of Rho family GTPases to the stimulation of lipid hydrolysis catalyzed by phospholipase C (PLC)-beta isozymes. To better define this relationship, members of a library of recombinant Rho GTPases were screened for their capacity to directly engage various purified PLC-beta isozymes. Of the 17 tested members of the Rho family, only the active isoforms of Rac (Rac1, Rac2, and Rac3) both stimulate PLC-beta activity in vivo and bind PLC-beta2 and PLC-beta3, but not PLC-beta1, in vitro. Furthermore, the recognition site for Rac GTPases was localized to the pleckstrin homology (PH) domain of PLC-beta2, and this PH domain is fully sufficient to selectively interact with the active versions of the Rac GTPases, but not with other similar Rho GTPases. Together, these findings present a quantitative evaluation of the direct interactions between Rac GTPases and PLC-beta isozymes and define a novel role for the PH domain of PLC-beta2 as a putative effector site for Rac GTPases.     

The Insert Region of RhoA Is Essential for Rho Kinase Activation and Cellular Transformation

RhoA is involved in multiple cellular processes, including cytoskeletal organization, gene expression, and transformation. These processes are mediated by a variety of downstream effector proteins. However, which effectors are involved in cellular transformation and how these proteins are activated following interaction with Rho remains to be established. A unique feature that distinguishes the Rho family from other Ras-related GTPases is the insert region, which may confer Rho-specific signaling events. Here we report that deletion of the insert region does not result in impaired effector binding. Instead, this insert deletion mutant (RhoDeltaRas, in which the insert helix has been replaced with loop 8 of Ras) acted in a dominant inhibitory fashion to block RhoA-induced transformation. Since RhoDeltaRas failed to promote stress fiber formation, we examined the ability of this mutant to bind to and subsequently activate Rho kinase. Surprisingly, RhoDeltaRas-GTP coprecipitated with Rho kinase but failed to activate it in vivo. These data suggested that the insert domain is not required for Rho kinase binding but plays a role in its activation. The constitutively active catalytic domain of Rho kinase did not promote focus formation alone or in the presence of Raf(340D) but cooperated with RhoDeltaRas to induce cellular transformation. This suggests that Rho kinase needs to cooperate with additional Rho effectors to promote transformation. Further, the Rho kinase catalytic domain reversed the inhibitory effect of RhoDeltaRas on Rho-induced transformation, suggesting that one of the downstream targets of Rho-induced transformation abrogated by RhoDeltaRas is indeed Rho kinase. In conclusion, we have demonstrated that the insert region of RhoA is required for Rho kinase activation but not for binding and that this kinase activity is required to induce morphologic transformation of NIH 3T3 cells.     

Phosphorylation-dependent formation of a quaternary complex at the c-fos SRE.

The rapid and transient induction of the human proto-oncogene c-fos in response to a variety of stimuli depends on the serum responses element (SRE). In vivo footprinting experiments show that this promoter element is bound by a multicomponent complex including the serum response factor (SRF) and a ternary complex factor such as Elk-1. SRF is thought to recruit a ternary complex factor monomer into an asymmetric complex. In this report, we describe a quaternary complex over the SRE which, in addition to an SRF dimer, contains two Elk-1 molecules. Its formation at the SRE is strictly dependent on phosphorylation of S-383 in the Elk-1 regulatory domain and appears to involve a weak intermolecular association between the two Elk-1 molecules. The influence of mutations in Elk-1 on quaternary complex formation in vitro correlates with their effect on the induction of c-fos reporter expression in response to mitogenic stimuli in vivo.     

Functional analysis of RhoGDI inhibitory activity on vacuole membrane fusion

RhoGDIs (Rho GDP-dissociation inhibitors) are the natural inhibitors of Rho GTPases. They interfere with Rho protein function by either blocking upstream activation or association with downstream signalling molecules. RhoGDIs can also extract membrane-bound Rho GTPases to form soluble cytosolic complexes. We have shown previously that purified yeast RhoGDI Rdi1p, can inhibit vacuole membrane fusion in vitro. In the present paper we functionally dissect Rdi1p to discover its mode of regulating membrane fusion. Overexpression of Rdi1p in vivo profoundly affected cell morphology including increased actin patches in mother cells indicative of polarity defects, delayed ALP (alkaline phosphatase) sorting and the presence of highly fragmented vacuoles indicative of membrane fusion defects. These defects were not caused by the loss of typical transport and fusion proteins, but rather were linked to the reduction of membrane localization and activation of Cdc42p and Rho1p. Subcellular fractionation showed that Rdi1p is predominantly a cytosolic monomer, free of bound Rho GTPases. Overexpression of endogenous Rdi1p, or the addition of exogenous Rdi1p, generated stable cytosolic complexes. Rdi1p structure-function analysis showed that membrane association via the C-terminal β-sheet domain was required for the functional inhibition of membrane fusion. Furthermore, Rdi1p inhibited membrane fusion through the binding of Rho GTPases independent from its extraction activity.     

Evidence for the genetic interaction between the actin-binding protein Vrp1 and the RhoGAP Rgd1 mediated through Rho3p and Rho4p in Saccharomyces cerevisiae

The non-essential RGD1 gene from Saccharomyces cerevisiae encodes a protein that has been characterized in vitro as a Rho GTPase activating protein (RhoGAP) for the Rho3 and Rho4 proteins. Rgd1p, which displays a conserved FCH-coiled coil-Rho-GAP domain organization, showed a patch-like distribution in the cell, including a localization in growing buds. Using a genetic screen, we found that rgd1delta and vrp1alpha mutations exhibited a synthetic lethality, thus revealing an interaction between these genes. The VRP1 product is an actin and myosin interacting protein involved in polarized growth. Using mutant forms of both Rho3 and Rho4 proteins, we provide evidence for the involvement of these two GTPases in RGD1-VRP1 co-lethality. In addition, these results strongly argue in favour of Rho3p and Rho4p being the targets of Rgd1p RhoGAP activity in vivo. Genetic relationships between either VRP1 or RGD1 and actin cytoskeleton-linked genes were also studied. These and other well-established data support the idea that Vrp1, Las17, Rvs167 proteins belong to the same complex. This protein structure might act with myosins in various actin cytoskeleton-based activities, in co-operation with a Rho3p/Rho4p signalling pathway that is negatively regulated by Rgd1p GAP activity.     

Members of a novel class of Arabidopsis Rho guanine nucleotide exchange factors control Rho GTPase-dependent polar growth

Rho family small GTPases are signaling switches controlling many eukaryotic cellular processes. Conversion from the GDP- to GTP-bound form is catalyzed by guanine nucleotide exchange factors (GEFs). Rho GEFs in animals fall into two structurally distinct classes containing DH and DOCKER catalytic domains. Using a plant Rho GTPase (ROP1) as bait in yeast two-hybrid screens, we identified a family of Rho GEFs, named RopGEFs. The Arabidopsis thaliana RopGEF family of 14 members contains a conserved central domain, the domain of unknown function 315 (DUF315), and variable N- and C-terminal regions. In vitro GEF assays show that DUF315 but not the full-length version of RopGEF1 has high GEF activity toward ROP1. Our data suggest that the variable regions of RopGEF1 are involved in regulation of RopGEF through an autoinhibitory mechanism. RopGEF1 overexpression in pollen tubes produced growth depolarization, as does a constitutively active ROP1 mutant. The RopGEF1 overexpression phenotype was suppressed by expression of a dominant-negative mutant of ROP1, probably by trapping RopGEF1. Deletion mutant analysis suggested a requirement of RopGEF activity for the function of RopGEF1 in polar growth. Green fluorescent protein-tagged RopGEF1 was localized to the tip of pollen tubes where ROP1 is activated. These results provide strong evidence that RopGEF1 activates ROP1 in control of polar growth in pollen tubes.     

Arabidopsis RopGAPs are a novel family of rho GTPase-activating proteins that require the Cdc42/Rac-interactive binding motif for rop-specific GTPase stimulation

The plant-specific Rop subfamily of Rho GTPases, most closely related to the mammalian Cdc42 and Rac GTPases, plays an important role in the regulation of calcium-dependent pollen tube growth, H(2)O(2)-mediated cell death, and many other processes in plants. In a search for Rop interactors using the two-hybrid method, we identified a family of Rho GTPase-activating proteins (GAP) from Arabidopsis, termed RopGAPs. In addition to a GAP catalytic domain, RopGAPs contain a Cdc42/Rac-interactive binding (CRIB) motif known to allow Cdc42/Rac effector proteins to bind activated Cdc42/Rac. This novel combination of a GAP domain with a CRIB motif is widespread in higher plants and is unique to the regulation of the Rop GTPase. A critical role for CRIB in the regulation of in vitro RopGAP activity was demonstrated using point and deletion mutations. Both types of mutants have drastically reduced capacities to stimulate the intrinsic Rop GTPase activity and to bind Rop. Furthermore, RopGAPs preferentially stimulate the GTPase activity of Rop, but not Cdc42 in a CRIB-dependent manner. In vitro binding assays show that the RopGAP CRIB domain interacts with GTP- and GDP-bound forms of Rop, as well as the transitional state of Rop mimicked by aluminum fluoride. The CRIB domain also promotes the association of the GAP domain with the GDP-bound Rop, as does aluminum fluoride. These results reveal a novel CRIB-dependent mechanism for the regulation of the plant-specific family of Rho GAPs. We propose that the CRIB domain facilitates the formation of or enhanced GAP-mediated stabilization of the transitional state of the Rop GTPase.     

PKNbeta interacts with the SH3 domains of Graf and a novel Graf related protein, Graf2, which are GTPase activating proteins for Rho family.

PKNbeta is a novel isoform of PKNalpha, which is one of the target protein kinases for the small GTPase Rho. By yeast two-hybrid screening of a human embryonic kidney 293 cell cDNA library with the PKNbeta linker region containing proline-rich motifs as a bait, clones encoding Graf (GAP for Rho Associated with Focal adhesion kinase) and a novel Graf-related protein, termed Graf2, were isolated. The full length of Graf2 contains a putative PH domain, a RhoGAP domain, and an SH3 domain as well as Graf. Northern and Western blot analyses demonstrated that Graf2 is expressed in several tissues, with the highest expression in skeletal muscle. Recombinant Graf2 exhibited GTPase-activating activity toward the small GTPase RhoA and Cdc42Hs, but not toward Rac1, in vitro. The SH3 domains of Graf and Graf2 purified from Escherichia coli bound directly to PKNbeta. Graf or Graf2 was co-immunoprecipitated with PKNbeta in COS-7 cells transiently transfected with Graf or Graf2 and PKNbeta expression constructs. The catalytically active form of PKNbeta phosphorylated Graf and Graf2 in vitro. The interplay of PKNbeta and the GTPase-activating proteins, Graf and Graf2, may offer a novel mechanism regulating the Rho-mediated signaling.