Cloning of a SH3 domain-containing proline-rich protein, p85SPR, and its localization in focal adhesion

3D matrix adhesion feedback controls nuclear force coupling to drive invasive cell migration

Cell invasion is a multi-step process, initiated by the acquisition of a migratory phenotype and the ability to move through complex 3D extracellular environments. We determine the composition of cell-matrix adhesion complexes of invasive breast cancer cells in 3D matrices and identify an interaction complex required for invasive migration. βPix and myosin18A (Myo18A) drive polarized recruitment of non-muscle myosin 2A (NM2A) to adhesion complexes at the tips of protrusions. Actomyosin force engagement then displaces the Git1-βPix complex from paxillin, establishing a feedback loop for adhesion maturation. We observe active force transmission to the nucleus during invasive migration that is needed to pull the nucleus forward. The recruitment of NM2A to adhesions creates a non-muscle myosin isoform gradient, which extends from the protrusion to the nucleus. We postulate that this gradient facilitates coupling of cell-matrix interactions at the protrusive cell front with nuclear movement, enabling effective invasive migration and front-rear cell polarity.

βPix-d promotes tubulin acetylation and neurite outgrowth through a PAK/Stathmin1 signaling pathway

Microtubules are a major cytoskeletal component of neurites, and the regulation of microtubule stability is essential for neurite morphogenesis. βPix (ARHGEF7) is a guanine nucleotide exchange factor for the small GTPases Rac1 and Cdc42, which modulate the organization of actin filaments and microtubules. βPix is expressed as alternatively spliced variants, including the ubiquitous isoform βPix-a and the neuronal isoforms βPix-b and βPix-d, but the function of the neuronal isoforms remains unclear. Here, we reveal the novel role of βPix neuronal isoforms in regulating tubulin acetylation and neurite outgrowth. At DIV4, hippocampal neurons cultured from βPix neuronal isoform knockout (βPix-NIKO) mice exhibit defects in neurite morphology and tubulin acetylation, a type of tubulin modification which often labels stable microtubules. Treating βPix-NIKO neurons with paclitaxel, which stabilizes the microtubules, or reintroducing either neuronal βPix isoform to the KO neurons overcomes the impairment in neurite morphology and tubulin acetylation, suggesting that neuronal βPix isoforms may promote microtubule stabilization during neurite development. βPix-NIKO neurons also exhibit lower phosphorylation levels for Stathmin1, a microtubule-destabilizing protein, at Ser16. Expressing either βPix neuronal isoform in the βPix-NIKO neurons restores Stathmin1 phosphorylation levels, with βPix-d having a greater effect than βPix-b. Furthermore, we find that the recovery of neurite length and Stathmin1 phosphorylation via βPix-d expression requires PAK kinase activity. Taken together, our study demonstrates that βPix-d regulates the phosphorylation of Stathmin1 in a PAK-dependent manner and that neuronal βPix isoforms promote tubulin acetylation and neurite morphogenesis during neuronal development.

Loss of ϐPix Causes Defects in Early Embryonic Development, and Cell Spreading and PlateletDerived Growth Factor-Induced Chemotaxis in Mouse Embryonic Fibroblasts

βPix is a guanine nucleotide exchange factor for the Rho family small GTPases, Rac1 and Cdc42. It is known to regulate focal adhesion dynamics and cell migration. However, the in vivo role of βPix is currently not well understood. Here, we report the production and characterization of βPix-KO mice. Loss of βPix results in embryonic lethality accompanied by abnormal developmental features, such as incomplete neural tube closure, impaired axial rotation, and failure of allantoischorion fusion. We also generated βPix-KO mouse embryonic fibroblasts (MEFs) to examine βPix function in mouse fibroblasts. βPix-KO MEFs exhibit decreased Rac1 activity, and defects in cell spreading and platelet-derived growth factor (PDGF)-induced ruffle formation and chemotaxis. The average size of focal adhesions is increased in βPix-KO MEFs. Interestingly, βPix-KO MEFs showed increased motility in random migration and rapid wound healing with elevated levels of MLC2 phosphorylation. Taken together, our data demonstrate that βPix plays essential roles in early embryonic development, cell spreading, and cell migration in fibroblasts.

βPix heterozygous mice have defects in neuronal morphology and social interaction

βPix activates Rho family small GTPases, Rac1 and Cdc42 as a guanine nucleotide exchange factor. Although overexpression of βPix in cultured neurons indicates that βPix is involved in spine morphogenesis and synapse formation in vitro, the in vivo role of βPix in the neuron is not well understood. Recently, we generated βPix knockout mice that showed lethality at embryonic day 9.5. Here, we investigate the neuronal role of βPix using βPix heterozygous mice that are viable and fertile. βPix heterozygous mice show decreased expression levels of βPix proteins in various tissues including the brain. Cultured hippocampal neurons from βPix heterozygous mice show a decrease in neurite length and complexity as well as synaptic density. Both excitatory and inhibitory synapse densities are decreased in these neurons. Golgi-staining of hippocampal tissues from the brain of these mice show reduced dendritic complexity and spine density in the hippocampal neurons. Expression levels of NMDA- and AMPA-receptor subunits and Git1 protein in hippocampal tissues are also decreased in these mice. Behaviorally, βPix heterozygous mice exhibit impaired social interaction. Altogether, these results indicate that βPix is required for neurite morphogenesis and synapse formation, and the reduced expression of βPix proteins results in a defect in social behavior.

Src-mediated phosphorylation of βPix-b regulates dendritic spine morphogenesis

PAK-interacting guanine nucleotide exchange factor (βPix) has been implicated in many actin-based cellular processes including spine morphogenesis in neurons. However, the molecular mechanisms by which βPix controls spine morphology remain elusive. Previously, we have reported the expression of several alternative spliced βPix isoforms in the brain. Here, we report a novel finding that the b isoform of βPix (βPix-b) mediates regulation of spine and synapse formation. We found that βPix-b, which is mainly expressed in neurons, enhances spine and synapse formation through preferential localization at spines. In neurons, glutamate treatment efficiently stimulates Rac1 GEF activity of βPix-b. The glutamate stimulation also promotes Src kinase-mediated phosphorylation of βPix-b in both AMPA receptor- and NMDA receptor-dependent manner. Tyrosine 598 (Y598) of βPix-b is identified as the major Src-mediated phosphorylation site. Finally, Y598 phosphorylation of βPix-b enhances its Rac1 GEF activity that is critical for spine and synapse formation. In conclusion, we provide a novel mechanism by which βPix-b regulates activity-dependent spinogenesis and synaptogenesis via Src-mediated phosphorylation.

Decrypting the PAK4 transcriptome profile in mammary tumor forming cells using Next Generation Sequencing

The p-21 Activated Kinase 4 (PAK4) protein kinase is implicated in many cancers, including breast cancer. Overexpression of PAK4 is sufficient to cause mouse mammary epithelial cells (iMMECs) to become tumorigenic. To gain insight into the long-term gene expression changes that occur downstream to PAK4, we performed Next Generation Sequencing of RNA collected from PAK4 overexpressing iMMECs and wild-type iMMECs. We identified a list of genes whose expression levels were altered in response to PAK4 overexpression in iMMECs. Some of these genes, including FoxC2 and ParvB, are consistent with a role for PAK4 in cancer. In addition, PAK4 regulates many genes that are frequently associated with the inflammatory response, raising the possibility that there is a connection between PAK4, inflammation, and the tumor microenvironment. This study delineates the PAK4 transcriptome profile in transformed mammary cells and can provide translational utility in other types of cancers as well.

Expanding functions of GIT Arf GTPase-activating proteins, PIX Rho guanine nucleotide exchange factors and GIT-PIX complexes

The GIT proteins, GIT1 and GIT2, are GTPase-activating proteins (inactivators) for the ADP-ribosylation factor (Arf) small GTP-binding proteins, and function to limit the activity of Arf proteins. The PIX proteins, α-PIX and β-PIX (also known as ARHGEF6 and ARHGEF7, respectively), are guanine nucleotide exchange factors (activators) for the Rho family small GTP-binding protein family members Rac1 and Cdc42. Through their multi-domain structures, GIT and PIX proteins can also function as signaling scaffolds by binding to numerous protein partners. Importantly, the constitutive association of GIT and PIX proteins into oligomeric GIT-PIX complexes allows these two proteins to function together as subunits of a larger structure that coordinates two distinct small GTP-binding protein pathways and serves as multivalent scaffold for the partners of both constituent subunits. Studies have revealed the involvement of GIT and PIX proteins, and of the GIT-PIX complex, in numerous fundamental cellular processes through a wide variety of mechanisms, pathways and signaling partners. In this Commentary, we discuss recent findings in key physiological systems that exemplify current understanding of the function of this important regulatory complex. Further, we draw attention to gaps in crucial information that remain to be filled to allow a better understanding of the many roles of the GIT-PIX complex in health and disease.

RhoJ interacts with the GIT-PIX complex and regulates focal adhesion disassembly

RhoJ is a Rho GTPase expressed in endothelial cells and tumour cells, which regulates cell motility, invasion, endothelial tube formation and focal adhesion numbers. This study aimed to further delineate the molecular function of RhoJ. Using timelapse microscopy RhoJ was found to regulate focal adhesion disassembly; small interfering RNA (siRNA)-mediated knockdown of RhoJ increased focal adhesion disassembly time, whereas expression of an active mutant (daRhoJ) decreased it. Furthermore, daRhoJ co-precipitated with the GIT–PIX complex, a regulator of focal adhesion disassembly. An interaction between daRhoJ and GIT1 was confirmed using yeast two-hybrid experiments, and this depended on the Spa homology domain of GIT1. GIT1, GIT2, β-PIX (also known as ARHGEF7) and RhoJ all colocalised in focal adhesions and depended on each other for their recruitment to focal adhesions. Functionally, the GIT–PIX complex regulated endothelial tube formation, with knockdown of both GIT1 and GIT2, or β-PIX phenocopying RhoJ knockdown. RhoJ-knockout mice showed reduced tumour growth and diminished tumour vessel density, identifying a role for RhoJ in mediating tumour angiogenesis. These studies give new insight into the molecular function of RhoJ in regulating cell motility and tumour vessel formation.

Role of βPix in the Kidney

Small GTPases function as molecular switches in cell signaling, alternating between an inactive, GDP-bound state, and active GTP-bound state. βPix is one of guanine nucleotide exchange factors (GEFs) that catalyze the exchange of bound GDP for ambient GTP. The central goal of this review article is to summarize recent findings on βPix and the role it plays in kidney pathology and physiology. Recent studies shed new light on several key questions concerning the signaling mechanisms mediated by βPix. This manuscript provides a review of the various mechanisms whereby βPix has been shown to function within the kidney through a wide range of actions. Both canonical GEF activity and non-canonical signaling pathways mediated by βPix are discussed. Distribution patterns of βPix in the kidney will be also covered. Much has yet to be discerned, but it is clear that βPix plays a significant role in the kidney.

Neuronal specific βPix-b stimulates actin-dependent processes via the interaction between its PRD and WH1 domain of N-WASP

βPix, a Pak-interacting nucleotide exchange factor (Cool-1/p85SPR), is a Cdc42/Rac1-specific guanine nucleotide exchange factor (GEF) involved in various actin-related processes. Many previous studies have focused on ubiquitously expressed βPix-a, while the role of the neuronal-specific isoform βPix-b is still unknown, especially whether its role is distinct from or similar to βPix-a. Here we show that unlike βPix-a, overexpression of βPix-b stimulates actin-dependent comet formation in BHK21 cells. This effect is attributed to the interaction between its proline-rich domain (PRD) and the WH1 domain of N-WASP. In addition, we show that overexpression of βPix-b stimulates actin-dependent dendritic spine formation in rat hippocampal neurons in culture, a formation that is blocked by co-expression of the WH1 domain of N-WASP or the PRD of βPix-b. Knocking-down endogenous expression of βPix-b by shRNA reduced the number of dendritic spines, which were rescued only by PRD-containing βPix-b mutants. GEF activity of βPix-b is also required for these effects. The results show that neuronal-specific βPix-b stimulates actin-dependent processes in cells via the interaction between its PRD and the WH1 domain of N-WASP. Our results identify N-WASP as the first protein shown to interact with the PRD of βPix-b, raising the possibility that, as an N-WASP WH1-binding protein, βPix-b may regulate N-WASP's activity in cells.

Interaction of microtubules and actin with the N-terminus of βPix-b(L) directs cellular pinocytosis

βPix is a Rac/Cdc42 guanine nucleotide exchange factor (GEF) that is known to be a regulator of actin cytoskeleton remodeling. Recently, a novel splicing isoform, βPix-b(L), was identified as an alternative translational product of the βPix-b mRNA with an extended N-terminus comprising a partial calponin homology (CH) domain and a serine-rich (SR) domain. However, the cellular function of βPix-b(L) is largely unknown. In the current study, we analyzed the genomic DNA structure and cellular functions of βPix-b(L). The results of this study demonstrate that βPix is composed of 24 exons and 21 introns spanning around 100 kb. RT-PCR experiments revealed that there are two forms of βPix mRNA with distinct 5' UTRs that are the result of alternative splicing of exon 1 and 2 from βPix genomic DNA. In addition, affinity chromatography analysis and a pull-down assay with the N-terminal region of βPix-b(L) revealed that βPix-b(L) interacts with tubulin and actin via its N-terminal CH and SR domains, respectively. Interaction with tubulin enabled βPix-b(L) to bundle the microtubule and form membrane protrusions. Furthermore, the N-terminus of βPix-b(L) was also critical for its localization to cellular vesicles. Functionally, βPix-b(L) induced pinocytosis through cooperative action of the CH and Dbl homology (DH) domains, demonstrating the role of βPix-b(L) in the regulation of membrane dynamics.

BetaPix up-regulates Na+/H+ exchanger 3 through a Shank2-mediated protein-protein interaction

Na(+)/H(+) exchanger 3 (NHE3) plays an important role in neutral Na(+) transport in mammalian epithelial cells. The Rho family of small GTPases and the PDZ (PSD-95/discs large/ZO-1) domain-based adaptor Shank2 are known to regulate the membrane expression and activity of NHE3. In this study we examined the role of betaPix, a guanine nucleotide exchange factor for the Rho GTPase and a strong binding partner to Shank2, in NHE3 regulation using integrated molecular and physiological approaches. Immunoprecipitation and pulldown assays revealed that NHE3, Shank2, and betaPix form a macromolecular complex when expressed heterologously in mammalian cells as well as endogenously in rat colon, kidney, and pancreas. In addition, these proteins co-segregated at the apical surface of rat colonic epithelial cells, as detected by immunofluorescence staining. When expressed in PS120/NHE3 cells, betaPix increased membrane expression and basal activity of NHE3. Interestingly, the effects of betaPix on NHE3 were abolished by cotransfection with dominant-negative Shank2 mutants and by treatment with Clostridium difficile toxin B, a Rho GTPase inhibitor, indicating that Shank2 and Rho GTPases are involved in betaPix-mediated NHE3 regulation. Knockdown of endogenous betaPix by RNA interference decreased Shank2-induced increase of NHE3 membrane expression in HEK 293T cells. These results indicate that betaPix up-regulates NHE3 membrane expression and activity by Shank2-mediated protein-protein interaction and by activating Rho GTPases in the apical regions of epithelial cells.

Genetic and cell biological analysis of integrin outside-in signaling

Integrins are cell surface transmembrane receptors that recognize and bind to extracellular matrix proteins and counter receptors. Binding of activated integrins to their ligands induces a vast number of structural and signaling changes within the cell. Large, multimolecular complexes assemble onto the cytoplasmic tails of activated integrins to engage and organize the cytoskeleton, and activate signaling pathways that ultimately lead to changes in gene expression. Additionally, integrin-mediated signaling intersects with growth factor-mediated signaling through various levels of cross-talk. This review discusses recent work that has tremendously broadened our understanding of the complexity of integrin-mediated signaling.

AlphaPIX Rho GTPase guanine nucleotide exchange factor regulates lymphocyte functions and antigen receptor signaling

AlphaPIX is a Rho GTPase guanine nucleotide exchange factor domain-containing signaling protein that associates with other proteins involved in cytoskeletal-membrane complexes. It has been shown that PIX proteins play roles in some immune cells, including neutrophils and T cells. In this study, we report the immune system phenotype of alphaPIX knockout mice. We extended alphaPIX expression experiments and found that whereas alphaPIX was specific to immune cells, its homolog betaPIX was expressed in a wider range of cells. Mice lacking alphaPIX had reduced numbers of mature lymphocytes and defective immune responses. Antigen receptor-directed proliferation of alphaPIX(-) T and B cells was also reduced, but basal migration was enhanced. Accompanying these defects, formation of T-cell-B-cell conjugates and recruitment of PAK and Lfa-1 integrin to the immune synapse were impaired in the absence of alphaPIX. Proximal antigen receptor signaling was largely unaffected, with the exception of reduced phosphorylation of PAK and expression of GIT2 in both T cells and B cells. These results reveal specific roles for alphaPIX in the immune system and suggest that redundancy with betaPIX precludes a more severe immune phenotype.

The PIX-GIT complex: a G protein signaling cassette in control of cell shape

Arf and Rho GTP-binding proteins coordinately regulate membrane dynamics and cytoskeletal rearrangements. The Cdc42/Rac guanine nucleotide exchange factor PIX and the Arf GTPase-activating protein GIT form a stable complex in cells. The PIX-GIT complex functions to integrate signaling among Arf, Cdc42, and Rac proteins in response to cues emanating from integrins, heterotrimeric G proteins, receptor tyrosine kinases, and cell-cell interactions. A concept that emerges from the literature is that the PIX-GIT complex serves as a cassette to elicit changes in cell shape essential for polarized cell responses in a wide range of biological contexts.

A betaPix Pak2a signaling pathway regulates cerebral vascular stability in zebrafish

The vasculature tailors to the needs of different tissues and organs. Molecular, structural, and functional specializations are observed in different vascular beds, but few genetic models give insight into how these differences arise. We identify a unique cerebrovascular mutation in the zebrafish affecting the integrity of blood vessels supplying the brain. The zebrafish bubblehead (bbh) mutant exhibits hydrocephalus and severe cranial hemorrhage during early embryogenesis, whereas blood vessels in other regions of the embryo appear intact. Here we show that hemorrhages are associated with poor cerebral endothelial-mesenchymal contacts and an immature vascular pattern in the head. Positional cloning of bbh reveals a hypomorphic mutation in betaPix, a binding partner for the p21-activated kinase (Pak) and a guanine nucleotide exchange factor for Rac and Cdc42. betaPix is broadly expressed during embryonic development and is enriched in the brain and in large blood vessels. By knockdown of specific betaPix splice variants, we show that they play unique roles in embryonic vascular stabilization or hydrocephalus. Finally, we show that Pak2a signaling is downstream of betaPix. These data identify an essential in vivo role for betaPix and Pak2a during embryonic development and illuminate a previously unrecognized pathway specifically involved in cerebrovascular stabilization.

Cool-1 functions as an essential regulatory node for EGF receptor- and Src-mediated cell growth

Cool-1 (cloned-out of library 1) has a key role in regulating epidermal growth factor receptor (EGFR) degradation. Here, we show that Cool-1 performs this function by functioning as both an upstream activator and downstream target for Cdc42. EGF-dependent phosphorylation of Cool-1 enables it to act as a nucleotide exchange factor for Cdc42 and to form a complex with the E3 ligase Cbl, thus regulating Cbl-catalysed EGFR degradation. The EGF-dependent phosphorylation is normally transient; however, Cool-1 phosphorylation is sustained in cells expressing v-Src and is essential for cellular transformation, as well as for v-Src-induced tumour formation in mice. These findings demonstrate that the regulated phosphorylation of Cool-1 is necessary to maintain the balance between normal signalling by EGFR and Src versus aberrant growth and transformation.

αPIX and βPIX and their role in focal adhesion formation

Alpha and betaPIX belong to the group of guanine nucleotide exchange factors (GEFs) that mediate activation of members of the Rho GTPase family, in particular Rac1 and Cdc42, by stimulating the exchange of GDP for GTP. Rho family proteins are well known as regulators of the actin cytoskeleton and have been implicated in the formation of various types of focal adhesion structures. However, the function of GEF proteins during focal adhesion formation is only beginning to emerge. Here, we highlight the recent findings on alpha and betaPIX and their involvement in integrin-dependent signaling and suggest models for the role of PIX proteins during focal adhesion turnover.

Molecular interaction of NADPH oxidase 1 with betaPix and Nox Organizer 1

It is well established that growth-factor-induced reactive oxygen species (ROS) act as second messengers in cell signaling. We have previously reported that betaPix, a guanine nucleotide exchange factor for Rac, interacts with NADPH oxidase 1 (Nox1) leading to EGF-induced ROS generation. Here, we report the identification of the domains of Nox1 and betaPix responsible for the interaction between the two proteins. GST pull-down assays show that the PH domain of betaPix binds to the FAD-binding region of Nox1. We also show that overexpression of the PH domain of betaPix results in inhibition of superoxide anion generation in response to EGF. Additionally, NADPH oxidase Organizer 1 (NoxO1) is shown to interact with the NADPH-binding region of Nox1. These results suggest that the formation of the complex consisting of Nox1, betaPix, and NoxO1 is likely to be a critical step in EGF-induced ROS generation.

Biochemical characterization of the Cool (Cloned-out-of-Library)/Pix (Pak-interactive exchange factor) proteins

The Cool (Cloned out of Library)/Pix (Pak interactive exchange factor) proteins have been implicated in a diversity of biological activities, ranging from pathways initiated by growth factors and chemoattractants to X-linked mental retardation. Initially discovered through yeast two-hybrid and biochemical analyses as binding partners for the Cdc42/Rac-target/effector, Pak (p21 activated kinase), the sequences for the Cool/Pix proteins revealed a DH (Dbl homology) domain. Because the DH domain is the limit functional unit for stimulating guanine nucleotide exchange on Rho family GTP-binding proteins, it was assumed that the Cool/Pix proteins would act as guanine nucleotide exchange factors (GEFs) for the Rho proteins. Of the three known isoforms, (p50Cool-1, p85Cool-1/beta-Pix, and 90Cool-2/alpha-Pix), only Cool-2/alpha-Pix has exhibited significant GEF activity. A number of experimental techniques have been used to characterize Cool-2, and in vitro analysis has revealed that its GEF activity is under tight control through intramolecular interactions involving several binding partners. Here we describe the biochemical methods used to study the Cool/Pix proteins and, in particular, the regulation of the GEF activity of Cool-2/alpha-Pix.

Crystal structure of the N-terminal SH3 domain of mouse βPIX, p21-activated kinase-interacting exchange factor

The mouse betaPIX-SH3 domain, residues 8-63 of P21-activated kinase interacting exchange factor, has been characterized by X-ray diffraction. Crystals belonging to space group P3(2)21 diffracted to 2.0 A and the structure was phased by the single-wavelength anomalous diffraction method. The domain is a compact beta-barrel with an overall conformation similar to the general SH3 structure. The X-ray structure shows mouse betaPIX-SH3 domain binding the way in which the betaPIX characteristic amino acids do so for an unconventional ligand binding surface. This arrangement provides a rationale for the unusual ligand recognition motif exhibited by mouse betaPIX-SH3 domain. Comparison with another SH3/peptide complex shows that the recognition mode of the mouse betaPIX-SH3 domain should be very similar to the RXXK ligand binding mode. The unique large and planar hydrophobic pocket may contribute to the promiscuity of betaPIX-SH3 domain resulting in its multiple biological functions.

Le complexe GIT-PIX : Une plate-forme de régulation des GTPases ARF et Rac/Cdc42

We recently described that the tumor suppressor factor Scribble anchors the PIX exchange factor for Rac/Cdc42 and the ARF-GAP GIT proteins at the plasma membrane. Because it has been postulated that the GIT-PIX proteins dimerize and tightly self-assemble to form a high molecular weight complex, this nexus may be capable of linking together important signalling molecules to control cytosqueleton polymerization and membrane dynamics. To date, most studies that have tempted to unravel the function of these proteins have found their implication in a great variety of cellular functions (receptor recycling, endo-exocytosis, cell migration, synapse formation...) but have mostly neglected to consider the multimeric organization of this hub. There is no doubt that our comprehension of physiopathological disorders such as cancers will be improved when the nature of the complex pathways integrated by the GIT-PIX nodule will be understood.

BetaPix-a enhances the activity of phospholipase Cgamma1 by binding SH3 domain in breast cancer

Phospholipase C-gamma1 (PLCgamma1) plays a critical role in cell growth and proliferation by generating the second messengers, diacylglycerol and 1, 4, 5-inositol triphosphate. To investigate the roles of Src homology domain 2 and domain 3 of PLCgamma1 in PLCgamma1-mediated cell signaling, we characterized some proteins binding to these domains in the MCF7 and MDA-MB-231 breast cancer cell lines. Of the several proteins that bind to glutathione-S-transferase-SH2/SH2/SH3, we identified an 85 kDa protein that binds to the SH3 domain of PLCgamma1 as the guanine nucleotide exchange factor, p21-activated protein kinase-interacting exchange factor-a (betaPix-a). BetaPix-a co-immunoprecipitated with PLCgamma1 in breast cancer tissues extracts and in MCF7 and MDA-MB-231 cell extracts. In addition, PDGF-stimulated PLCgamma1 activity was elevated in betaPix-a-overexpressing NIH3T3 cells. Our results suggest that betaPix-a binds to the Src homology domain 3 of PLCgamma1 and promotes tumor growth in breast cancer by enhancing the activity PLCgamma1.

p85 beta-PIX is required for cell motility through phosphorylations of focal adhesion kinase and p38 MAP kinase

Lysophosphatidic acid (LPA) mediates diverse biological responses, including cell migration, through the activation of G-protein-coupled receptors. Recently, we have shown that LPA stimulates p21-activated kinase (PAK) that is critical for focal adhesion kinase (FAK) phosphorylation and cell motility. Here, we provide the direct evidence that p85 beta-PIX is required for cell motility of NIH-3T3 cells by LPA through FAK and p38 MAP kinase phosphorylations. LPA induced p85 beta-PIX binding to FAK in NIH-3T3 cells that was inhibited by pretreatment of the cells with phosphoinositide 3-kinase inhibitor, LY294002. Furthermore, the similar inhibition of the complex formation was also observed, when the cells were transfected with either p85 beta-PIX mutant that cannot bind GIT or dominant negative mutants of Rac1 (N17Rac1) and PAK (PAK-PID). Transfection of the cells with specific p85 beta-PIX siRNA led to drastic inhibition of LPA-induced FAK phosphorylation, peripheral redistribution of p85 beta-PIX with FAK and GIT1, and cell motility. p85 beta-PIX was also required for p38 MAP kinase phosphorylation induced by LPA. Finally, dominant negative mutant of Rho (N19Rho)-transfected cells did not affect PAK activation, while the cells stably transfected with p85 beta-PIX siRNA or N17Rac1 showed the reduction of LPA-induced PAK activation. Taken together, the present data suggest that p85 beta-PIX, located downstream of Rac1, is a key regulator for the activations of FAK or p38 MAP kinase and plays a pivotal role in focal complex formation and cell motility induced by LPA.

Novel regulatory mechanisms for the Dbl family guanine nucleotide exchange factor Cool-2/alpha-Pix

The Cool-2 (cloned-out of library-2) protein (identical to alpha-Pix for Pak-interactive exchange factor) has been implicated in various biological responses including chemoattractant signaling and in certain forms of mental retardation. We show that when Cool-2 exists as a dimer, it functions as a Rac-specific guanine nucleotide exchange factor (GEF). Dimerization of Cool-2 enables its Dbl (diffuse B-cell lymphoma) and pleckstrin homology domains to work together (in trans) to bind specifically to Rac-GDP. Dissociation of dimeric Cool-2 into its monomeric form allows it to act as a GEF for Cdc42 as well as for Rac. The binding of either PAK (p21-activated kinase) or Cbl (Casitas B-lymphoma) to the SH3 domain of monomeric Cool-2 is necessary for the functional interactions between GDP-bound Cdc42 or Rac and the Cool-2 monomer. The betagamma subunit complex of large GTP-binding proteins, by interacting with PAK, stimulates the dissociation of the Cool-2 dimer and activates its GEF activity for Cdc42. Overall, these findings highlight novel mechanisms by which extracellular signals can direct the specific activation of Rac versus Cdc42 by Cool-2/alpha-Pix.

betaPix-b(L), a novel isoform of betaPix, is generated by alternative translation

betaPix (Pak-interacting exchange factor) isoforms are recently identified guanine nucleotide exchange factors (GEFs) for Rho family GTPases, Rac/Cdc42, that are key players in the regulation of actin dynamics. Here we show that a novel 105-kDa betaPix isoform, betaPix-bL, is generated by alternative translation of betaPix-b mRNA. Translation of betaPix-bL starts at an atypical initiation site, GTG, that is located 57 nucleotides downstream from the newly identified 5' end of betaPix-b cDNA. The expression of two isoforms, betaPix-b and betaPix-bL, from betaPix-b mRNA is controlled by an internal ribosome entry site (IRES)-driven mechanism. Comparing to betaPix-b, betaPix-bL contains additional 105 amino acids composed of a calponin homology (CH) domain and a serine-rich sequence in the N-terminus. The expression of betaPix-bL in rat brain is developmentally regulated and high in the embryonic stages, suggesting that the function of betaPix-bL is more heavily required during the early stages of brain development.

Crystal structure of the Shank PDZ-ligand complex reveals a class I PDZ interaction and a novel PDZ-PDZ dimerization

The Shank/proline-rich synapse-associated protein family of multidomain proteins is known to play an important role in the organization of synaptic multiprotein complexes. For instance, the Shank PDZ domain binds to the C termini of guanylate kinase-associated proteins, which in turn interact with the guanylate kinase domain of postsynaptic density-95 scaffolding proteins. Here we describe the crystal structures of Shank1 PDZ in its peptide free form and in complex with the C-terminal hexapeptide (EAQTRL) of guanylate kinase-associated protein (GKAP1a) determined at 1.8- and 2.25-A resolutions, respectively. The structure shows the typical class I PDZ interaction of PDZ-peptide complex with the consensus sequence -X-(Thr/Ser)-X-Leu. In addition, Asp-634 within the Shank1 PDZ domain recognizes the positively charged Arg at -1 position and hydrogen bonds, and salt bridges between Arg-607 and the side chains of the ligand at -3 and -5 positions contribute further to the recognition of the peptide ligand. Remarkably, whether free or complexed, Shank1 PDZ domains form dimers with a conserved beta B/beta C loop and N-terminal beta A strands, suggesting a novel model of PDZ-PDZ homodimerization. This implies that antiparallel dimerization through the N-terminal beta A strands could be a common configuration among PDZ dimers. Within the dimeric structure, the two-peptide binding sites are arranged so that the N termini of the bound peptide ligands are in close proximity and oriented toward the 2-fold axis of the dimer. This configuration may provide a means of facilitating dimeric organization of PDZ-target assemblies.

The Cbl proteins are binding partners for the Cool/Pix family of p21-activated kinase-binding proteins

Members of the Cool protein family contain SH3, Dbl, and pleckstrin homology domains and are binding partners for the p21-activated kinase (PAK). Using the yeast two-hybrid screen, we identified Cbl-b as a Cool family binding partner. We co-immunoprecipitated endogenous Cool and Cbl-b from a variety of breast cancer cell lines. The Cool-Cbl-b interaction requires the SH3 domain of Cool and competes with the binding of PAK to Cool proteins. Expression of Cbl-b effectively blocks the ability of Cool-2 to stimulate PAK, thus providing an additional mechanism, aside from catalyzing receptor ubiquitination, by which Cbl-b acts as a negative regulator for signaling activities requiring PAK activation.

The Shank family of postsynaptic density proteins interacts with and promotes synaptic accumulation of the beta PIX guanine nucleotide exchange factor for Rac1 and Cdc42

The Shank/ProSAP family of multidomain proteins is known to play an important role in organizing synaptic multiprotein complexes. Here we report a novel interaction between Shank and beta PIX, a guanine nucleotide exchange factor for the Rac1 and Cdc42 small GTPases. This interaction is mediated by the PDZ domain of Shank and the C-terminal leucine zipper domain and the PDZ domain-binding motif at the extreme C terminus of beta PIX. Shank colocalizes with beta PIX at excitatory synaptic sites in cultured neurons. In brain, Shank forms a complex with beta PIX and beta PIX-associated signaling molecules including p21-associated kinase (PAK), an effector kinase of Rac1/Cdc42. Importantly, overexpression of Shank in cultured neurons promotes synaptic accumulation of beta PIX and PAK. Considering the involvement of Rac1 and PAK in spine dynamics, these results suggest that Shank recruits beta PIX and PAK to spines for the regulation of postsynaptic structure.

Overexpression of betaPix-a in human breast cancer tissues

Pak interacting exchange factor (betaPix) is a recently cloned protein that contains a multidomain with many potential binding sites and is known to be involved in the regulation of Cdc42/Rac GTPases and Pak kinase activity. These domains of betaPix appear to play a critical role in the regulation of the cytoskeletal organization. The overexpression of betaPix enhances the activation of p38, which is thought to be an important downstream effector of the Rho GTPase family (Rac, Cdc42), which are involved in increased membrane ruffling and cell motility. This increase of cell mobility is an important feature of cancer invasion. We examined the expression of betaPix-a in human breast cancer tissues and adjacent normal tissues obtained from 39 breast cancer patients. Immunoblot analysis and RT-PCR revealed that betaPix-a expression was significantly increased in 37 of the 39 breast cancer tissues (94.9%) versus normal breast tissues. Immunohistochemical analysis showed that breast cancer tissues have consistently stronger immunoreactivity to betaPix-a antibodies than normal tissues. betaPix-a overexpression was inversely associated with extensive intraductal component (P<0.001). In conclusion, betaPix-a expression was found to be higher in human breast cancer tissues than in normal breast tissues, which implies a role for betaPix-a in human breast tumorigenesis. We suggest that betaPix-a may be a useful marker of malignant disease in the breast.

EhPAK, a member of the p21-activated kinase family, is involved in the control of Entamoeba histolytica migration and phagocytosis

Entamoeba histolytica migration is essential for the development of amoebiasis, a human disease characterised by invasion and destruction of tissues. Amoebic motility requires both polarisation of the cell and formation of a predominant pseudopod. As p21-activated kinases PAKs are known to regulate eukaryotic cell motility and morphology, we investigated the role of PAK in E. histolytica. We showed that the C-terminal domain of EhPAK comprised a constitutive kinase activity in vitro and that overproduction of this fragment, in E. histolytica, caused a significant reduction in amoeboid migration, as measured by dynamic image analysis, indicating an involvement of EhPAK in this process. A dramatic loss of polarity, as indicated by the increased number of membrane extensions all around E. histolytica, was also observed, suggesting that the N-terminal domain of EhPAK was necessary for maintenance of cell polarity. To support this view, we showed that despite the absence of the consensus motif to bind to Rac and Cdc42, the N-terminal domain of EhPAK bound to Rac1, suggesting that the N-terminal region was a regulatory domain. In addition, we also found an increased rate of human red blood cell phagocytosis, suggesting for the first time an active role for a PAK protein in this process. Taking together, the results suggest strongly that EhPAK is a key regulatory element in polarity, motility and phagocytosis of E. histolytica.

Phosphorylation of p85 beta PIX, a Rac/Cdc42-specific guanine nucleotide exchange factor, via the Ras/ERK/PAK2 pathway is required for basic fibroblast growth factor-induced neurite outgrowth

Guanine nucleotide exchange factors (GEFs) have been implicated in growth factor-induced neuronal differentiation through the activation of small GTPases. Although phosphorylation of these GEFs is considered an activation mechanism, little is known about the upstream of PAK-interacting exchange factor (PIX), a member of the Dbl family of GEFs. We report here that phosphorylation of p85 betaPIX/Cool/p85SPR is mediated via the Ras/ERK/PAK2 pathway. To understand the role of p85 betaPIX in basic fibroblast growth factor (bFGF)-induced neurite outgrowth, we established PC12 cell lines that overexpress the fibroblast growth factor receptor-1 in a tetracycline-inducible manner. Treatment with bFGF induces the phosphorylation of p85 betaPIX, as determined by metabolic labeling and mobility shift upon gel electrophoresis. Interestingly, phosphorylation of p85 betaPIX is inhibited by PD98059, a specific MEK inhibitor, suggesting the involvement of the ERK cascade. PAK2, a major PAK isoform in PC12 cells as well as a binding partner of p85 betaPIX, also functions upstream of p85 betaPIX phosphorylation. Surprisingly, PAK2 directly binds to ERK, and its activation is dependent on ERK. p85 betaPIX specifically localizes to the lamellipodia at neuronal growth cones in response to bFGF. A mutant form of p85 betaPIX (S525A/T526A), in which the major phosphorylation sites are replaced by alanine, shows significant defect in targeting. Moreover, expression of the mutant p85 betaPIX efficiently blocks PC12 cell neurite outgrowth. Our study defines a novel signaling pathway for bFGF-induced neurite outgrowth that involves activation of the PAK2-p85 betaPIX complex via the ERK cascade and subsequent translocation of this complex.

Analysis of the subcellular distribution of avian p95-APP2, an ARF-GAP orthologous to mammalian paxillin kinase linker

We describe here the identification and characterization of avian p95-APP2, a multi-domain protein of a recently identified family of ADP-ribosylation factor (ARF)-GTPase-activating proteins (GAPs) including mammalian G protein-coupled receptor kinases (GRK)-interactor 1 (GIT1), paxillin kinase linker (PKL), and GIT2, as well as avian p95-APP1. The p95-APP2 is eluted from Rac-GTP-gamma-S, but not from Rac-GDP-beta-S columns. As other members of the family, p95-APP2 has binding regions for the focal adhesion protein paxillin, and for the Rac exchanging factor PIX. Sequence comparison indicates that p95-APP2 is the avian orthologue of mammalian PKL. Expression studies showed a largely diffuse distribution of the full length p95-APP2, without evident effects on cell morphology. We observed a dramatic difference between the localization of the amino-terminal portion of the protein, including the ARF-GAP domain and the three ankyrin repeats, and the carboxy-terminal portion including the paxillin-binding site. Moreover, the expression of truncated carboxy-terminal polypeptides including both the PIX- and paxillin-binding regions leads to a marked localization of the protein together with paxillin at large vesicles. Comparison of the expression of corresponding ARF-GAP-deficient constructs from p95-APP2 and p95-APP1 shows their distribution at distinct endocytic compartments. Altogether, these data support a role of distinct members of this family of ARF-GAPs in the regulation of different steps of membrane traffic during cell motility, and suggest that p95-APP2 may shuttle between an intracellular compartment and the cell periphery, although, further work will be needed to address this point.

Regulation of the Cool/Pix proteins: key binding partners of the Cdc42/Rac targets, the p21-activated kinases

The Cool (cloned-out of library)/Pix (for PAK-interactive exchange factor) proteins directly bind to members of the PAK family of serine/threonine kinases and regulate their activity. Three members of the Cool/Pix family have shown distinct regulatory activities: (i) p50(Cool-1) inhibits Cdc42/Rac-stimulated PAK activity, (ii) p85(Cool-1)/beta-Pix has a permissive effect on Cdc42/Rac-stimulated activity, and (iii) p90(Cool-2)/alpha-Pix strongly activates PAK. We initially suspected that these different functional effects were due to a binding interaction that occurs at the carboxyl-terminal ends of the larger Cool/Pix proteins, thus enabling them to stimulate (or at least permit) rather than inhibit PAK activity. This led to the identification of the Cat proteins (for Cool-associated tyrosine phosphosubstrates). However, here we show that the Cat proteins bind to the carboxyl-terminal ends of p85(Cool-1) (residues 523-546) and Cool-2 (residues 647-670), and that the binding of Cat to Cool-2 in fact is not necessary for the Cool-2-mediated activation of PAK. Rather, an 18-amino acid region, designated T1, that is present in the Cool-1 proteins, but missing in Cool-2, is essential for controlling the regulation of PAK activity by Cool-1/beta-Pix in vivo. Deletion of T1 yielded a p85(Cool-1) molecule that mimicked the Cool-2 protein and was capable of strongly stimulating PAK activity. However, when T1 was added to Cool-2, the ability of Cool-2 to directly activate PAK was lost. We conclude that T1 represents a novel regulatory domain that accounts for the specific functional effects on PAK activity exhibited by the different members of the Cool/Pix family.

Molecular mechanisms regulating the subcellular localization of p95-APP1 between the endosomal recycling compartment and sites of actin organization at the cell surface

Cell migration requires coordination between adhesion, actin organization and membrane traffic. Rac and ARF6 have been shown to cooperate for the organization of actin at the cell surface. Recently, the GIT family of ARF-GAPs has been identified, which includes proteins that can functionally interact with both ARF and Rac GTPases. The p95-APP1 protein is a member of this family, isolated as part of a multi-molecular complex interacting with GTP-Rac. Our previous work has indicated that this protein may be part of the machinery redirecting membrane recycling towards sites of protrusion during locomotion. By analyzing the distribution and the effects of truncated forms of p95-APP1, we show here that the lack of the ARF-GAP domain of p95-APP1 dramatically shifts its localization to large vesicles. The use of several markers of the endocytic pathway has revealed that the truncated p95-APP1 localizes specifically to a Rab11-, transferrin receptor-positive compartment. Other markers are excluded from the p95-APP1-positive vesicles, while known components of the multi-molecular complex colocalize with truncated p95-APP1 in this compartment. Coexpression of a constitutively active form of Rac induces the redistribution of the truncated constructs and of the associated PIX, PAK, and paxillin to peripheral sites of Rac-mediated actin organization, and the disassembly of the large Rab11-positive vesicles. Together, the data presented indicate that p95-APP1 is part of a complex that shuttles between the plasma membrane and the endocytic recycling compartment, and suggest that the dynamic redistribution of the p95-APP1-containing complex is mediated both by the ARF-GAP domain, and by the recruitment of the complex at the cell surface at sites of Rac activation.

β1PIX, the PAK-interacting exchange factor, requires localization via a coiled-coil region to promote microvillus-like structures and membrane ruffles

PIX is a Rho-family guanine nucleotide exchange factor that binds PAK. We previously described two isoforms of PIX that differ in their N termini. Here, we report the identification of a new splice variant of βPIX, designated β2PIX, that is the dominant species in brain and that lacks the region of ∼120 residues with predicted coiled-coil structure at the C terminus of β1PIX. Instead, β2PIX contains a serine-rich C terminus. To determine whether these splice variants differ in their cellular function, we studied the effect of expressing these proteins in HeLa cells. We found that the coiled-coil region plays a key role in the localization of β1PIX to the cell periphery and is also responsible for PIX dimerization. Overexpression of β1, but not β2PIX, drives formation of membrane ruffles and microvillus-like structures (via activation of Rac1 and Cdc42, respectively), indicating that its function requires localized activation of these GTPases. Thus, β1PIX, like other RhoGEFs, exerts specific morphological functions that are dependent on its intracellular location and are mediated by its C-terminal dimerization domain.

Involvement of alpha-PAK-interacting exchange factor in the PAK1-c-Jun NH(2)-terminal kinase 1 activation and apoptosis induced by benzo[a]pyrene

Benzo[a]pyrene [B(a)P], a potent procarcinogen found in combustion products such as diesel exhaust and cigarette smoke, has been recently shown to activate the c-Jun NH(2)-terminal kinase 1 (JNK1) and induce caspase-3-mediated apoptosis in Hepa1c1c7 cells. However, the molecules of the signaling pathway that control the mitogen-activated protein kinase cascades induced by B(a)P and the interaction between those and apoptosis by B(a)P have not been well defined. We report here that B(a)P promoted Cdc42/Rac1, p21-activated kinase 1 (PAK1), and JNK1 activities in 293T and HeLa cells. Moreover, alpha-PAK-interacting exchange factor (alpha PIX) mRNA and its protein expression were upregulated by B(a)P. While overexpression of an active mutant of alpha PIX (DeltaCH) facilitated B(a)P-induced activation of Cdc42/Rac1, PAK1, and JNK1, overexpression of mutated alphaPIX (L383R, L384S), which lacks guanine nucleotide exchange factor activity, SH3 domain-deleted alphaPIX (Delta SH3), which lacks the ability to bind PAK, kinase-negative PAK1 (K299R), and kinase-negative SEK1 (K220A, K224L) inhibited B(a)P-triggered JNK1 activation. Interestingly, overexpression of alphaPIX (Delta CH) and a catalytically active mutant PAK1 (T423E) accelerated B(a)P-induced apoptosis in HeLa cells, whereas alphaPIX (Delta SH3), PAK1 (K299R), and SEK 1 (K220A, K224L) inhibited B(a)P-initiated apoptosis. Finally, a preferential caspase inhibitor, Z-Asp-CH2-DCB, strongly blocked the alphaPIX (Delta CH)-enhanced apoptosis in cells treated with B(a)P but did not block PAK1/JNK1 activation. Taken together, these results indicate that alphaPIX plays a crucial role in B(a)P-induced apoptosis through activation of the JNK1 pathway kinases.

Searching new targets for anticancer drug design: the families of Ras and Rho GTPases and their effectors

The Ras superfamily of low-molecular-weight GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell growth and development, apoptosis, lipid metabolism, cytoarchitecture, membrane trafficking, and transcriptional regulation. More than 100 genes of this superfamily grouped in six subfamilies have been described so far, pointing to the complexities and specificities of their cellular functions. Dysregulation of members of at least two of these families (the Ras and the Rho families) is involved in the events that lead to the uncontrolled proliferation and invasiveness of human tumors. In recent years, the cloning and characterization of downstream effectors for Ras and Rho proteins have given crucial clues to the specific pathways that lead to aberrant cellular growth and ultimately to tumorigenesis. A direct link between the functions of some of these effectors with the appearance of transformed cells and their ability to proliferate and invade surrounding tissues has been made. Accordingly, drugs that specifically alter their functions display antineoplasic properties, and some of these drugs are already under clinical trials. In this review, we survey the progress made in understanding the underlying molecular connections between carcinogenesis and the specific cellular functions elicited by some of these effectors. We also discuss new drugs with antineoplastic or antimetastatic activity that are targeted to specific effectors for Ras or Rho proteins.

BetaPix-enhanced p38 activation by Cdc42/Rac/PAK/MKK3/6-mediated pathway. Implication in the regulation of membrane ruffling

betaPix (PAK-interacting exchange factor) is a recently identified guanine nucleotide exchange factor for Rho family small G protein Cdc42/Rac. The protein interacts with p21-activated protein kinase (PAK) through its SH3 domain. We examined the effect of betaPix on MAP kinase signaling and cytoskeletal rearrangement in NIH3T3 fibroblast cells. Overexpression of betaPix enhanced the activation of p38 in the absence of other stimuli and also induced translocation of p38 to the nucleus. This betaPix-induced p38 activation was blocked by coexpression of dominant-negative Cdc42/Rac or kinase-inactive PAK, indicating that the effect of betaPix on p38 is exerted through the Cdc42/Rac-PAK pathway and requires PAK kinase activity. The essential role of betaPix in growth factor-stimulated p38 activation was evidenced by the blocking of platelet-derived growth factor-induced p38 activation in the cells expressing betaPix SH3m (W43K) and betaPix DHm (L238R,L239R). In addition, SB203580, a p38 inhibitor, and kinase-inactive p38 (T180A,Y182F) blocked membrane ruffling induced by betaPix, suggesting that p38 might be involved in mediating betaPix-induced membrane ruffling. The results in this study suggest that betaPix might have a role in nuclear signaling, as well as in actin cytoskeleton regulation, and that some part of these cellular functions is possibly mediated by p38 MAP kinase.

Cell migration: GAPs between membrane traffic and the cytoskeleton

During cell migration, coordination between membrane traffic, cell substrate adhesion and actin reorganization is required for protrusive activity to occur at the leading edge. Actin organization is regulated by Rho family GTPases and, with a contribution from the endocytic cycle, serves to extend the cell front. The details of the molecular mechanisms that direct membrane traffic at sites of adhesion and rearrange actin at the cell edge are still unknown. However, recent findings show that a number of multi-domain proteins characterized by an ArfGAP domain interact with both actin-regulating and integrin-binding proteins, as well as affecting Rac-mediated protrusive activity and cell migration. Some of these proteins have been shown to localize to endocytic compartments and to have a role in regulating endocytosis. Given the participation of Arf proteins in regulating membrane traffic, one appealing hypothesis is that the ArfGAPs act as molecular devices that coordinate membrane traffic and cytoskeletal reorganization during cell motility.

Leucine zipper-mediated homodimerization of the p21-activated kinase-interacting factor, beta Pix. Implication for a role in cytoskeletal reorganization

Pix, a p21-activated kinase-interacting exchange factor, is known to be involved in the regulation of Cdc42/Rac GTPases. The 85-kDa betaPix-a protein contains an Src homology 3 domain, the tandem Dbl homology and Pleckstrin homology domains, a proline-rich region, and a GIT1-binding domain. In addition to those domains, betaPix-a also contains a putative leucine zipper domain at the C-terminal end. In this study, we demonstrate that the previously identified putative leucine zipper domain mediates the formation of betaPix-a homodimers. Using in vitro and in vivo methodologies, we show that deletion of the leucine zipper domain is sufficient to abolish betaPix-a homodimerization. In NIH3T3 fibroblast cells, expression of wild type betaPix-a induces the formation of membrane ruffles. However, cells expressing the leucine zipper domain deletion mutant could not form membrane ruffle structures. Moreover, platelet-derived growth factor-mediated cytoskeletal changes were completely blocked by the leucine zipper domain deletion mutant. The results suggest that the leucine zipper domain enables betaPix-a to homodimerize, and homodimerization is essential for betaPix-a signaling functions leading to the cytoskeletal reorganization.

Evolution and characterization of tetraonine endogenous retrovirus: a new virus related to avian sarcoma and leukosis viruses

In a previous study, we found avian sarcoma and leukosis virus (ASLV) gag genes in 19 species of birds in the order Galliformes including all grouse and ptarmigan (Tetraoninae) surveyed. Our data suggested that retroviruses had been transmitted horizontally among some host species. To further investigate these elements, we sequenced a replication-defective retrovirus, here named tetraonine endogenous retrovirus (TERV), from Bonasa umbellus (ruffed grouse). This is the first report of a complete, replication-defective ASLV provirus sequence from any bird other than the domestic chicken. We found a replication-defective proviral sequence consisting of putative Gag and Env proteins flanked by long terminal repeats. Reverse transcription-PCR analysis showed that retroviral gag sequences closely related to TERV are transcribed, supporting the hypothesis that TERV is an active endogenous retrovirus. Phylogenetic analyses suggest that TERV may have arisen via recombination between different retroviral lineages infecting birds. Southern blotting using gag probes showed that TERV occurs in tetraonines but not in chickens or ducks, suggesting that integration occurred after the earliest phasianid divergences but prior to the radiation of tetraonine birds.

Isolation and characterization of a novel human gene (NESH) which encodes a putative signaling molecule similar to e3B1 protein

Using a conventional cloning technique, a novel full-length cDNA was isolated and sequenced from a human placental cDNA library. This cDNA consists of 2129 bp and has a predicted open reading frame encoding 366 amino acids. It possesses a Src homology 3 (SH3) motif, proline-rich region, serine-rich region and no catalytic domain, suggesting that it seems to be a signaling protein most similar to e3B1, an eps8 SH3 binding protein. PCR-based mapping with both a monochromosomal hybrid panel and radiation hybrid cell panels placed the gene to human chromosome 17q21.3 near the marker D17S1795.

p95-APP1 links membrane transport to Rac-mediated reorganization of actin

Motility requires protrusive activity at the cellular edge, where Rho family members regulate actin dynamics. Here we show that p95-APP1 (ArfGAP-putative, Pix-interacting, paxillin-interacting protein 1), a member of the GIT1/PKL family, is part of a complex that interacts with Rac. Wild-type and truncated p95-APP1 induce actin-rich protrusions mediated by Rac and ADP-ribosylation factor 6 (Arf6). Distinct p95-APP1-derived polypeptides have different distributions, indicating that p95-APP1 cycles between the cell surface and endosomes. Our results show that p95-APP1 functionally interacts with Rac and localizes to endosomal compartments, thus identifying p95-APP1 as a molecular link between actin organization, adhesion, and membrane transport during cell motility.

The GIT family of ADP-ribosylation factor GTPase-activating proteins. Functional diversity of GIT2 through alternative splicing

We recently characterized a novel protein, GIT1, that interacts with G protein-coupled receptor kinases and possesses ADP-ribosylation factor (ARF) GTPase-activating protein activity. A second ubiquitously expressed member of the GIT protein family, GIT2, has been identified in data base searches. GIT2 undergoes extensive alternative splicing and exists in at least 10 and potentially as many as 33 distinct forms. The longest form of GIT2 is colinear with GIT1 and shares the same domain structure, whereas one major splice variant prominent in immune tissues completely lacks the carboxyl-terminal domain. The other 32 potential variants arise from the independent alternative splicing of five internal regions in the center of the molecule but share both the amino-terminal ARF GTPase-activating protein domain and carboxyl-terminal domain. Both the long and short carboxyl-terminal variants of GIT2 are active as GTPase-activating proteins for ARF1, and both also interact with G protein-coupled receptor kinase 2 and with p21-activated kinase-interacting exchange factors complexed with p21-activated kinase but not with paxillin. Cellular overexpression of the longest variant of GIT2 leads to inhibition of beta(2)-adrenergic receptor sequestration, whereas the shortest splice variant appears inactive. Although GIT2 shares many properties with GIT1, it also exhibits both structural and functional diversity due to tissue-specific alternative splicing.

Molecular cloning of neuronally expressed mouse betaPix isoforms

Pix, a Pak-interacting exchange factor, is known to be involved in the regulation of Cdc42/Rac GTPases and Pak kinase activity. In this study, we cloned the cDNAs encoding two betaPix isoforms from mouse brain cDNA library. Both of the cloned genes, designated betaPix-b and betaPix-c (GenBank Accession Nos. AF247654 and AF247655, respectively), have a novel insert region consisting of 59 amino acid residues. In betaPix-c, 75 amino acid residues are deleted in the proline-rich region at the carboxyl-terminus of betaPix. In situ hybridization studies with insert region-specific probe in rat embryo show that insert region-containing isoforms are expressed mainly in the central nervous system. Moreover, temporal expression pattern of isoforms is correlated with the active neurogenesis period in the cerebral cortex and cerebellum. These results strongly suggest that betaPix isoforms may play important roles in the cellular events required for brain development such as neuronal migration.

Proteolytic cleavage of phospholipase C-gamma1 during apoptosis in Molt-4 cells

Apoptosis is a cell suicide mechanism that requires the activation of cellular death proteases for its induction. We examined whether the progress of apoptosis involves cleavage of phospholipase C-gamma1 (PLC-gamma1), which plays a pivotal role in mitogenic signaling pathway. Pretreatment of T leukemic Molt-4 cells with PLC inhibitors such as U-73122 or ET-18-OCH(3) potentiated etoposide-induced apoptosis in these cells. PLC-gamma1 was fragmented when Molt-4 cells were treated with several apoptotic stimuli such as etoposide, ceramides, and tumor necrosis factor alpha. Cleavage of PLC-gamma1 was blocked by overexpression of Bcl-2 and by specific inhibitors of caspases such as Z-DEVD-CH(2)F and YVAD-cmk. Purified caspase-3 and caspase-7, group II caspases, cleaved PLC-gamma1 in vitro and generated a cleavage product of the same size as that observed in vivo, suggesting that PLC-gamma1 is cleaved by group II caspases in vivo. From point mutagenesis studies, Ala-Glu-Pro-Asp(770) was identified to be a cleavage site within PLC-gamma1. Epidermal growth factor receptor (EGFR) -induced tyrosine phosphorylation of PLC-gamma1 resulted in resistance to cleavage by caspase-3 in vitro. Furthermore, cleaved PLC-gamma1 could not be tyrosine-phosphorylated by EGFR in vitro. In addition, tyrosine-phosphorylated PLC-gamma1 was not significantly cleaved during etoposide-induced apoptosis in Molt-4 cells. This suggests that the growth factor-induced tyrosine phosphorylation may suppress apoptosis-induced fragmentation of PLC-gamma1. We provide evidence for the biochemical relationship between PLC-gamma1-mediated signal pathway and apoptotic signal pathway, indicating that the defect of PLC-gamma1-mediated signaling pathway can facilitate an apoptotic progression.

Molecular cloning and characterization of a novel human gene (NESCA) which encodes a putative adapter protein containing SH3

A full-length cDNA encoding a novel protein was isolated and sequenced from a human placental cDNA library. This cDNA consists of 1990 bp and has a predicted open reading frame encoding 433 amino acids. It possesses an Src homology 3 (SH3) motif, a leucine zipper motif and no catalytic domain, suggesting that it seems to be an adapter protein. PCR-based mapping with both a monochromosomal hybrid panel and radiation hybrid cell panels placed the gene to human chromosome 1q21-22.