A novel serine kinase activated by rac1/CDC42Hs-dependent autophosphorylation is related to PAK65 and STE20

RhoG GTPase controls a pathway that independently activates Rac1 and Cdc42Hs

RhoG is a member of the Rho family of GTPases that shares 72% and 62% sequence identity with Rac1 and Cdc42Hs, respectively. We have expressed mutant RhoG proteins fused to the green fluorescent protein and analyzed subsequent changes in cell surface morphology and modifications of cytoskeletal structures. In rat and mouse fibroblasts, green fluorescent protein chimera and endogenous RhoG proteins colocalize according to a tubular cytoplasmic pattern, with perinuclear accumulation and local concentration at the plasma membrane. Constitutively active RhoG proteins produce morphological and cytoskeletal changes similar to those elicited by a simultaneous activation of Rac1 and Cdc42Hs, i.e., the formation of ruffles, lamellipodia, filopodia, and partial loss of stress fibers. In addition, RhoG and Cdc42Hs promote the formation of microvilli at the cell apical membrane. RhoG-dependent events are not mediated through a direct interaction with Rac1 and Cdc42Hs targets such as PAK-1, POR1, or WASP proteins but require endogenous Rac1 and Cdc42Hs activities: coexpression of a dominant negative Rac1 impairs membrane ruffling and lamellipodia but not filopodia or microvilli formation. Conversely, coexpression of a dominant negative Cdc42Hs only blocks microvilli and filopodia, but not membrane ruffling and lamellipodia. Microtubule depolymerization upon nocodazole treatment leads to a loss of RhoG protein from the cell periphery associated with a reversal of the RhoG phenotype, whereas PDGF or bradykinin stimulation of nocodazole-treated cells could still promote Rac1- and Cdc42Hs-dependent cytoskeletal reorganization. Therefore, our data demonstrate that RhoG controls a pathway that requires the microtubule network and activates Rac1 and Cdc42Hs independently of their growth factor signaling pathways.

Delineation of the Cdc42/Rac-binding domain of p21-activated kinase

p21-activated kinases (PAKs) serve as effector proteins for the GTP-binding proteins Cdc42 and Rac. They are serine/threonine kinases containing the Cdc42/Rac interactive binding (CRIB) motif. The main aim of this study was to define the minimal domain of alphaPAK required for Cdc42/Rac binding. Eight stable PAK fragments of varying lengths, each containing the CRIB motif (residues 75-88), were expressed in Escherichia coli, and their ability to interact with Cdc42 and Rac was assessed using scintillation proximity assays, isothermal titration calorimetry, and fluorescence techniques. The shortest fragments examined (residues 70-94 and 75-94) bound only weakly to either Cdc42 or Rac. A longer fragment starting at residue 75 and ending at residue 105 showed binding to Q61L Rac.GTP with Kd = 1.9 microM. Highest affinity binding (Kd approximately 0.05 microM) was seen with longer fragments ending at residue 118 or 132. A small increase in affinity was seen with those fragments starting at residue 70 rather than residue 75. PAK fragments bound with approximately 3-10-fold higher affinity to Cdc42 than to Rac and bound Q61L variants with 5-10-fold higher affinity than wild type. The dissociation rates of Q61L Rac.mant-GTP and of Q61L Cdc42. mant-GTP from PAK fragment residues 70-132 were measured to be 0.66 and 0.25 min-1, respectively, which are 100-fold lower than dissociation rates for Ras:Ras-effector domains, although their affinities are similar. Calorimetric measurements revealed that binding was associated with a relatively slow heat change. It is suggested that these PAK fragments (in the absence of Cdc42 or Rac) might exist predominantly in an inactive conformation that slowly interconverts with an active conformation and/or a slow conformational change may occur upon binding to Cdc42/Rac. In conclusion, the PAK CRIB motif itself is insufficient for high-affinity binding to Cdc42/Rac, but a 30 amino acid region of PAK (residues 75-105), containing this motif, is sufficient.

A role for Pak protein kinases in Schwann cell transformation

Neurofibromatosis type 1 (NF1), a common autosomal dominant disorder caused by loss of the NF1 gene, is characterized clinically by neurofibromas and more rarely by neurofibrosarcomas. Neurofibromin, the protein encoded by NF1, possesses an intrinsic GTPase accelerating activity for the Ras proto-oncogene. Through this activity, it is a negative regulator of Ras. The Pak protein kinase is a candidate for a downstream signaling protein that may mediate Ras signals because it is activated by Rac and Cdc42, two small G proteins required for Ras signaling. Here, we use Pak mutants to explore the role of Pak in Ras signaling in Schwann cells, the cells affected in NF1. Whereas an activated Pak mutant does not transform cells, dominant negative Pak mutants are potent inhibitors of Ras transformation of rat Schwann cells and of a neurofibrosarcoma cell line from an NF1 patient. Although activated Pak stimulated jun-N-terminal kinase, inhibition of Ras transformation by dominant negative Pak did not require inhibition of jun-N-terminal kinase. Instead, the Pak mutants appeared to inhibit transformation by preventing Ras activation of the ERK/mitogen-activated protein kinase cascade. These results have implications for our understanding of NF1 because a neurofibrosarcoma cell line derived from a patient with NF1 was reverted by stable expression of the Pak dominant negative mutants.

Interaction of Rac1 with GTPase-activating proteins and putative effectors. A comparison with Cdc42 and RhoA

The intrinsic GTPase activity of the Rho family GTP-binding protein Rac1 is drastically stimulated upon interaction with its GTPase-activating proteins (GAPs) and is significantly inhibited when coupled to certain effector targets such as the p21-activated kinases (PAKs) and IQGAPs. Here we have characterized the interaction of Rac1 with a panel of mammalian GAPs and putative effectors by measuring the kinetic and binding parameters involved and made comparisons with similar interactions for Cdc42 and RhoA. In contrast with Cdc42 (for which the GAP domain of p50RhoGAP is 50-fold more efficient than those of p190, Bcr, and 3BP-1) and with RhoA (toward which only p50RhoGAP and p190 displayed high efficiencies), the catalytic efficiencies (Kcat/Km) of the GAP domains of p50RhoGAP, p190, Bcr, and 3BP-1 on Rac1 are found to be comparable in a range between 0.9 and 2.6 min-1 microM-1. However, similar to the cases of Cdc42 and RhoA, the Km values of the GAP domains on Rac1 compare well to the binding affinity to the guanylyl imidodiphosphate-bound Rac1, which ranges from 10.5 to 40.5 microM, suggesting a rapid equilibrium reaction mechanism. The dissociation constants of the p21-binding domains of PAK1, PAK2, and the RasGAP-related domain of IQGAP1, which all cause significant reduction of the intrinsic rate of GTP hydrolysis upon binding to Rac1-GTP, are found to be 0.71, 0.26, and 2.13 microM for Rac1-GTP, compared with that determined for Cdc42-GTP at 2.9, 20.5, and 0.39 microM, respectively, under similar conditions. These results suggest that p50RhoGAP, p190, Bcr, and 3BP-1 are all capable of acting as a negative regulator for Rac1-mediated signaling, and that, although PAK1 and IQGAP1 can couple tightly with both Rac1 and Cdc42, PAK2 is likely to be a specific effector for Rac1 instead of Cdc42.

A conserved negative regulatory region in alphaPAK: inhibition of PAK kinases reveals their morphological roles downstream of Cdc42 and Rac1

AlphaPAK in a constitutively active form can exert morphological effects (E. Manser, H.-Y. Huang, T.-H. Loo, X.-Q. Chen, J.-M. Dong, T. Leung, and L. Lim, Mol. Cell. Biol. 17:1129-1143, 1997) resembling those of Cdc42G12V. PAK family kinases, conserved from yeasts to humans, are directly activated by Cdc42 or Rac1 through interaction with a conserved N-terminal motif (corresponding to residues 71 to 137 in alphaPAK). alphaPAK mutants with substitutions in this motif that resulted in severely reduced Cdc42 binding can be recruited normally to Cdc42G12V-driven focal complexes. Mutation of residues in the C-terminal portion of the motif (residues 101 to 137), though not affecting Cdc42 binding, produced a constitutively active kinase, suggesting this to be a negative regulatory region. Indeed, a 67-residue polypeptide encoding alphaPAK83-149 potently inhibited GTPgammaS-bound Cdc42-mediated kinase activation of both alphaPAK and betaPAK. Coexpression of this PAK inhibitor with Cdc42G12V prevented the formation of peripheral actin microspikes and associated loss of stress fibers normally induced by the p21. Coexpression of PAK inhibitor with Rac1G12V also prevented loss of stress fibers but not ruffling induced by the p21. Coexpression of alphaPAK83-149 completely blocked the phenotypic effects of hyperactive alphaPAKL107F in promoting dissolution of focal adhesions and actin stress fibers. These results, coupled with previous observations with constitutively active PAK, demonstrate that these kinases play an important role downstream of Cdc42 and Rac1 in cytoskeletal reorganization.

The small GTP-binding protein Cdc42 is required for nerve growth factor withdrawal-induced neuronal death

An increase in the level of the c-Jun transcription factor and of its phosphorylation has previously been shown to be essential for nerve growth factor (NGF) withdrawal-induced apoptosis of rat sympathetic neurons (SCG). The Rho-like GTPases Cdc42 and Rac1 are involved in the regulation of a number of cellular processes, including activation of the c-Jun NH2-terminal kinase (JNK) pathway. Therefore, we have investigated the role of these GTPases in this process. Overexpression of activated Rac1 or Cdc42 in SCG neurons maintained in the presence of NGF induced apoptosis, whereas expression of dominant negative mutants of Cdc42 or Rac1 blocked apoptosis following NGF withdrawal. Cdc42 activation produced an increase in the level of c-Jun and of its phosphorylation. Furthermore, Cdc42-induced death was prevented by coexpressing the c-Jun dominant negative FLAGDelta169. Thus, Cdc42 appears to function as an initiator of neuronal cell death by activating a transcriptional pathway regulated by c-Jun.

The Rho family G proteins play a critical role in muscle differentiation

The Rho family GTP-binding proteins play a critical role in a variety of cytoskeleton-dependent cell functions. In this study, we examined the role of Rho family G proteins in muscle differentiation. Dominant negative forms of Rho family proteins and RhoGDI, a GDP dissociation inhibitor, suppressed transcription of muscle-specific genes, while mutationally activated forms of Rho family proteins strongly activated their transcription. C2C12 cells overexpressing RhoGDI (C2C12RhoGDI cells) did not differentiate into myotubes, and expression levels of myogenin, MRF4, and contractile protein genes but not MyoD and myf5 genes were markedly reduced in C2C12RhoGDI cells. The promoter activity of the myogenin gene was suppressed by dominant negative mutants of Rho family proteins and was reduced in C2C12RhoGDI cells. Expression of myocyte enhancer binding factor 2 (MEF2), which has been reported to be required for the expression of the myogenin gene, was reduced at the mRNA and protein levels in C2C12RhoGDI cells. These results suggest that the Rho family proteins play a critical role in muscle differentiation, possibly by regulating the expression of the myogenin and MEF2 genes.

Epidermal growth factor induction of the c-jun promoter by a Rac pathway

The c-jun proto-oncogene encodes a transcription factor which is activated by mitogens both transcriptionally and by phosphorylation by Jun N-terminal kinase (JNK). We have investigated the cellular signalling pathways involved in epidermal growth factor (EGF) induction of the c-jun promoter. We find that two sequence elements, which bind ATF1 and MEF2D transcription factors, are required in HeLa cells, although they are not sufficient for maximal induction. Activated forms of Ras, RacI, Cdc42Hs, and MEKK increased expression of the c-jun promoter, while dominant negative forms of Ras, RacI, and MEK kinase (MEKK) inhibited EGF induction. These and previously published results suggest that EGF activates the c-jun promoter by a Ras-to-Rac-to-MEKK pathway. This pathway is similar to that used for posttranslational activation of c-jun by JNK.

Multiple interactions of PRK1 with RhoA. Functional assignment of the Hr1 repeat motif

PRK1 (PKN) is a serine/threonine kinase that has been shown to be activated by RhoA (Amano, M., Mukai, H., Ono, Y., Chihara, K., Matsui, T., Hamajima, Y., Okawa, K., Iwamatsu, A., and Kaibuchi, K. (1996) Science 271, 648-650). Detailed analysis of the PRK1 region involved in RhoA binding has revealed that two homologous sequences within the HR1 domain (HR1a and HR1b) both bind to RhoA; the third repeat within this domain, HR1cPRK1, does not bind RhoA. The related HR1 motif is also found to confer RhoA binding activity to the only other fully cloned member of this kinase family, PRK2. Furthermore, the predictive value of this motif is established for an HR1a sequence derived from a Caenorhabditis elegans open reading frame encoding a protein kinase of unknown function. Interestingly, the HR1aPRK1 and HR1bPRK1 subdomains are shown to display a distinctive nucleotide dependence for RhoA binding. HRIaPRK1 is entirely GTP-dependent, while HR1bPRK1 binds both GTP- and GDP-bound forms of RhoA. This distinction indicates that there are two sites of contact between RhoA and PRK1, one contact through a region that is conformationally dependent upon the nucleotide-bound state of RhoA and one that is not. Analysis of binding to Rho/Rac chimera provides evidence for a HR1aPRK1 but not HR1bPRK1 interaction in the central third of Rho. Additionally, it is observed that the V14RhoA mutant binds HR1a but does not bind HR1b. This distinct binding behavior corroborates the conclusion that there are independent contacts on RhoA for the HR1aPRK1 and HR1bPRK1 motifs.

Genetic analysis of the actin cytoskeleton in the Drosophila ovary

The Drosophila ovary provides a favorable model system in which to study cellular morphogenesis. The development of a mature egg involves a syncytium of 16 germline cells and over 1000 somatically derived follicle cells. Intercellular transport, stable intercellular bridges, cell migrations, cell shape changes, and specific subcellular localization of many embryonic patterning determinants contribute to egg development and require a dynamic cytoskeleton. We discuss many of the recent genetic and cell biological studies that have led to insights into how the actin cytoskeleton is assembled and regulated during the morphogenesis of the Drosophila egg.

Identification of the thyroid transcription factor-1 as a target for rat MST2 kinase

Thyroid transcription factor-1 (TTF-1) is a homeodomain-containing transcription factor that is required for thyroid-specific expression of the thyroglobulin and thyroperoxidase genes as well as for lung-specific expression of the surfactant protein A, B, and C and the CC10 and the HNF-3 alpha genes. TTF-1 is a phosphoprotein, and the phosphorylation of TTF-1 has been studied already. However, the kinase(s) that could be responsible for this phosphorylation have not been known. In this paper we report the identification by in-gel kinase assay of a 56-kDa serine/threonine kinase that is able to phosphorylate TTF-1 in thyroid cells. The cloning of this kinase revealed that we had identified the rat homolog of the human MST2 kinase. The pathway in which human MST2 functions is not known; however, it does not appear to involve either mitogen-activated protein kinases such as Erk1 and Erk2 nor the stress-activated protein kinases such as JNK and p38. We show that the activity responsible for TTF-1 phosphorylation is constitutive in thyroid cells. Furthermore, we demonstrate that TTF-1 is phosphorylated in vivo by rMST2 at the same residues that had been identified previously as the major phosphorylation sites. Thus, TTF-1 represents the first identified target of this class of protein kinases.

p140Sra-1 (specifically Rac1-associated protein) is a novel specific target for Rac1 small GTPase

Rac1 small GTPase plays pivotal roles in various cell functions such as cell morphology, cell polarity, and cell proliferation. We have previously identified IQGAP1 from bovine brain cytosol as a target for Rac1 by an affinity purification method. By using the same method, we purified a specifically Rac1-associated protein with a molecular mass of about 140 kDa (p140) from bovine brain cytosol. This protein interacted with guanosine 5'-(3-O-thio)triphosphate (GTPgammaS).glutathione S-transferase (GST)-Rac1 but not with the GDP.GST-Rac1, GTPgammaS.GST-Cdc42, or GTPgammaS.GST-RhoA. The amino acid sequences of this protein revealed that p140 is identified as a product of KIAA0068 gene. We denoted this protein as Sra-1 (Specifically Rac1-associated protein). Recombinant Sra-1 interacted with GTPgammaS.GST-Rac1 and weakly with GDP.Rac1 but not with GST-Cdc42 or GST-RhoA. The N-terminal domain of Sra-1 (1-407 amino acids) was responsible for the interaction with Rac1. Myc-tagged Sra-1 and the deletion mutant capable of interacting with Rac1, but not the mutants unable to bind Rac1, were colocalized with dominant active Rac1(Val-12) and cortical actin filament at the Rac1(Val-12)-induced membrane ruffling area in KB cells. Sra-1 was cosedimented with filamentous actin (F-actin), indicating that Sra-1 directly interacts with F-actin. These results suggest that Sra-1 is a novel and specific target for Rac1.

Myotonic dystrophy kinase-related Cdc42-binding kinase acts as a Cdc42 effector in promoting cytoskeletal reorganization

The Rho GTPases play distinctive roles in cytoskeletal reorganization associated with growth and differentiation. The Cdc42/Rac-binding p21-activated kinase (PAK) and Rho-binding kinase (ROK) act as morphological effectors for these GTPases. We have isolated two related novel brain kinases whose p21-binding domains resemble that of PAK whereas the kinase domains resemble that of myotonic dystrophy kinase-related ROK. These approximately 190-kDa myotonic dystrophy kinase-related Cdc42-binding kinases (MRCKs) preferentially phosphorylate nonmuscle myosin light chain at serine 19, which is known to be crucial for activating actin-myosin contractility. The p21-binding domain binds GTP-Cdc42 but not GDP-Cdc42. The multidomain structure includes a cysteine-rich motif resembling those of protein kinase C and n-chimaerin and a putative pleckstrin homology domain. MRCK alpha and Cdc42V12 colocalize, particularly at the cell periphery in transfected HeLa cells. Microinjection of plasmid encoding MRCK alpha resulted in actin and myosin reorganization. Expression of kinase-dead MRCK alpha blocked Cdc42V12-dependent formation of focal complexes and peripheral microspikes. This was not due to possible sequestration of the p21, as a kinase-dead MRCK alpha mutant defective in Cdc42 binding was an equally effective blocker. Coinjection of MRCK alpha plasmid with Cdc42 plasmid, at concentrations where Cdc42 plasmid by itself elicited no effect, led to the formation of the peripheral structures associated with a Cdc42-induced morphological phenotype. These Cdc42-type effects were not promoted upon coinjection with plasmids of kinase-dead or Cdc42-binding-deficient MRCK alpha mutants. These results suggest that MRCK alpha may act as a downstream effector of Cdc42 in cytoskeletal reorganization.

PAK kinases are directly coupled to the PIX family of nucleotide exchange factors

The PAK family of kinases are regulated through interaction with the small GTPases Cdc42 and Rac1, but little is known of the signaling components immediately upstream or downstream of these proteins. We have purified and cloned a new class of Rho-p21 guanine nucleotide exchange factor binding tightly through its N-terminal SH3 domain to a conserved proline-rich PAK sequence with a Kd of 24 nM. This PAK-interacting exchange factor (PIX), which is widely expressed and enriched in Cdc42- and Rac1-driven focal complexes, is required for PAK recruitment to these sites. PIX can induce membrane ruffling, with an associated activation of Rac1. Our results suggest a role for PIX in Cdc42-to-Rac1 signaling, involving the PIX/PAK complex.

Determinants for substrate phosphorylation by p21-activated protein kinase (gamma-PAK)

gamma-PAK, originally designated PAK I and subsequently identified as a member of the p21-activated protein kinase family, has been shown to have cytostatic properties and to be involved in maintaining cells in a nondividing state [Rooney, R. D., et al., (1996) J. Biol. Chem. 271, 21498-21504]. The determinants for phosphorylation of substrates by gamma-PAK have been identified by examining the kinetics of phosphorylation of a series of synthetic peptides patterned after the sequence KKRKSGL, which is the site phosphorylated by gamma-PAK in the Rous sarcoma virus nucleocapsid protein NC in vivo and in vitro. With these peptides, the recognition sequence for gamma-PAK has been shown to contain two basic amino acids in the -2 and -3 positions, as represented by (K/R)RXS, in which the -2 position is an arginine, the -3 position is an arginine or a lysine, and X can be an acidic, basic, or neutral amino acid. A basic amino acid in the -1 or -4 position improves the rate of phosphorylation by increasing the Vmax and decreasing the Km. An acidic amino acid in the -1 position increases the rate (2.5-fold), as does an acidic residue in the -4 position, although to a lower extent (1.6-fold). Proline in the -1 or +1 position has a deleterious effect and inhibits phosphorylation by gamma-PAK. The substrate requirements of protein kinases that recognize basic amino acids on the N-terminal side of the phosphorylatable residue such as cAMP-dependent protein kinase (PKA) and Ca2+/phospholipid-dependent protein kinase (PKC) have been compared with gamma-PAK using the same peptides. An acidic residue in the -1 position negatively affects PKA and PKC; thus, peptides containing the sequence KRES can be used to identify gamma-PAK.

Activation of hPAK65 by caspase cleavage induces some of the morphological and biochemical changes of apoptosis

Apoptosis is a highly regulated form of cell death, characterized by distinctive features such as cellular shrinkage and nuclear condensation. We demonstrate here that proteolytic activation of hPAK65, a p21-activated kinase, induces morphological changes and elicits apoptosis. hPAK65 is cleaved both in vitro and in vivo by caspases at a single site between the N-terminal regulatory p21-binding domain and the C-terminal kinase domain. The C-terminal cleavage product becomes activated, with a kinetic profile that parallels caspase activation during apoptosis. This C-terminal hPAK65 fragment also activates the c-Jun N-terminal kinase pathway in vivo. Microinjection or transfection of this truncated hPAK65 causes striking alterations in cellular and nuclear morphology, which subsequently promotes apoptosis in both CHO and Hela cells. Conversely, apoptosis is delayed in cells expressing a dominant-negative form of hPAK65. These findings provide a direct evidence that the activated form of hPAK65 generated by caspase cleavage is a proapoptotic effector that mediates morphological and biochemical changes seen in apoptosis.

The phosphorylation site for Ste20p-like protein kinases is essential for the function of myosin-I in yeast

The budding yeast Saccharomyces cerevisiae has two functionally redundant myosin-I isoforms encoded by the MYO3 and MYO5 genes. The function shared by these myosin proteins is required for proper yeast budding. Serine residue 357 in the head domain of Myo3p, conserved among myosin-I proteins including yeast Myo5p, was identified as a unique phosphorylation site for the serine/threonine protein kinase Ste20p and its closely related isoform Cla4p. These protein kinases share a function that is also essential for budding. Replacement of serine 357 with alanine disrupted the in vivo function of Myo3p, whereas this function was maintained by changing the serine residue to aspartate. This mutant version failed to compensate the growth defect of cells which lack both Ste20p and Cla4p, suggesting that myosin-I is not the only essential target of these protein kinases. Our results suggest that phosphorylation of the head domain by Ste20p-like protein kinases plays an essential role in the function of myosin-I in yeast cells.

Regulation of cross-linking of actin filament by IQGAP1, a target for Cdc42

We have previously shown that IQGAP1, a recently identified target for Cdc42 and Rac1 small GTPases, showed a distribution similar to that of cortical actin cytoskeleton at the membrane ruffling area induced by insulin and Rac1(val12) (Kuroda, S., Fukata, M., Kobayashi, K., Nakafuku, M., Nomura, N., Iwamatsu, A., and Kaibuchi, K. (1996) J. Biol. Chem. 271, 23363-23367). Here we identified an IQGAP1-interacting molecule with molecular mass of 43 kDa (p43) from bovine brain cytosol, using glutathione S-transferase (GST)-IQGAP1 affinity column chromatography. The amino acid sequencing of the protein revealed that p43 was identical to beta- and gamma-actin. IQGAP1 was cosedimentated with filamentous actin (F-actin). The amino-terminal domain (amino acids 1-216) of IQGAP1 was responsible for the interaction with F-actin. Falling ball viscometry assay revealed that IQGAP1 cross-linked the F-actin. This IQGAP1 activity was further enhanced by guanosine 5'-(3-O-thio)triphosphate (GTPgammaS).GST-Cdc42 but not by GDP.GST-Cdc42. The gel filtration analysis of IQGAP1 revealed that IQGAP1 appeared as oligomers and that GTPgammaS.GST-Cdc42 but not GDP.GST-Cdc42 enhanced the oligomerization of IQGAP1. These results strongly suggest that IQGAP1, acting downstream of Cdc42, can cross-link the actin filament through its oligomerization.

Regulation of cell-cell adhesion of MDCK cells by Cdc42 and Rac1 small GTPases

Rac1, a member of the Rho small GTPases family, has recently been shown to be involved in the regulation of cell-cell adhesion mediated by cadherin. Here we showed that Cdc42, another member of Rho family, accumulated at cell-cell contact sites. Microinjection of Rho GDI, a negative regulator of the Rho family members, into Madin-Darby canine kidney (MDCK) cells resulted in perturbation of epithelial cell morphology and of cell-cell and cell-substratum adhesions, and comicroinjection of dominant active Cdc42 or Rac1 reversed the action of Rho GDI, suggesting that the active form of Cdc42 or Rac1 is required for maintaining the cell-cell and cell-substratum adhesions. These observations suggest that Cdc42, in addition to Rac1, can regulate the cell-cell adhesion.

Novel Cdc42-binding proteins Gic1 and Gic2 control cell polarity in yeast

Cdc42p, a Rho-related GTP-binding protein, regulates cytoskeletal polarization and rearrangements in eukaryotic cells, but the effectors mediating this control remain unknown. Through the use of the complete yeast genomic sequence, we have identified two novel Cdc42p targets, Gic1p and Gic2p, which contain consensus Cdc42/Rac interactive-binding (CRIB) domains and bind specifically to Cdc42p-GTP. Gic1p and Gic2p colocalize with Cdc42p as cell polarity is established during the cell cycle and during mating in response to pheromones. Cells deleted for both GIC genes exhibit defects in actin and microtubule polarization similar to those observed in cdc42 mutants. Finally, the interaction of the Gic proteins and Cdc42p is essential, as mutations in the CRIB domain of Gic2p that eliminate Cdc42p binding disrupt Gic2p localization and function. Thus, Gic1p and Gic2p define a novel class of Cdc42p targets that are specifically required for cytoskeletal polarization in vivo.

Activation of the Ste20-like oxidant stress response kinase-1 during the initial stages of chemical anoxia-induced necrotic cell death. Requirement for dual inputs of oxidant stress and increased cytosolic [Ca2+]

Signal transduction mechanisms activated during the early stages of necrotic cell death are poorly characterized. We have recently identified the Sterile 20 (Ste20)-like oxidant stress response kinase-1, SOK-1, which is a member of the Ste20 kinase family. We report that SOK-1 is markedly activated as early as 20 min after chemical anoxia induced by exposure of Madin-Darby canine kidney or LLC-PK1 renal tubular epithelial cells to 2-deoxyglucose (2-DG) and any one of three inhibitors of the electron transport chain, cyanide (CN), rotenone, or antimycin A. Since oxidant stress activates SOK-1, we postulated that reactive oxygen species (ROS), which are produced by the electron transport chain during chemical anoxia, might be responsible for SOK-1 activation. The time course of CN/2-DG-induced SOK-1 activation and of production of ROS, measured in cells loaded with dichlorofluorescein, were compatible with a role for ROS in SOK-1 activation. Furthermore, preincubation of LLC-PK1 cells with three unrelated scavengers of ROS, pyrrolidine dithiocarbamate, pyruvate, or nordihydroguaiaretic acid, reduced both cellular oxidant stress and activation of SOK-1 by CN/2-DG. An increase in cytosolic free [Ca2+] ([Ca2+]i) was necessary but not sufficient for CN/2-DG-induced activation of SOK-1. Preincubation of cells with BAPTA-AM prevented activation of SOK-1. Incubation of cells with thapsigargin or the calcium ionophore, A23187, had no effect on SOK-1 activity, but preincubation of cells with either of these agents markedly enhanced CN/2-DG-induced activation of SOK-1 (20-fold versus 7-fold). In summary, chemical anoxia activates SOK-1 via an oxidant stress-dependent mechanism that is both critically dependent upon and markedly amplified by an increase in [Ca2+]i. This requirement for dual inputs of oxidant stress and an increase in [Ca2+]i may prevent inappropriate activation of the kinase by milder degrees of oxidant stress, which are insufficient to generate an increase in [Ca2+]i. The activation of SOK-1 may be one of the cell's earliest responses to inducers of necrotic cell death.

Cloning and characterization of a human STE20-like protein kinase with unusual cofactor requirements

We cloned and characterized a novel human member of the STE20 serine/threonine protein kinase family named mst-3. Based on its domain structure, mst-3 belongs to the SPS1 subgroup of STE20-like proteins, which includes germinal center (GC) kinase, hematopoietic progenitor kinase (HPK), kinase homologous to STE20/SPS-1 (KHS), kinases responsive to stress (KRS1/2), the mammalian STE20-like kinases (mst1/2), and the recently published STE20/oxidant stress response kinase SOK-1. mst-3 is most closely related to SOK-1, with 88% amino acid similarity in the kinase domain. The similarity of the mst-3 kinase domain to STE20 is 42%. The mst-3 transcript is ubiquitously expressed, and the protein was found in all human, mouse, and monkey cell lines tested. An in vitro kinase assay showed that mst-3 can phosphorylate basic exogenous substrates as well as itself. Interestingly, mst-3 prefers Mn2+ to Mg2+ as a divalent cation and can use both GTP and ATP as phosphate donors. Like SOK-1, mst-3 is activated by autophosphorylation. However, a physiological stimulus of mst-3 activity was not identified. mst-3 activity does not change upon exposure to several mitogenic and stress stimuli. Overexpression of mst-3 wild-type or kinase dead protein affects neither the extracellular signal-regulated kinases (ERK1/2 or ERK6), c-Jun N-terminal kinase (JNK), p38, nor pp70S6 kinase, suggesting that mst-3 is part of a novel signaling pathway.

Localization of p21-activated kinase 1 (PAK1) to pinocytic vesicles and cortical actin structures in stimulated cells

The mechanisms through which the small GTPases Rac1 and Cdc42 regulate the formation of membrane ruffles, lamellipodia, and filopodia are currently unknown. The p21-activated kinases (PAKs) are direct targets of active Rac and Cdc42 which can induce the assembly of polarized cytoskeletal structures when expressed in fibroblasts, suggesting that they may play a role in mediating the effects of these GTPases on cytoskeletal dynamics. We have examined the subcellular localization of endogenous PAK1 in fibroblast cell lines using specific PAK1 antibodies. PAK1 is detected in submembranous vesicles in both unstimulated and stimulated fibroblasts that colocalize with a marker for fluid-phase uptake. In cells stimulated with PDGF, in v-Src-transformed fibroblasts, and in wounded cells, PAK1 redistributed into dorsal and membrane ruffles and into the edges of lamellipodia, where it colocalizes with polymerized actin. PAK1 was also colocalized with F-actin in membrane ruffles extended as a response to constitutive activation of Rac1. PAK1 appears to precede F-actin in translocating to cytoskeletal structures formed at the cell periphery. The association of PAK1 with the actin cytoskeleton is prevented by the actin filament-disrupting agent cytochalasin D and by the phosphatidylinositol 3-kinase inhibitor wortmannin. Co-immunoprecipitation experiments demonstrate an in vivo interaction of PAK1 with filamentous (F)-actin in stimulated cells. Microinjection of a constitutively active PAK1 mutant into Rat-1 fibroblasts overexpressing the insulin receptor (HIRcB cells) induced the formation of F-actin- and PAK1-containing structures reminiscent of dorsal ruffles. These data indicate a close correlation between the subcellular distribution of endogenous PAK1 and the formation of Rac/Cdc42-dependent cytoskeletal structures and support an active role for PAK1 in regulating cortical actin rearrangements.

Stress-activated protein kinases: activation, regulation and function

The response of cells to extracellular stimuli is mediated in part by a number of intracellular kinase and phosphatase enzymes. Within this area of research the activation of the p42 and p44 isoforms of mitogen-activated protein (MAP) kinases have been extensively described and characterised as central components of the signal transduction pathways stimulated by both growth factors and G-protein-coupled receptor agonists. Signaling events mediated by these kinases are fundamental to cellular functions such as proliferation and differentiation. More recently, homologues of the p42 and p44 isoforms of MAP kinase have been described, namely the stress-activated protein kinases (SAPKs) or alternatively the c-jun N-terminal kinases (JNKs) and p38 MAP kinase (the mammalian homologue of yeast HOG1). These MAP kinase homologues are integral components of parallel MAP kinase cascades activated in response to a number of cellular stresses including inflammatory cytokines (e.g., Interleukin-1 (Il-1) and tumour necrosis factor-alpha (TNF-alpha), heat and chemical shock, bacterial endotoxin and ischaemia/cellular ATP depletion. Activation of these MAP kinase homologues mediates the transduction of extracellular signals to the nucleus and are pivotal events in the regulation of the transcription events that determine functional outcome in response to such stresses. In this review we highlight the identification and characterisation of the stress-activated MAP kinase homologues, their role as components of parallel MAP kinase pathways and the regulation of cellular responses following exposure to cellular stress.

Characterization of the interactions between the small GTPase Cdc42 and its GTPase-activating proteins and putative effectors. Comparison of kinetic properties of Cdc42 binding to the Cdc42-interactive domains

The small GTPase Cdc42 interacts with multiple factors to transduce diverse intracellular signals. The factors that preferentially recognize the GTP-bound, active state of Cdc42 include a panel of GTPase-activating proteins (GAPs), the Cdc42/Rac interactive binding (CRIB) motif-containing molecules, and the RasGAP domain containing IQGAP1 and IQGAP2. In the present study, we have determined the kinetic parameters underlying the functional interactions between the Cdc42-binding domains of some of these factors and Cdc42 by monitoring the continuous release of gammaPi and have compared the ability of the domains to bind to Cdc42. The catalytic efficiencies (Kcat/Km) of the GAP domains of Bcr, 3BP-1, and p190 on Cdc42 are found to be 60-, 160-, and over 500-fold less than that of Cdc42GAP, respectively, and the differences are due, to a large part, to differences in Km. The Km values of the GAP domains compare well to the binding affinity to the guanylyl imidodiphosphate-bound Cdc42, suggesting a rapid equilibrium reaction mechanism. The affinity of the Cdc42-binding domains of the CRIB motif of Wiskott-Aldrich Syndrome protein and p21(cdc42/rac)-activated kinase 1, and the RasGAP-related domain of IQGAP1, which all inhibit the intrinsic rate of GTP hydrolysis of Cdc42, are found to be 4, 0.7, and 0.08 microM, respectively. These quantitative analysis provide insight that Cdc42GAP functions as an effective negative regulator of Cdc42 by fast, relatively tight binding to the GTP-bound Cdc42, whereas IQGAP1 interacts with Cdc42 as a putative effector with over 10-fold higher affinity than the CRIB domains and GAPs, and suggest that various GAPs and effectors employ distinct mechanism to play roles in Cdc42-mediated signaling pathways.

Identification by mass spectrometry of the phosphorylated residue responsible for activation of the catalytic domain of myosin I heavy chain kinase, a member of the PAK/STE20 family

Myosin I heavy chain kinase from Acanthamoeba castellanii is activated in vitro by autophosphorylation (8-10 mol of P per mol). The catalytically active C-terminal domain produced by trypsin cleavage of the phosphorylated kinase contains 2-3 mol of P per mol. However, the catalytic domain expressed in a baculovirus-insect cell system is fully active as isolated without autophosphorylation in vitro. We now show that the expressed catalytic domain is inactivated by incubation with acid phosphatase and regains activity upon autophosphorylation. The state of phosphorylation of all of the hydroxyamino acids in the catalytic domain were determined by mass spectrometry of unfractionated protease digests. Ser-627 was phosphorylated in the active, expressed catalytic domain, lost its phosphate when the protein was incubated with phosphatase, and was rephosphorylated when the dephosphorylated protein was incubated with ATP. No other residue was significantly phosphorylated in any of the three samples. Thus, phosphorylation of Ser-627, which is in the same position as the Ser and Thr residues that are phosphorylated in many other kinases, is necessary and sufficient for full activity of the catalytic domain. Ser-627 is also phosphorylated when full-length, native kinase is activated by autophosphorylation.

Inhibition of neutrophil oxidative burst and granule secretion by wortmannin: potential role of MAP kinase and renaturable kinases

Exposure of neutrophils to a variety of agonists including soluble chemoattractant peptides and cytokines results in degranulation and activation of the oxidative burst (effector functions) that are required for bacterial killing. At present, the signaling pathways regulating these important functions are incompletely characterized. Mitogen-activated protein (MAP) kinases (MAPK) as well as members of a family of "renaturable kinases" are rapidly activated in neutrophils in response to diverse physiological agonists, suggesting that they may regulate cell activation. Antagonists of phosphatidyl inositol-3-(OH) kinase (PI3-kinase) such as wortmannin (Wtmn) inhibit these effector responses as well as certain of the above-mentioned kinases, leading to the suggestion that these enzymes lie downstream of PI3-kinase in the pathway regulating the oxidative burst and granule secretion. However, an apparent discrepancy exists in that, while virtually obliterating activity of PI3-kinase and the oxidase at low concentrations (ID50 < 20 nM), Wtmn has only variable inhibitory effects on MAPK even at substantially higher concentrations (75-100 nM). This raises the possibility that the inhibitory effects of Wtmn are mediated via other enzyme systems. The purpose of the current study was therefore to compare the effects of Wtmn on PI3-kinase activity and on the chemoattractant-activated kinases, and to determine the potential relationship of these pathways to microbicidal responses. In human neutrophils, both the oxidative burst and granule secretion induced by fMLP were inhibited by Wtmn but at markedly different concentrations: the oxidative burst was inhibited with an ID50 of < 5 nM while granule secretion was only partially inhibited at concentrations exceeding 75 nM. Activation of both MEK-1 and MAPK in response to fMLP was only partially inhibited by high doses of Wtmn (ID50 of > 100 nM and approximately 75 nM, respectively). In contrast, Wtmn potently inhibited fMLP-induced activation of the 63 and 69 kDa renaturable kinases (ID50 approximately 5-10 nM). We speculate that the renaturable kinases may be involved in the regulation of the oxidative burst, whereas the MAPK pathway may play a role in other neutrophil functions such as granule secretion.

Phosphatidic acid-mediated phosphorylation of the NADPH oxidase component p47-phox. Evidence that phosphatidic acid may activate a novel protein kinase

Phosphatidic acid (PA), generated by phospholipase D activation, has been linked to the activation of the neutrophil respiratory burst enzyme, NADPH oxidase; however, the intracellular enzyme targets for PA remain unclear. We have recently shown (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935) that a PA-activated protein kinase is involved in the activation of NADPH oxidase in a cell-free system. This protein kinase phosphorylates numerous endogenous proteins, including p47-phox, a component of the NADPH oxidase complex. Phospholipids other than PA were less effective at inducing endogenous protein phosphorylation. Several of these endogenous substrates were also phosphorylated during stimulation of intact cells by opsonized zymosan, an agonist that induces phospholipase D activation. We sought to identify the PA-activated protein kinase that phosphorylates p47-phox. The PA-dependent protein kinase was shown to be cytosolic. cis-Unsaturated fatty acids were poor inducers of protein kinase activity, suggesting that the PA-activated protein kinase is not a fatty acid-regulated protein kinase (e.g. protein kinase N). Chromatographic techniques separated the PA-activated protein kinase from a number of other protein kinases known to be activated by PA or to phosphorylate p47-phox. These included isoforms of protein kinase C, p21 (Cdc42/Rac)-activated protein kinase, and mitogen-activated protein kinase. Gel filtration chromatography indicated that the protein kinase has an apparent molecular size of 125 kDa. Screening of cytosolic fractions from several cell types and rat brain suggested the enzyme has widespread cell and tissue distribution. The partially purified protein kinase was sensitive to the same protein kinase inhibitors that diminished NADPH oxidase activation and was independent of guanosine 5'-3-O-(thio)triphosphate and Ca2+. Phosphoamino acid analysis showed that serine and tyrosine residues were phosphorylated on p47-phox by this kinase(s). These data indicate that one or more potentially novel protein kinases are targets for PA in neutrophils and other cell types. Furthermore, a PA-activated protein kinase is likely to be an important regulator of the neutrophil respiratory burst by phosphorylation of the NADPH oxidase component p47-phox.

The Nef protein of human immunodeficiency virus type 1 enhances serine phosphorylation of the viral matrix

The human immunodeficiency virus type 1 matrix (MA) protein is phosphorylated during virion maturation on its C-terminal tyrosine and on several serine residues. Whereas MA tyrosine phosphorylation facilitates viral nuclear import, the significance of MA serine phosphorylation remains unclear. Here, we report that MA serine but not tyrosine phosphorylation is strongly enhanced by Nef. Mutations that abrogated the membrane association of Nef and its ability to bind a cellular serine/threonine kinase greatly diminished the extent of virion MA serine phosphorylation. Correspondingly, a protein kinase coimmunoprecipitated with Nef could phosphorylate MA on serine in vitro, producing a phosphopeptide pattern reminiscent of that of virion MA. Recombinant p21-activated kinase hPAK65, a recently proposed relative of the Nef-associated kinase, achieved a comparable result. Taken together, these data suggest that MA is a target of the Nef-associated serine kinase.

Molecular cloning and characterization of a novel putative STE20-like kinase in guinea pigs

Protein kinases play a key role in cell growth and differentiation. We have isolated the cDNA of a novel protein serine/threonine kinase (referred to as STE20-like kinase (SLK) from a guinea pig liver cDNA library with a probe generated by a cloning approach based on the polymerase chain reaction. The encoded polypeptide (1231 amino acids, M(r) 141,079) contains all conserved subdomains characteristic of the protein serine threonine kinase family. A hemagglutinin-tagged SLK expressed artificially in COS7 cells was hyperphosphorylated by anisomycin. By Northern blot analysis, SLK mRNA was detected in all organs examined: brain, lung heart, liver, kidney, spleen, testis, and eosinophils. Sequence comparisons of its catalytic domain related SLK to p21-activated kinase family of protein serine/threonine kinases. Its noncatalytic domain comprises several intriguing structural features, including the acidic region and the nuclear targeting sequence. This noncatalytic domain exhibited no extended similarity with other proteins. Thus, SLK is a protein serine/threonine kinase which contains an unknown regulatory domain(s).

Emerging from the Pak: the p21-activated protein kinase family

The p21-activated protein kinases (PAKs) are members of a growing family of regulatory enzymes that may play roles in diverse phenomena such as cellular morphogenesis, the stress response and the pathogenesis of AIDS. PAKs were initially discovered as binding partners for small (21 kDa) GTPases that regulate actin polymerization, and recent evidence has shown that some members of the PAK family may be effectors for related GTPases that are involved in intracellular vesicle trafficking. Because the downstream signalling pathways for all such GTPases are poorly understood, intense studies are under way to discern the role of PAK and its cousins. In this review, the authors highlight some of the established properties of the extended PAK family and discuss current controversies regarding their possible roles as GTPase effectors.

Identification of two essential phosphorylated threonine residues in the catalytic domain of Mekk1. Indirect activation by Pak3 and protein kinase C

The 78-kDa protein kinase Mekk1 plays an important role in the stress response pathway that involves the activation of downstream kinases Sek1 and stress-activated protein kinase/c-Jun NH2-terminal kinase. Conserved serine and threonine residues located between the kinase subdomains VII and VIII of many protein kinases are phosphorylated for maximal kinase activation. Two threonine residues within this region in Mekk1 at positions 560 and 572, but not the serine at 557, were shown to be essential for catalytic activity in this study. When these threonine residues were replaced with alanine, there was a significant loss in phosphotransferase activity toward the primary substrate, Sek1, and a large decrease in autophosphorylation activity. Site-directed mutagenesis demonstrated that these threonine residues cannot be replaced with either serine or glutamic acid for preservation of phosphotransferase activity. Further examination of the Mekk1 mutants isolated from 32P-labeled transfected COS cells showed that Thr-560 and Thr-572 were indeed phosphorylated after two-dimensional tryptic-chymotryptic phosphopeptide analysis. Additional determinants in the NH2-terminal domain of Mekk1 also play a role in the regulation of Mekk1 activity. Although Pak3 and PKC can activate Mekk1 in vivo, this interaction is indirect and independent, since there was no direct phosphorylation of Mekk1 by Pak3 or PKC or of Pak3 by PKC, respectively.

p21-activated kinase has substrate specificity similar to Acanthamoeba myosin I heavy chain kinase and activates Acanthamoeba myosin I

Acanthamoeba class I myosins are unconventional, single-headed myosins that express actin-activated Mg2+-ATPase and in vitro motility activities only when a single serine or threonine in the heavy chain is phosphorylated by myosin I heavy chain kinase (MIHCK). Some other, but not most, class I myosins have the same consensus phosphorylation site sequence, and the two known class VI myosins have a phosphorylatable residue in the homologous position, where most myosins have an aspartate or glutamate residue. Recently, we found that the catalytic domain of Acanthamoeba MIHCK has extensive sequence similarity to the p21-activated kinase (PAK)/STE20 family of kinases from mammals and yeast, which are activated by small GTP-binding proteins. The physiological substrates of the PAK/STE20 kinases are not well characterized. In this paper we show that PAK1 has similar substrate specificity as MIHCK when assayed against synthetic substrates and that PAK1 phosphorylates the heavy chain (1 mol of P(i) per mol) and activates Acanthamoeba myosin I as MIHCK does. These results, together with the known involvement of Acanthamoeba myosin I, yeast myosin I, STE20, PAK, and small GTP-binding proteins in membrane- and cytoskeleton-associated morphogenetic transformations and activities, suggest that myosins may be physiological substrates for the PAK/STE20 family and thus mediators of these events.

Activation of the nuclear factor-kappaB by Rho, CDC42, and Rac-1 proteins

The Rho family of small GTPases are critical elements involved in the regulation of signal transduction cascades from extracellular stimuli to the cell nucleus, including the JNK/SAPK signaling pathway, the c-fos serum response factor, and the p70 S6 kinase. Here we report a novel signaling pathway activated by the Rho proteins that may be responsible for their biological activities, including cytoskeleton organization, transformation, apoptosis, and metastasis. The human RhoA, CDC42, and Rac-1 proteins efficiently induce the transcriptional activity of nuclear factor kappaB (NF-kappaB) by a mechanism that involves phosphorylation of Ikappa Balpha and translocation of p50/p50 and p50/p65 dimers to the nucleus, but independent of the Ras GTPase and the Raf-1 kinase. We also show that activation of NF-kappaB by TNFalpha depends on CDC42 and RhoA, but not Rac-1 proteins, because this activity is drastically inhibited by their respective dominant-negative mutants. In contrast, activation of NF-kappaB by UV light was not affected by Rho, CDC42, or Rac-1 dominant-negative mutants. Thus, members of the Rho family of GTPases are involved specifically in the regulation of NF-kappaB-dependent transcription.

Use of a fluorescence spectroscopic readout to characterize the interactions of Cdc42Hs with its target/effector, mPAK-3

The family of p21-activated kinases (PAKs) has been shown to contain a domain that can independently bind to the Ras-like proteins Cdc42Hs and Rac. We have expressed a 72 amino acid recombinant form of this p21-binding domain (PBD) from mPAK-3 in Escherichia Coli for use in structure-function studies. The protein can be purified on a nickel affinity resin due to a hexa-His tag that is incorporated onto the amino terminus of the domain. PBD binds to Cdc42Hs in a guanine nucleotide-dependent manner as demonstrated by a novel fluorescence assay that takes advantage of the spectroscopic properties of N-methylanthraniloyl (Mant)-guanine nucleotides. Ionic strength has little effect on the affinity of PBD for Cdc42Hs, but alkaline pH values tend to weaken the interaction. We have shown that the inhibition of the GTPase activity of Cdc42Hs, as well as a previously undescribed inhibition of guanine nucleotide dissociation, is mediated by the PBD portion of the mPAK-3 molecule. These findings suggest that PBD binding alters the geometry of the guanine nucleotide binding site on Cdc42Hs, perhaps as an outcome of the target/effector molecule binding in close proximity to the nucleotide domain. We therefore tested if mutations in the effector region of Cdc42Hs (32-40), which in Ras are very close to the guanine nucleotide binding site, had any effect on PBD binding. Changing tyrosine 32 to lysine (Y32K) resulted in a small (5-fold) inhibition of PBD binding, but the very conservative mutation D38E yielded at least a 50-fold decrease in affinity. Finally, the catalytic domain of the GTPase activating protein, Cdc42-GAP, was shown to inhibit PBD binding in a competitive manner, indicating that this target molecule and the negative regulator (GAP) bind to overlapping sites on the Cdc42Hs molecule.

MEKKs, GCKs, MLKs, PAKs, TAKs, and tpls: upstream regulators of the c-Jun amino-terminal kinases?

Regulation of the mitogen-activated protein kinase (MAPK) family members - which include the extracellular response kinases (ERKs), p38/HOG1, and the c-Jun amino-terminal kinases (JNKs) - plays a central role in mediating the effects of diverse stimuli encompassing cytokines, hormones, growth factors and stresses such as osmotic imbalance, heat shock, inhibition of protein synthesis and irradiation. A rapidly increasing number of kinases that activate the JNK pathways has been described recently, including the MAPK/ERK kinase kinases, p21-activated kinases, germinal center kinase, mixed lineage kinases, tumor progression locus 2, and TGF-beta-activated kinase. Thus, regulation of the JNK pathway provides an interesting example of how many different stimuli can converge into regulating pathways critical for the determination of cell fate.

Activation of Pak by membrane localization mediated by an SH3 domain from the adaptor protein Nck

BACKGROUND

The adaptor protein Nck consists of three Src homology 3 (SH3) domains followed by one SH2 domain. Like the Grb2 adaptor protein, which is known to couple receptor tyrosine kinases to the small GTPase Ras, Nck is presumed to bind to tyrosine-phosphorylated proteins using its SH2 domain and to downstream effector proteins using its SH3 domain. Little is known, however, about the specific biological function of Nck. The Pak family of serine/threonine kinases are known to be activated by binding to the GTP-bound form of Cdc42 or Rac1, which are small GTPases of the Rho family that are involved in regulating the organization of the actin cytoskeleton.

RESULTS

We present evidence that Nck can mediate the relocalization and subsequent activation of the Pak1 kinases. We show that Nck associates in vivo with Pak using the second of its three SH3 domains, and that localization of this individual Nck SH3 domain, or of Pak kinase itself, to the membrane results in activation of Pak and stimulation of downstream mitogen activated protein kinase cascades. Activation of downstream signaling by the membrane-localized Nck SH3 domain is blocked by a kinase-inactive mutant form of Pak1.

CONCLUSION

These results demonstrate that localization of Pak1 to the membrane in the absence of other signals is sufficient for its activation, and imply that the Nck adaptor protein could function to link changes in tyrosine phosphorylation of cellular proteins to the Cdc42/Pak signaling pathway.

Angiotensin II stimulates c-Jun NH2-terminal kinase in cultured cardiac myocytes of neonatal rats

Many lines of evidence have suggested that angiotensin II (Ang II)plays an important role in cardiac hypertrophy. Ang II not only increases protein synthesis but also induces the reprogramming of gene expression in cultured cardiac myocytes. In the present study, to elucidate the mechanism by which Ang II regulates gene expression in cardiac myocytes, we examined whether Ang II activates c-Jun NH2-terminal kinase (JNK), which is a member of the mitogen-activated protein kinase family and activates the transcription factor, activator protein-1 (AP-1). The activity of JNK increased 5 minutes after the addition of Ang II, peaked at 20 minutes, and gradually decreased thereafter. Examination of the Ang II dose-response relation revealed detectable JNK activation at 10(-9) mol/L and maximal activation at 10(-6) mol/L. Ang II activated JNK through the AT1 receptor, and the activation was attenuated by the downregulation of protein kinase C or the chelation of intracellular Ca2+. Although the addition of either Ca2+ ionophore or phorbol ester resulted in little or no activation of JNK, simultaneous addition of both Ca2+ ionophore and phorbol ester markedly activated JNK. Slight expressions of the c-jun gene were observed in unstimulated cardiac myocytes, and Ang II increased expressions of the c-jun gene as well as the c-fos gene. Ang II increased transcription of the endothelin-1 gene through the AP-1 binding site. In conclusion, Ang II may activate JNK in cultured cardiac myocytes through an increase in intracellular Ca2+ and activation of protein kinase C, and the activated JNK may regulate gene expression by activating AP-1 during Ang II-induced cardiac hypertrophy.

Requirement of CDC42 for Salmonella-induced cytoskeletal and nuclear responses

The bacterial pathogen Salmonella typhimurium triggers host cell signaling pathways that lead to cytoskeletal and nuclear responses required for pathogenesis. Here, the role of the small guanosine triphosphate (GTP)-binding protein CDC42Hs in these responses was examined. Expression of a dominant interfering mutant of CDC42 (CDC42HsN17) prevented S. typhimurium-induced cytoskeletal reorganization and subsequent macropinocytosis and bacterial internalization into host cells. Cells expressing constitutively active CDC42 (CDC42HsV12) internalized an S. typhimurium mutant unable to trigger host cell responses. Furthermore, expression of CDC42HsN17 prevented S. typhimurium-induced JNK kinase activation. These results indicate that CDC42 is required for bacterial invasion and induction of nuclear responses in host cells.

Functional analysis of the interaction between the small GTP binding protein Cdc42 and the Ste20 protein kinase in yeast

STE20 encodes a protein kinase related to mammalian p65Pak which functions in several signal transduction pathways in yeast, including those involved in pseudohyphal and invasive growth, as well as mating. In addition, Ste20 plays an essential role in cells lacking Cla4, a kinase with significant homology to Ste20. It is not clear how the activity of Ste20 is regulated in response to these different signals in vivo, but it has been demonstrated recently that binding of the small GTP binding protein Cdc42 is able to activate Ste20 in vitro. Here we show that Ste20 functionally interacts with Cdc42 in a GTP-dependent manner in vivo: Ste20 mutants that can no longer bind Cdc42 were unable to restore growth of ste20 cla4 mutant cells. They were also defective for pseudohyphal growth and agar invasion, and displayed reduced mating efficiency when mated with themselves. Surprisingly, however, the kinase activity of such Ste20 mutants was normal when assayed in vitro. Furthermore, these alleles were able to fully activate the MAP kinase pathway triggered by mating pheromones in vivo, suggesting that binding of Cdc42 and Ste20 was not required to activate Ste20. Wild-type Ste20 protein was visualized as a crescent at emerging buds during vegetative growth and at shmoo tips in cells arrested with alpha-factor. In contrast, a Ste20 mutant protein unable to bind Cdc42 was found diffusely throughout the cytoplasm, suggesting that Cdc42 is required to localize Ste20 properly in vivo.

Activation of myosin-I by members of the Ste20p protein kinase family

The heavy chain of myosin-ID isolated from Dictyostelium was identified as an in vitro substrate for members of the Ste20p family of serine/threonine protein kinases which are thought to regulate conserved mitogen-activated protein kinase pathways. Yeast Ste20p and Cla4p and mammalian p21-activated protein kinase (PAK) phosphorylated the heavy chain to 0.5-0.6 mol of Pi/mol and stimulated the actin-dependent Mg2+-ATPase activity to an extent equivalent to that of the Ste20p-like myosin-I heavy chain kinase isolated from Dictyostelium. PAK purified from rat brain required GTPgammaS-Cdc42 to express full activity, whereas recombinant mouse mPAK3 fused to glutathione S-transferase and purified from bacteria, and Ste20p and Cla4p purified from yeast extracts were fully active without GTPgammaS-Cdc42. These results suggest, together with the high degree of structural and functional conservation of Ste20p family members and myosin-I isoforms, that myosin-I activation by Ste20p family protein kinases may contribute to the regulation of morphogenetic processes in organisms ranging from yeast to mammalian cells.

Regulation of phosphorylation pathways by p21 GTPases. The p21 Ras-related Rho subfamily and its role in phosphorylation signalling pathways

The oncogenic Ras p21 GTPases regulate phosphorylation pathways that underlie a wealth of activities, including growth and differentiation, in organisms ranging from yeast to human. In metazoa, growth factors trigger conversion of Ras from an inactive GDP-bound form to an active GTP-bound form. This activation of Ras leads to activation of Raf. Raf is one of the initial kinases in the cytoplasmic mitogen-activated protein kinase (MAPK) cascade, involving extracellular-signal-regulated kinases (ERK), which culminates in nuclear transcription. The Ras-related subfamily of Rho p21s, including Rho, Rac and Cdc42 are similarly active in their GTP-bound forms. These p21s mediate growth-factor-induced morphological changes involving actin-based cellular structures. For example, in mammalian fibroblasts, Rho mediates the formation of cytoskeletal stress fibres induced by lysophosphatidic acid, while Rac mediates the formation of membrane ruffles induced by platelet-derived growth factor, and Cdc42 mediates the formation of peripheral filopodia by bradykinin. In some cases, factor-induced Rac activation results in Rho activation, and factor-induced Cdc42 activation leads to Rac activation, as determined by specific morphological changes. Although separate Cdc42/Rac and Rac/Rho hierarchies exist, these might not extend into a linear form (i.e. Cdc42-->Rac-->Rho) since Cdc42 and Rho activities may be competitive or even antagonistic. Thus Cdc42-mediated formation of filopodia is accompanied by loss of stress fibres (whose formation is mediated by Rho). Recently, mammalian kinases that bind to the GTP-bound forms of Rho p21s have been isolated. These kinases include the p21-activated serine/threonine kinase (PAK), which is stimulated by binding to Cdc42 and Rac, and the Rho-binding serine/threonine kinase (ROK), which is not as strongly stimulated by binding. These kinases act as effectors for their p21 partners since they can directly affect the reorganization of the relevant actin-containing structures. ROK promotes the formation of Rho-induced actin-containing stress fibres and focal-adhesion complexes, to which the ends of the stress fibres attach. PAK stimulates the disassembly of stress fibres, which has been shown to accompany formation of Cdc42-induced peripheral-actin-containing structures, including filopodia, which with Rac-induced membrane ruffles play a role in cell movement. PAK also fosters loss of focal-adhesion complexes. Thus, there is cooperation between different Rho p21s as well as antagonism, with their associated kinases having a role in the integration of the reorganization of the actin cytoskeleton. The similarity of PAK to the Saccharomyces cerevisiae kinase Ste20p, which initiates the yeast mating/pheromone MAPK cascade, led to experiments showing that Cdc42 regulates Ste20p in this MAPK pathway. This similarity has also led to the demonstration that mammalian Cdc42 and Rac can signal to the nucleus through MAPK pathways. However, c-Jun N-terminal kinase (JNK, stress-activated protein kinase) rather than ERK, is involved. PAK have been implicated in the JNK pathway, but their exact roles are uncertain. Thus members of the Rho subfamily, and kinases that bind to these p21s are intimately involved in immediate morphological processes as well as long-term transcriptional events.

CDC42 and Rac1 are implicated in the activation of the Nef-associated kinase and replication of HIV-1

BACKGROUND

The negative factor (Nef) of human and simian immunodeficiency viruses (HIV-1, HIV-2 and SIV) is required for high levels of viremia and progression to AIDS. Additionally, Nef leads to cellular activation, increased viral infectivity and decreased expression of CD4 on the cell surface. Previously, we and others demonstrated that Nef associates with a cellular serine kinase (NAK) activity. Recently, it was demonstrated that NAK bears structural and functional similarity to p21-activated kinases (PAKs).

RESULTS

In this study, we demonstrate that Nef not only binds to but also activates NAK via the small GTPases CDC42 and Rac1. First, the dominant-negative PAK (PAKR), via its GTPase-binding domain, and dominant-negative GTPases (CDC42Hs-N17 and Rac1-N17) block the ability of Nef to associate with and activate NAK. Second, constitutively active small GTPases (CDC42Hs-V12 and Rac1-V12) potentiate the effects of Nef. Third, interactions between Nef and NAK result in several cellular effector functions, such as activation of the serum-response pathway. And finally, PAKR, CDC42Hs-N17 and Rac1-N17 decrease levels of HIV-1 production to those of virus from which the nef gene is deleted.

CONCLUSIONS

By activating NAK via small GTPases and their downstream effectors, Nef interacts with regulatory pathways required for cell growth, cytoskeletal rearrangement and endocytosis. Thus, NAK could participate in the budding of new virions, the modification of viral proteins and the increased endocytosis of surface molecules such as CD4. Moreover, blocking the activity of these GTPases could lead to new therapeutic interventions against AIDS.

RhoGDI-3 is a new GDP dissociation inhibitor (GDI). Identification of a non-cytosolic GDI protein interacting with the small GTP-binding proteins RhoB and RhoG

RhoB is a small GTP-binding protein highly homologous to the RhoA protein. While RhoA is known to regulate the assembly of focal adhesions and stress fibers in response to growth factors, the function of RhoB remains unknown. We have reported that the transient expression of the endogenous RhoB protein is regulated during the cell cycle, contrasting with the permanent RhoA protein expression (). Using the yeast two-hybrid system to characterize proteins interacting with RhoB, we identified a new mouse Rho GDP dissociation inhibitor, referenced as RhoGDI-3. The NH2-terminal alpha helix of RhoGDI-3 is strongly amphipatic and differs thus from that found in previously described bovine, human, and yeast RhoGDI proteins and mouse and human D4/Ly-GDIs. Contrary to the cytosolic localization of all known GDI proteins, acting on Rab or Rho, RhoGDI-3 is associated to a Triton X-100-insoluble membranous or cytoskeletal subcellular fraction. In the two-hybrid system, RhoGDI-3 interacts specifically with GDP- and GTP-bound forms of post-translationally processed RhoB and RhoG proteins, both of which show a growth-regulated expression in mammalian cells. No interaction is found with RhoA, RhoC, or Rac1 proteins. We show that GDI-3 is able to inhibit GDP/GTP exchange of RhoB and to release GDP-bound but not GTP-bound RhoB from cell membranes.

The highly conserved skb1 gene encodes a protein that interacts with Shk1, a fission yeast Ste20/PAK homolog

The Shk1 protein kinase, a homolog of Saccharomyces cerevisiae Ste20 and mammalian p21Cdc42/Rac-activated kinases, is an essential component of a Ras- and Cdc42-dependent signaling cascade required for cell viability, normal morphology, and mitogen-activated protein kinase-mediated sexual responses in the fission yeast, Schizosaccharomyces pombe. To identify S. pombe proteins that modulate or mediate Shk1 functions, we conducted a two-hybrid screen for Shk1-interacting proteins. One of the genes identified as a result of this screen was skb1. We show that Skb1 interacts with a region of the N-terminal regulatory domain of Shk1 distinct from that to which Cdc42 binds, and that Shk1, Cdc42, and Skb1 are able to form a ternary complex in vivo. S.pombe cells carrying an skb1 null mutation are less elongate in morphology than wild-type cells and exhibit a moderate growth defect. The morphology defect of the skb1 deletion mutant is suppressed by overexpression of Shk1. Overexpression of Skb1 causes wild-type S. pombe cells to become hyperelongated. Additional genetic analyses described herein suggest that Skb1 is a component of the morphology control branch of the Ras signaling cascade in S. pombe and that it positively modulates Shk1 function. Homologs of Skb1 are encoded by open reading frames in the genomes of S. cerevisiae and Caenorhabditis elegans and by an uncharacterized human cDNA sequence. Thus, skb1 may be the first well-characterized member of a highly conserved family of genes encoding potential p21Cdc42/Rac-activated kinase regulators.