Breakpoint cluster region gene product-related domain of n-chimaerin. Discrimination between Rac-binding and GTPase-activating residues by mutational analysis

The regulation of class IA PI 3-kinases by inter-subunit interactions

Phosphoinositide 3-kinases (PI 3-kinases) are activated by growth factor and hormone receptors, and regulate cell growth, survival, motility, and responses to changes in nutritional conditions (Engelman et al. 2006). PI 3-kinases have been classified according to their subunit composition and their substrate specificity for phosphoinositides (Vanhaesebroeck et al. 2001). The class IA PI 3-kinase is a heterodimer consisting of one regulatory subunit (p85α, p85β, p55α, p50α, or p55γ) and one 110-kDa catalytic subunit (p110α, β or δ). The Class IB PI 3-kinase is also a dimer, composed of one regulatory subunit (p101 or p87) and one catalytic subunit (p110γ) (Wymann et al. 2003). Class I enzymes will utilize PI, PI[4]P, or PI[4,5]P2 as substrates in vitro, but are thought to primarily produce PI[3,4,5]P3 in cells.The crystal structure of the Class IB PI 3-kinase catalytic subunit p110γ was solved in 1999 (Walker et al. 1999), and crystal or NMR structures of the Class IA p110α catalytic subunit and all of the individual domains of the Class IA p85α regulatory subunit have been solved (Booker et al. 1992; Günther et al. 1996; Hoedemaeker et al. 1999; Huang et al. 2007; Koyama et al. 1993; Miled et al. 2007; Musacchio et al. 1996; Nolte et al. 1996; Siegal et al. 1998). However, a structure of an intact PI 3-kinase enzyme has remained elusive. In spite of this, studies over the past 10 years have lead to important insights into how the enzyme is regulated under physiological conditions. This chapter will specifically discuss the regulation of Class IA PI 3-kinase enzymatic activity, focusing on regulatory interactions between the p85 and p110 subunits and the modulation of these interactions by physiological activators and oncogenic mutations. The complex web of signaling downstream from Class IA PI 3-kinases will be discussed in other chapters in this volume.

Inhibition of Rac by the GAP activity of centralspindlin is essential for cytokinesis

During cytokinesis, the guanosine triphosphatase (GTPase) RhoA orchestrates contractile ring assembly and constriction. RhoA signaling is controlled by the central spindle, a set of microtubule bundles that forms between the separating chromosomes. Centralspindlin, a protein complex consisting of the kinesin-6 ZEN-4 and the Rho family GTPase activating protein (GAP) CYK-4, is required for central spindle assembly and cytokinesis in Caenorhabditis elegans. However, the importance of the CYK-4 GAP activity and whether it regulates RhoA remain unclear. We found that two separation-of-function mutations in the GAP domain of CYK-4 lead to cytokinesis defects that mimic centralspindlin loss of function. These defects could be rescued by depletion of the GTPase Rac or its effectors, but not by depletion of RhoA. Thus, inactivation of Rac by centralspindlin functions in parallel with RhoA activation to drive contractile ring constriction during cytokinesis.

Rac-GAP alpha-chimerin regulates motor-circuit formation as a key mediator of EphrinB3/EphA4 forward signaling

The ephrin/Eph system plays a central role in neuronal circuit formation; however, its downstream effectors are poorly understood. Here we show that alpha-chimerin Rac GTPase-activating protein mediates ephrinB3/EphA4 forward signaling. We discovered a spontaneous mouse mutation, miffy (mfy), which results in a rabbit-like hopping gait, impaired corticospinal axon guidance, and abnormal spinal central pattern generators. Using positional cloning, transgene rescue, and gene targeting, we demonstrated that loss of alpha-chimerin leads to mfy phenotypes similar to those of EphA4(-/-) and ephrinB3(-/-) mice. alpha-chimerin interacts with EphA4 and, in response to ephrinB3/EphA4 signaling, inactivates Rac, which is a positive regulator of process outgrowth. Moreover, downregulation of alpha-chimerin suppresses ephrinB3-induced growth cone collapse in cultured neurons. Our findings indicate that ephrinB3/EphA4 signaling prevents growth cone extension in motor circuit formation via alpha-chimerin-induced inactivation of Rac. They also highlight the role of a Rho family GTPase-activating protein as a key mediator of ephrin/Eph signaling.

Chimaerins: GAPs that bridge diacylglycerol signalling and the small G-protein Rac

Chimaerins are the only known RhoGAPs (Rho GTPase-activating proteins) that bind phorbol ester tumour promoters and the lipid second messenger DAG (diacylglycerol), and show specific GAP activity towards the small GTPase Rac. This review summarizes our knowledge of the structure, biochemical and biological properties of chimaerins. Recent findings have established that chimaerins are regulated by tyrosine kinase and GPCRs (G-protein-coupled receptors) via PLC (phospholipase C) activation and DAG generation to promote Rac inactivation. The finding that chimaerins, along with some other proteins, are receptors for DAG changed the prevalent view that PKC (protein kinase C) isoenzymes are the only cellular molecules regulated by DAG. In addition, vigorous recent studies have begun to decipher the critical roles of chimaerins in the central nervous system, development and tumour progression.

The zebrafish homologue of mammalian chimerin Rac-GAPs is implicated in epiboly progression during development

In this paper, we report an in vivo model for the chimerins, a family of Rac GTPase-activating proteins (Rac-GAPs) that are uniquely regulated by the lipid second messenger diacylglycerol and have been implicated in the control of actin dynamics, migration, and proliferation. We cloned the zebrafish homologue of mammalian alpha2-chimerin (chn1) and determined that it possesses Rac-GAP activity and a C1 domain with phorbol ester/diacylglycerol-binding capability. chn1 morpholino knockdown embryos exhibit severe abnormalities, including the development of round somites, lack of yolk extension, and a kinked posterior notochord. These zebrafish morphants show Rac hyperactivation and progress faster through epiboly, leading to tailbud-stage embryos that have a narrow axis and an enlarged tailbud with expanded bmp4 and shh expression. Phenotypic rescue was achieved by mRNA microinjection of chn1 or an active chimerin Rac-GAP domain into the yolk syncytial layer but not by a chn1 mutant deficient in Rac-GAP activity, suggesting that the lack of chn1 Rac-GAP activity in the yolk syncytial layer was causative of the misbalance in morphogenetic movements. Our results reveal a crucial role for chn1 in early development and implicate Rac as a key regulator of morphogenetic movements during zebrafish epiboly.

The diacylglycerol-binding protein alpha1-chimaerin regulates dendritic morphology

The morphological and functional differentiation of neuronal dendrites is controlled through transcriptional programs and cell-cell signaling. Synaptic activity is thought to play an important role in the maturation of dendritic arbors, but the signaling pathways that couple neuronal activity and morphological changes in dendrites are not well understood. We explored the function of alpha1-chimaerin, a neuronal diacylglycerol-binding protein with a Rho GTPase-activating protein domain that inactivates Rac1. We find that stimulation of phospholipase Cbeta-coupled cell surface receptors recruits alpha1-chimaerin to the plasma membrane of cultured hippocampal neurons. We further show that alpha1-chimaerin protein levels are controlled by synaptic activity and that increased alpha1-chimaerin expression results in the pruning of dendritic spines and branches. This pruning activity requires both the diacylglycerol-binding and Rac GTPase-activating protein activity of alpha1-chimaerin. Suppression of alpha1-chimaerin expression resulted in increased process growth from the dendritic shaft and from spine heads. Our data suggest that alpha1-chimaerin is an activity-regulated Rho GTPase regulator that is activated by phospholipase Cbeta-coupled cell surface receptors and contributes to pruning of dendritic arbors.

Targeting protein kinase C and "non-kinase" phorbol ester receptors: emerging concepts and therapeutic implications

Phorbol esters, natural compounds that mimic the action of the lipid second messenger diacylglycerol (DAG), are known to exert their biological actions through the activation of classical and novel protein kinase C (PKC) isozymes. Phorbol esters, via binding to the PKC C1 domains, cause major effects on mitogenesis by controlling the activity of cyclin-cdk complexes and the expression of cdk inhibitors. In the last years it became clear that phorbol esters activate other molecules having a C1 domain in addition to PKCs. One of the most interesting families of "non-kinase" phorbol ester receptors is represented by the chimaerins, lipid-regulated Rac-GAPs that modulate actin cytoskeleton reorganization, migration, and proliferation. The discovery of the chimaerins and other "non-kinase" phorbol ester receptors has major implications in the design of agents for cancer therapy.

Biology of chronic myelogenous leukemia--signaling pathways of initiation and transformation

Chronic myeloid leukemia (CML) is caused by the Bcr-Abl oncoprotein,the product of the t(9;22) chromosomal translocation that generates the Philadelphia chromosome. Different disease phenotypes are associated with each of the three Bcr-Abl isoforms: p190Bcr-Abl, p210Bcr-Abl, and p230Bcr-Abl all of which have a constitutively activated tyrosine kinase. Mechanisms associated with malignant transformation include altered cellular adhesion, activation of mitogenic signaling pathways, inhibition of apoptosis, and proteasomal degradation of physiologically important cellular proteins.CML is subject to an inexorable progression from an "indolent" chronic phase to a terminal blast crisis. Disease progression is presumed to be associated with the phenomenon of genomic instability.

A RhoGEF and Rho family GTPase-activating protein complex links the contractile ring to cortical microtubules at the onset of cytokinesis

The mechanism that positions the cytokinetic contractile ring is unknown, but derives from the spindle midzone. We show that an interaction between the Rho GTP exchange factor, Pebble, and the Rho family GTPase-activating protein, RacGAP50C, connects the contractile ring to cortical microtubules at the site of furrowing in D. melanogaster cells. Pebble regulates actomyosin organization, while RacGAP50C and its binding partner, the Pavarotti kinesin-like protein, regulate microtubule bundling. All three factors are required for cytokinesis. As furrowing begins, these proteins colocalize to a cortical equatorial ring. We propose that RacGAP50C-Pavarotti complexes travel on cortical microtubules to the cell equator, where they associate with the Pebble RhoGEF to position contractile ring formation and coordinate F-actin and microtubule remodeling during cytokinesis.

BCR/ABL genes and leukemic phenotype: from molecular mechanisms to clinical correlations

The Philadelphia chromosome (Ph), a minute chromosome that derives from the balanced translocation between chromosomes 9 and 22, was first described in 1960 and was for a long time the only genetic lesion consistently associated with human cancer. This chromosomal translocation results in the fusion between the 5' part of BCR gene, normally located on chromosome 22, and the 3' part of the ABL gene on chromosome 9 giving origin to a BCR/ABL fusion gene which is transcribed and then translated into a hybrid protein. Three main variants of the BCR/ABL gene have been described, that, depending on the length of the sequence of the BCR gene included, encode for the p190(BCR/ABL), P210(BCR/ABL), and P230(BCR/ABL) proteins. These three main variants are associated with distinct clinical types of human leukemias. Herein we review the data on the correlations between the type of BCR/ABL gene and the corresponding leukemic clinical features. Lastly, drawing on experimental data, we provide insight into the different transforming power of the three hybrid BCR/ABL proteins.

Cdc42Hs facilitates cytoskeletal reorganization and neurite outgrowth by localizing the 58-kD insulin receptor substrate to filamentous actin

Cdc42Hs is involved in cytoskeletal reorganization and is required for neurite outgrowth in N1E-115 cells. To investigate the molecular mechanism by which Cdc42Hs regulates these processes, a search for novel Cdc42Hs protein partners was undertaken by yeast two-hybrid assay. Here, we identify the 58-kD substrate of the insulin receptor tyrosine kinase (IRS-58) as a Cdc42Hs target. IRS-58 is a brain-enriched protein comprising at least four protein-protein interaction sites: a Cdc42Hs binding site, an Src homology (SH)3-binding site, an SH3 domain, and a tryptophan, tyrptophan (WW)-binding domain. Expression of IRS-58 in Swiss 3T3 cells leads to reorganization of the filamentous (F)-actin cytoskeleton, involving loss of stress fibers and formation of filopodia and clusters. In N1E-115 cells IRS-58 induces neurite outgrowth with high complexity. Expression of a deletion mutant of IRS-58, which lacks the SH3- and WW-binding domains, induced neurite extension without complexity in N1E-115 cells. In Swiss 3T3 cells and N1E-115 cells, IRS-58 colocalizes with F-actin in clusters and filopodia. An IRS-58(1267N) mutant unable to bind Cdc42Hs failed to localize with F-actin to induce neurite outgrowth or significant cytoskeletal reorganization. These results suggest that Cdc42Hs facilitates cytoskeletal reorganization and neurite outgrowth by localizing protein complexes via adaptor proteins such as IRS-58 to F-actin.

DRacGAP, a novel Drosophila gene, inhibits EGFR/Ras signalling in the developing imaginal wing disc

We have identified a novel Drosophila gene, DRacGAP, which behaves as a negative regulator of Ρ-family GTPases DRac1 and DCdc42. Reduced function of DRacGAP or increased expression of DRac1 in the wing imaginal disc cause similar effects on vein and sensory organ development and cell proliferation. These effects result from enhanced activity of the EGFR/Ras signalling pathway. We find that in the wing disc, DRac1 enhances EGFR/Ras-dependent activation of MAP Kinase in the prospective veins. Interestingly, DRacGAP expression is negatively regulated by the EGFR/Ras pathway in these regions. During vein formation, local DRacGAP repression would ensure maximal activity of Rac and, in turn, of Ras pathways in vein territories. Additionally, maximal expression of DRacGAP at the vein/intervein boundaries would help to refine the width of the veins. Hence, control of DRacGAP expression by the EGFR/Ras pathway is a previously undescribed feedback mechanism modulating the intensity and/or duration of its signalling during Drosophila development.

MgcRacGAP is involved in the control of growth and differentiation of hematopoietic cells

In a search for key molecules that prevent murine M1 leukemia cells from undergoing interleukin (IL)-6–induced differentiation into macrophages, we isolated an antisense complementary DNA (cDNA) that encodes full-length mouse MgcRac-GTPase-activating protein (GAP) through functional cloning. Forced expression of this antisense cDNA profoundly inhibited IL-6–induced differentiation of M1 cells into macrophage lineages. We also isolated a full-length human MgcRacGAP cDNA, which encodes an additional N-terminal polypeptide of 105 amino acid residues compared with the previously published human MgcRacGAP. In human HL-60 leukemic cells, overexpression of the full-length form of human MgcRacGAP alone induced growth suppression and macrophage differentiation associated with hypervacuolization and de novo expression of the myelomonocytic marker CD14. Analyses using a GAP-inactive mutant and 2 deletion mutants of MgcRacGAP indicated that the GAP activity was dispensable, but the myosin-like domain and the cysteine-rich domain were indispensable for growth suppression and macrophage differentiation. The present results indicated that MgcRacGAP plays key roles in controlling growth and differentiation of hematopoietic cells through mechanisms other than regulating Rac GTPase activity.

MgcRacGAP is involved in the control of growth and differentiation of hematopoietic cells

In a search for key molecules that prevent murine M1 leukemia cells from undergoing interleukin (IL)-6–induced differentiation into macrophages, we isolated an antisense complementary DNA (cDNA) that encodes full-length mouse MgcRac-GTPase-activating protein (GAP) through functional cloning. Forced expression of this antisense cDNA profoundly inhibited IL-6–induced differentiation of M1 cells into macrophage lineages. We also isolated a full-length human MgcRacGAP cDNA, which encodes an additional N-terminal polypeptide of 105 amino acid residues compared with the previously published human MgcRacGAP. In human HL-60 leukemic cells, overexpression of the full-length form of human MgcRacGAP alone induced growth suppression and macrophage differentiation associated with hypervacuolization and de novo expression of the myelomonocytic marker CD14. Analyses using a GAP-inactive mutant and 2 deletion mutants of MgcRacGAP indicated that the GAP activity was dispensable, but the myosin-like domain and the cysteine-rich domain were indispensable for growth suppression and macrophage differentiation. The present results indicated that MgcRacGAP plays key roles in controlling growth and differentiation of hematopoietic cells through mechanisms other than regulating Rac GTPase activity.

Evidence for a novel Cdc42GAP domain at the carboxyl terminus of BNIP-2

We recently identified BNIP-2, a previously cloned Bcl-2- and E1B-associated protein, as a putative substrate of the FGF receptor tyrosine kinase and showed that it possesses GTPase-activating activity toward Cdc42 despite the lack of homology to previously described catalytic domains of GTPase-activating proteins (GAPs). BNIP-2 contains many arginine residues at the carboxyl terminus, which includes the region of homology to the noncatalytic domain of Cdc42GAP, termed BNIP-2 and Cdc42GAP homology (BCH) domain. Using BNIP-2 glutathione S-transferase recombinants, it was found that its BCH bound Cdc42, and contributed the GAP activity. This domain was predicted to fold into alpha-helical bundles similar to the topology of the catalytic GAP domain of Cdc42GAP. Alignment of exposed arginine residues in this domain helped to identify Arg-235 and Arg-238 as good candidates for catalysis. Arg-238 matched well to the arginine "finger" required for enhanced GTP hydrolysis in homodimerized Cdc42. Site-directed mutagenesis confirmed that an R235K or R238K mutation severely impaired the BNIP-2 GAP activity without affecting its binding to Cdc42. From deletion studies, a region adjacent to the arginine patch ((288)EYV(290) on BNIP-2) and the Switch I and Rho family-specific "Insert" region on Cdc42 are involved in the binding. The results indicate that the BCH domain of BNIP-2 represents a novel GAP domain that employs an arginine patch motif similar to that of the Cdc42-homodimer.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

Structure and function of phosphoinositide 3-kinases

Phosphoinositide kinases (PI3Ks) play an important role in mitogenic signaling and cell survival, cytoskeletal remodeling, metabolic control and vesicular trafficking. Here we summarize the structure-function relationships delineating the activation process of class I PI3Ks involving various domains of adapter subunits, Ras, and interacting proteins. The resulting product, PtdIns(3,4,5)P3, targets Akt/protein kinase B (PKB), Bruton's tyrosine kinase (Btk), phosphoinositide-dependent kinases (PDK), integrin-linked kinase (ILK), atypical protein kinases C (PKC), phospholipase Cgamma and more. Surface receptor-activated PI3Ks function in mammals, insects, nematodes and slime mold, but not yeast. While many members of the class II family have been identified and characterized biochemically, it is presently unknown how these C2-domain containing PI3Ks are activated, and which PI substrate they phosphorylate in vivo. PtdIns 3-P is produced by Vps34p/class III PI3Ks and operates via the PtdIns 3-P-binding proteins early endosomal antigen (EEA1), yeast Vac1p, Vps27p, Pip1p in lysosomal protein targeting. Besides the production of D3 phosphorylated lipids, PI3Ks have an intrinsic protein kinase activity. For trimeric GTP-binding protein-activated PI3Kgamma, protein kinase activity seems to be sufficient to trigger mitogen-activated protein kinase (MAPK). Recent disruption of PI3K genes in slime mold, Caenorhabditis elegans, Drosophila melanogaster and mice further underlines the importance of PI3K signaling systems and elucidates the role of PI3K signaling in multicellular organisms.

Biochemical studies of the mechanism of action of the Cdc42-GTPase-activating protein

The small GTP-binding proteins Rac, Rho, and Cdc42 were shown to mediate a variety of signaling pathways including cytoskeletal rearrangements, cell-cycle progression, and transformation. Key to the proper function of these GTP-binding proteins is an efficient shut-off mechanism that ensures the decay of the signal. Regulatory proteins termed GAPs (GTPase-activating proteins) enhance the intrinsic GTP hydrolysis of the GTP-binding proteins, thereby ensuring signal termination. We have used site-specific mutagenesis to elucidate the limit domain for GAP activity in Cdc42-GAP, and show that in addition to the known GAP-homology domain (three conserved boxes), a C-terminal region outside that domain is also essential for GAP activity. In addition, we have replaced the conserved arginine (Arg305), which was suggested by structural studies to be a key catalytic residue, with an alanine and found that the R305A Cdc42-GAP mutant has a greatly diminished catalytic capacity but is still able to bind Cdc42 with high affinity. Thus, a key catalytic role for this residue is confirmed. However, we also present evidence for the involvement of an additional residue(s), since the R305A Cdc42-GAP mutant still exhibits measurable activity. Some of this residual activity might result from a neighboring arginine, since a double mutant R305A/R306A shows a further decrease in catalytic activity.

Cryptic Rac-binding and p21(Cdc42Hs/Rac)-activated kinase phosphorylation sites of NADPH oxidase component p67(phox)

Rac1 is a member of the Rho family of small molecular mass GTPases that act as molecular switches to control actin-based cell morphology as well as cell growth and differentiation. Rac1 and Rac2 are specifically required for superoxide formation by components of the NADPH oxidase. In binding assays, Rac1 interacts directly with p67(phox), but not with the other oxidase components: cytochrome b, p40(phox), or p47(phox) (Prigmore, E., Ahmed, S., Best, A., Kozma, R. , Manser, E., Segal, A. W., and Lim, L. (1995) J. Biol. Chem. 270, 10717-10722). Here, the Rac1/2 interaction with p67(phox) has been characterized further. Rac1 and Rac2 can bind to p67(phox) amino acid residues 170-199, and the N terminus (amino acids 1-192) of p67(phox) can be used as a specific inhibitor of Rac signaling. Deletion of p67(phox) C-terminal sequences (amino acids 193-526), the C-terminal SH3 domain (amino acids 470-526), or the polyproline-rich motif (amino acids 226-236) stimulates Rac1 binding by approximately 8-fold. p21(Cdc42Hs/Rac)-activated kinase (PAK) phosphorylates p67(phox) amino acid residues adjacent to the Rac1/2-binding site, and this phosphorylation is stimulated by deletion of the C-terminal SH3 domain or the polyproline-rich motif. These data suggest a role for cryptic Rac-binding and PAK phosphorylation sites of p67(phox) in control of the NADPH oxidase.

MgcRacGAP, a new human GTPase-activating protein for Rac and Cdc42 similar to Drosophila rotundRacGAP gene product, is expressed in male germ cells

In a search for new partners of the activated form of Rac GTPase, we have isolated through a two-hybrid cloning procedure a human cDNA encoding a new GTPase-activating protein (GAP) for Rho family GTPases. A specific mRNA of 3.2 kilobases was detected in low abundance in many cell types and found highly expressed in testis. A protein of the predicted size 58 kDa, which we call MgcRacGAP, was detected in human testis as well as in germ cell tumor extracts by immunoblotting with antibodies specific to recombinant protein. In vitro, the GAP domain of MgcRacGAP strongly stimulates Rac1 and Cdc42 GTPase activity but is almost inactive on RhoA. N-terminal to its GAP domain, MgcRacGAP contains a cysteine-rich zinc finger-like motif characteristic of the Chimaerin family of RhoGAPs. The closest homolog of MgcRacGAP is RotundRacGAP, a product of the Drosophila rotund locus. In situ hybridization experiments in human testis demonstrate a specific expression of mgcRacGAP mRNA in spermatocytes similar to that of rotundRacGAP in Drosophila testis. Therefore, protein sequence similarity and analogous developmental and tissue specificities of gene expression support the hypothesis that RotundRacGAP and MgcRacGAP have equivalent functions in insect and mammalian germ cells. Since rotundRacGAP deletion leads to male sterility in the fruit fly, the mgcRacGAP gene may prove likewise to play a key role in mammalian male fertility.

Fluoride activation of the Rho family GTP-binding protein Cdc42Hs

Aluminum tetrafluoride (AlF4-) activation of heterotrimeric G-protein alpha-subunits is a well established aspect of the biochemistry of these proteins; however, until recently it has been thought that AlF4- does not mediate effects on the Ras superfamily of low molecular weight GTP-binding proteins. Recent work demonstrating aluminum fluoride-induced complex formation between Ras and its GTPase-activating proteins (RasGAP and NF1) has provided important insights into the mechanism of GAP-stimulated GTP hydrolysis. We have characterized the AlF4--induced complex formation between the GDP-bound form of the Rho subfamily G-protein Cdc42Hs and a limit functional domain of the Cdc42-GAP using a variety of biochemical techniques. Our results indicate that the apparent affinity of GAP for the AlF4--mediated complex is similar to the affinity observed for the activated (GTP-bound) form of Cdc42 and that beryllium (Be) can replace aluminum in mediating fluoride-induced complex formation. Additionally, the AlF4--induced interaction is weakened significantly by the catalytically compromised GAP(R305A) mutant, indicating that this arginine is critical in transition state stabilization. Unlike Ras, we find that AlF4- and BeF3- mediate complex formation between Cdc42Hs.GDP and downstream target/effector molecules, indicating that there are important differences in the mechanism of effector binding between the Ras and Rho subfamily G-proteins.

The rat myosin myr 5 is a GTPase-activating protein for Rho in vivo: essential role of arginine 1695

myr 5 is an unconventional myosin (class IX) from rat that contains a Rho-family GTPase-activating protein (GAP) domain. Herein we addressed the specificity of the myr 5 GAP activity, the molecular mechanism by which GAPs activate GTP hydrolysis, the consequences of myr 5 overexpression in living cells, and its subcellular localization. The myr 5 GAP activity exhibits a high specificity for Rho. To achieve similar rates of GTPase activation for RhoA, Cdc42Hs, and Rac1, a 100-fold or 1000-fold higher concentration of recombinant myr 5 GAP domain was needed for Cdc42Hs or Rac1, respectively, as compared with RhoA. Cell lysates from Sf9 insect cells infected with recombinant baculovirus encoding myr 5 exhibited increased GAP activity for RhoA but not for Cdc42Hs or Rac1. Analysis of Rho-family GAP domain sequences for conserved arginine residues that might contribute to accelerate GTP hydrolysis revealed a single conserved arginine residue. Mutation of the corresponding arginine residue in the myr 5 GAP domain to a methionine (M1695) virtually abolished Rho-GAP activity. Expression of myr 5 in Sf9 insect cells induced the formation of numerous long thin processes containing occasional varicosities. Such morphological changes were dependent on the myr 5 Rho-GAP activity, because they were induced by expression the myr 5 tail or just the myr 5 Rho-GAP domain but not by expressing the myr 5 myosin domain. Expression of myr 5 in mammalian normal rat kidney (NRK) or HtTA-1 HeLa cells induced a loss of actin stress fibers and focal contacts with concomitant morphological changes and rounding up of the cells. Similar morphological changes were observed in HtTA-1 HeLa cells expressing just the myr 5 Rho-GAP domain but not in cells expressing myr 5 M1695. These morphological changes induced by myr 5 were inhibited by coexpression of RhoV14, which is defective in GTP hydrolysis, but not by RhoI117. myr 5 was localized in dynamic regions of the cell periphery, in the perinuclear region in the Golgi area, along stress fibers, and in the cytosol. These results demonstrate that myr 5 has in vitro and in vivo Rho-GAP activity. No evidence for a Rho effector function of the myr 5 myosin domain was obtained.

Crystal structure of the breakpoint cluster region-homology domain from phosphoinositide 3-kinase p85 alpha subunit

Proteins such as the product of the break-point cluster region, chimaerin, and the Src homology 3-binding protein 3BP1, are GTPase activating proteins (GAPs) for members of the Rho subfamily of small GTP-binding proteins (G proteins or GTPases). A 200-residue region, named the breakpoint cluster region-homology (BH) domain, is responsible for the GAP activity. We describe here the crystal structure of the BH domain from the p85 subunit of phosphatidylinositol 3-kinase at 2.0 A resolution. The domain is composed of seven helices, having a previously unobserved arrangement. A core of four helices contains most residues that are conserved in the BH family. Their packing suggests the location of a G-protein binding site. This structure of a GAP-like domain for small GTP-binding proteins provides a framework for analyzing the function of this class of molecules.

The Ras-related GTPase Rac1 binds tubulin

The Ras-related Rho family are involved in controlling actin-based changes in cell morphology. Microinjection of Rac1, RhoA, and Cdc42Hs into Swiss 3T3 cells induces pinocytosis and membrane ruffling, stress fiber formation, and filopodia formation, respectively. To identify target proteins involved in these signaling pathways cell extracts immobilized on nitrocellulose have been probed with [gamma-32P]GTP-labeled Rac1, RhoA, and Cdc42Hs. We have identified two 55-kDa brain proteins which bind Rac1 but not RhoA or Cdc42Hs. These 55-kDa proteins were abundant, had pI values of around 5.5, and could be purified by Q-Sepharose chromatography. The characteristics on two-dimensional gel analysis suggested the proteins comprised alpha- and beta-tubulin. Indeed, beta-tubulin specific antibodies detected one of the purified 55-kDa proteins. Rac1 bound pure tubulin (purified by cycles of polymerization and depolymerization) only in the GTP-bound state. The GTPase negative Rac1 point mutants, G12V and Q61L, did not significantly affect the ability of Rac1 to interact with tubulin while the "effector-site" mutant D38A prevented interaction. These results suggest that the Rac1-tubulin interaction may play a role in Rac1 function.

A 68-kDa kinase and NADPH oxidase component p67phox are targets for Cdc42Hs and Rac1 in neutrophils

Cdc42Hs and Rac1 are members of the Ras superfamily of small molecular weight (p21) GTP binding proteins. Cdc42Hs induces filopodia formation in Swiss 3T3 fibroblasts while Rac1 induces membrane ruffling. Rac1 also activates superoxide production by the components (cytochrome b, p40phox, p67phox, and p47phox) of the neutrophil oxidase. To isolate target proteins involved in these signaling pathways, we have probed proteins from neutrophil cytosol immobilized on nitrocellulose with Cdc42Hs labeled with [gamma-32P]GTP. Cdc42Hs probe detected binding protein(s) of 66-68 kDa in neutrophil cytosol. Rac1 probe also detected the 66-68-kDa proteins, suggesting the possibility that p67phox may be a binding protein for both of these p21 proteins. Indeed, Cdc42Hs and Rac1 were found to bind specifically to purified recombinant p67phox but not the other oxidase components. A 68-kDa Cdc42Hs binding protein was purified from neutrophil cytosol and found to be related to the recently described p65pak kinase from brain. These results suggest that the p68 kinase and p67phox are targets for Cdc42Hs and Rac1 in neutrophils.

Increased neutrophil respiratory burst in bcr-null mutants

Philadelphia (Ph)-positive leukemias invariably contain a chromosomal translocation fusing BCR to ABL. The BCR-ABL protein is responsible for leukemogenesis. Here we show that exposure of bcr-null mutant mice to gram-negative endotoxin led to severe septic shock and increased tissue injury by neutrophils. Neutrophils of bcr (-/-) mice showed a pronounced increase in reactive oxygen metabolite production upon activation and were more sensitive to priming stimuli. Activated (-/-) neutrophils displayed a 3-fold increased p21rac2 membrane translocation compared with (+/+) neutrophils. These results connect Bcr in vivo with the regulation of Rac-mediated superoxide production by the NADPH-oxidase system of leukocytes and suggest a link between Bcr function and the cell type affected in Ph-positive leukemia.

GAPs for rho-related GTPases

Ras-related GTP-binding proteins (GTPases) of the rho subfamily play important roles in regulating the organization of the actin cytoskeleton. A large number of multifunctional proteins that can stimulate their intrinsic GTPase activity have been identified. Here, we discuss the nature of such GTPase-activating proteins (GAPs) and their potential importance for cell signalling.