Genghis Khan (Gek) as a putative effector for Drosophila Cdc42 and regulator of actin polymerization

The conserved Pelado/ZSWIM8 protein regulates actin dynamics by promoting linear actin filament polymerization

Actin filament polymerization can be branched or linear, which depends on the associated regulatory proteins. Competition for actin monomers occurs between proteins that induce branched or linear actin polymerization. Cell specialization requires the regulation of actin filaments to allow the formation of cell type-specific structures, like cuticular hairs in Drosophila, formed by linear actin filaments. Here, we report the functional analysis of CG34401/pelado, a gene encoding a SWIM domain-containing protein, conserved throughout the animal kingdom, called ZSWIM8 in mammals. Mutant pelado epithelial cells display actin hair elongation defects. This phenotype is reversed by increasing actin monomer levels or by either pushing linear actin polymerization or reducing branched actin polymerization. Similarly, in hemocytes, Pelado is essential to induce filopodia, a linear actin-based structure. We further show that this function of Pelado/ZSWIM8 is conserved in human cells, where Pelado inhibits branched actin polymerization in a cell migration context. In summary, our data indicate that the function of Pelado/ZSWIM8 in regulating actin cytoskeletal dynamics is conserved, favoring linear actin polymerization at the expense of branched filaments.

Cdc42/Rac Interactive Binding Containing Effector Proteins in Unicellular Protozoans With Reference to Human Host: Locks of the Rho Signaling

Small GTPases are the key to actin cytoskeleton signaling, which opens the lock of effector proteins to forward the signal downstream in several cellular pathways. Actin cytoskeleton assembly is associated with cell polarity, adhesion, movement and other functions in eukaryotic cells. Rho proteins, specifically Cdc42 and Rac, are the primary regulators of actin cytoskeleton dynamics in higher and lower eukaryotes. Effector proteins, present in an inactive state gets activated after binding to the GTP bound Cdc42/Rac to relay a signal downstream. Cdc42/Rac interactive binding (CRIB) motif is an essential conserved sequence found in effector proteins to interact with Cdc42 or Rac. A diverse range of Cdc42/Rac and their effector proteins have evolved from lower to higher eukaryotes. The present study has identified and further classified CRIB containing effector proteins in lower eukaryotes, focusing on parasitic protozoans causing neglected tropical diseases and taking human proteins as a reference point to the highest evolved organism in the evolutionary trait. Lower eukaryotes’ CRIB containing proteins fall into conventional effector molecules, PAKs (p21 activated kinase), Wiskoit-Aldrich Syndrome proteins family, and some have unique domain combinations unlike any known proteins. We also highlight the correlation between the effector protein isoforms and their selective specificity for Cdc42 or Rac proteins during evolution. Here, we report CRIB containing effector proteins; ten in Dictyostelium and Entamoeba, fourteen in Acanthamoeba, one in Trypanosoma and Giardia. CRIB containing effector proteins that have been studied so far in humans are potential candidates for drug targets in cancer, neurological disorders, and others. Conventional CRIB containing proteins from protozoan parasites remain largely elusive and our data provides their identification and classification for further in-depth functional validations. The tropical diseases caused by protozoan parasites lack combinatorial drug targets as effective paradigms. Targeting signaling mechanisms operative in these pathogens can provide greater molecules in combatting their infections.

Functional requirements of protein kinases and phosphatases in the development of the Drosophila melanogaster wing

Protein kinases and phosphatases constitute a large family of conserved enzymes that control a variety of biological processes by regulating the phosphorylation state of target proteins. They play fundamental regulatory roles during cell cycle progression and signaling, among other key aspects of multicellular development. The complement of protein kinases and phosphatases includes approximately 326 members in Drosophila, and they have been the subject of several functional screens searching for novel components of signaling pathways and regulators of cell division and survival. These approaches have been carried out mostly in cell cultures using RNA interference to evaluate the contribution of each protein in different functional assays, and have contributed significantly to assign specific roles to the corresponding genes. In this work we describe the results of an evaluation of the Drosophila complement of kinases and phosphatases using the wing as a system to identify their functional requirements in vivo. We also describe the results of several modifying screens aiming to identify among the set of protein kinases and phosphatases additional components or regulators of the activities of the Epidermal Growth Factor and Insulin receptors signaling pathways.

MRCK: a master regulator of tissue remodeling or another 'ROCK' in the epithelial block?

The epithelium forms a smart barrier to the external environment that can remodel whilst maintaining tissue integrity, a feature important for development, homeostasis, and function. Its dysregulation can lead to diseases ranging from cancer to vision loss. Epithelial remodeling requires reorganization of a thin sheet of actomyosin cortex under the plasma membrane of polarized cells that form basolateral contacts with neighboring cells and the extracellular matrix (ECM). Rho GTPases act as spatiotemporal molecular switches in this process, controlling localized actomyosin dynamics. However, the molecular mechanisms that control actomyosin dynamics at the apical cortex are poorly understood. This review focusses on a growing body of evidence that suggest myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK) plays a conserved role in morphogenetic signaling at the apical cortex in diverse cell and tissue remodeling processes. The possible molecular and mechanistic basis for the diverse functions of MRCK at the apical pole will also be discussed.

The cytoskeletal motor proteins Dynein and MyoV direct apical transport of Crumbs

Crumbs (Crb in Drosophila; CRB1-3 in mammals) is a transmembrane determinant of epithelial cell polarity and a regulator of Hippo signalling. Crb is normally localized to apical cell-cell contacts, just above adherens junctions, but how apical trafficking of Crb is regulated in epithelial cells remains unclear. We use the Drosophila follicular epithelium to demonstrate that polarized trafficking of Crb is mediated by transport along microtubules by the motor protein Dynein and along actin filaments by the motor protein Myosin-V (MyoV). Blocking transport of Crb-containing vesicles by Dynein or MyoV leads to accumulation of Crb within Rab11 endosomes, rather than apical delivery. The final steps of Crb delivery and stabilisation at the plasma membrane requires the exocyst complex and three apical FERM domain proteins - Merlin, Moesin and Expanded - whose simultaneous loss disrupts apical localization of Crb. Accordingly, a knock-in deletion of the Crb FERM-binding motif (FBM) also impairs apical localization. Finally, overexpression of Crb challenges this system, creating a sensitized background to identify components involved in cytoskeletal polarization, apical membrane trafficking and stabilisation of Crb at the apical domain.

Varroa destructor parasitism has a greater effect on proteome changes than the deformed wing virus and activates TGF-β signaling pathways

Honeybee workers undergo metamorphosis in capped cells for approximately 13 days before adult emergence. During the same period, Varroa mites prick the defenseless host many times. We sought to identify proteome differences between emerging Varroa-parasitized and parasite-free honeybees showing the presence or absence of clinical signs of deformed wing virus (DWV) in the capped cells. A label-free proteomic analysis utilizing nanoLC coupled with an Orbitrap Fusion Tribrid mass spectrometer provided a quantitative comparison of 2316 protein hits. Redundancy analysis (RDA) showed that the combination of Varroa parasitism and DWV clinical signs caused proteome changes that occurred in the same direction as those of Varroa alone and were approximately two-fold higher. Furthermore, proteome changes associated with DWV signs alone were positioned above Varroa in the RDA. Multiple markers indicate that Varroa activates TGF-β-induced pathways to suppress wound healing and the immune response and that the collective action of stressors intensifies these effects. Furthermore, we indicate JAK/STAT hyperactivation, p53-BCL-6 feedback loop disruption, Wnt pathway activation, Wnt/Hippo crosstalk disruption, and NF-κB and JAK/STAT signaling conflict in the Varroa–honeybee–DWV interaction. These results illustrate the higher effect of Varroa than of DWV at the time of emergence. Markers for future research are provided.

Myotonic dystrophy kinase-related Cdc42-binding kinases (MRCK), the ROCK-like effectors of Cdc42 and Rac1

Cdc42 is a member of the Rho GTPase protein family that plays key roles in local F-actin organization through a number of kinase and non-kinase effector proteins. The myotonic dystrophy kinase-related Cdc42-binding kinases (MRCKs), and the RhoA binding coiled-coil containing kinases (ROCKs) are widely expressed members of the Dystrophia myotonica protein kinase (DMPK) family. The MRCK proteins are ∼190 kDa multi-domain proteins expressed in all cells and coordinate certain acto-myosin networks. Notably MRCK is a key regulator of myosin18A and myosin IIA/B, and through phosphorylation of their common regulatory light chains (MYL9 or MLC2) to promote actin stress fiber contractility. The MRCK kinases are regulated by Cdc42, which is required for cell polarity and directional migration; MRCK links to the acto-myosin complex through interaction with a coiled-coil containing adaptor proteins LRAP35a/b. The biological activities of MRCK in model organisms such as worms and flies confirm it as a myosin II activator. In mammalian cell culture MRCK can be critical for cancer cell migration and neurite outgrowth. We review the current literatures regarding MRCK and highlight the similarities and differences between MRCK and ROCK kinases.

The actin-myosin regulatory MRCK kinases: regulation, biological functions and associations with human cancer

The contractile actin-myosin cytoskeleton provides much of the force required for numerous cellular activities such as motility, adhesion, cytokinesis and changes in morphology. Key elements that respond to various signal pathways are the myosin II regulatory light chains (MLC), which participate in actin-myosin contraction by modulating the ATPase activity and consequent contractile force generation mediated by myosin heavy chain heads. Considerable effort has focussed on the role of MLC kinases, and yet the contributions of the myotonic dystrophy-related Cdc42-binding kinases (MRCK) proteins in MLC phosphorylation and cytoskeleton regulation have not been well characterized. In contrast to the closely related ROCK1 and ROCK2 kinases that are regulated by the RhoA and RhoC GTPases, there is relatively little information about the CDC42-regulated MRCKα, MRCKβ and MRCKγ members of the AGC (PKA, PKG and PKC) kinase family. As well as differences in upstream activation pathways, MRCK and ROCK kinases apparently differ in the way that they spatially regulate MLC phosphorylation, which ultimately affects their influence on the organization and dynamics of the actin-myosin cytoskeleton. In this review, we will summarize the MRCK protein structures, expression patterns, small molecule inhibitors, biological functions and associations with human diseases such as cancer.

Multi-tissue microarray analysis identifies a molecular signature of regeneration

The inability to functionally repair tissues that are lost as a consequence of disease or injury remains a significant challenge for regenerative medicine. The molecular and cellular processes involved in complete restoration of tissue architecture and function are expected to be complex and remain largely unknown. Unlike humans, certain salamanders can completely regenerate injured tissues and lost appendages without scar formation. A parsimonious hypothesis would predict that all of these regenerative activities are regulated, at least in part, by a common set of genes. To test this hypothesis and identify genes that might control conserved regenerative processes, we performed a comprehensive microarray analysis of the early regenerative response in five regeneration-competent tissues from the newt Notophthalmus viridescens. Consistent with this hypothesis, we established a molecular signature for regeneration that consists of common genes or gene family members that exhibit dynamic differential regulation during regeneration in multiple tissue types. These genes include members of the matrix metalloproteinase family and its regulators, extracellular matrix components, genes involved in controlling cytoskeleton dynamics, and a variety of immune response factors. Gene Ontology term enrichment analysis validated and supported their functional activities in conserved regenerative processes. Surprisingly, dendrogram clustering and RadViz classification also revealed that each regenerative tissue had its own unique temporal expression profile, pointing to an inherent tissue-specific regenerative gene program. These new findings demand a reconsideration of how we conceptualize regenerative processes and how we devise new strategies for regenerative medicine.

The cytoskeletal regulator Genghis khan is required for columnar target specificity in the Drosophila visual system

A defining characteristic of neuronal cell type is the growth of axons and dendrites into specific layers and columns of the brain. Although differences in cell surface receptors and adhesion molecules are known to cause differences in synaptic specificity, differences in downstream signaling mechanisms that determine cell type-appropriate targeting patterns are unknown. Using a forward genetic screen in Drosophila, we identify the GTPase effector Genghis khan (Gek) as playing a crucial role in the ability of a subset of photoreceptor (R cell) axons to innervate appropriate target columns. In particular, single-cell mosaic analyses demonstrate that R cell growth cones lacking Gek function grow to the appropriate ganglion, but frequently fail to innervate the correct target column. Further studies reveal that R cell axons lacking the activity of the small GTPase Cdc42 display similar defects, providing evidence that these proteins regulate a common set of processes. Gek is expressed in all R cells, and a detailed structure-function analysis reveals a set of regulatory domains with activities that restrict Gek function to the growth cone. Although Gek does not normally regulate layer-specific targeting, ectopic expression of Gek is sufficient to alter the targeting choices made by another R cell type, the targeting of which is normally Gek independent. Thus, specific regulation of cytoskeletal responses to targeting cues is necessary for cell type-appropriate synaptic specificity.

A Syd-1 homologue regulates pre- and postsynaptic maturation in Drosophila

A proteomics approach identifies Drosophila Syd-1 as a Bruchpilot binding partner that controls maturation on both sides of the neuromuscular junction.

Active zones (AZs) are presynaptic membrane domains mediating synaptic vesicle fusion opposite postsynaptic densities (PSDs). At the Drosophila neuromuscular junction, the ELKS family member Bruchpilot (BRP) is essential for dense body formation and functional maturation of AZs. Using a proteomics approach, we identified Drosophila Syd-1 (DSyd-1) as a BRP binding partner. In vivo imaging shows that DSyd-1 arrives early at nascent AZs together with DLiprin-α, and both proteins localize to the AZ edge as the AZ matures. Mutants in dsyd-1 form smaller terminals with fewer release sites, and release less neurotransmitter. The remaining AZs are often large and misshapen, and ectopic, electron-dense accumulations of BRP form in boutons and axons. Furthermore, glutamate receptor content at PSDs increases because of excessive DGluRIIA accumulation. The AZ protein DSyd-1 is needed to properly localize DLiprin-α at AZs, and seems to control effective nucleation of newly forming AZs together with DLiprin-α. DSyd-1 also organizes trans-synaptic signaling to control maturation of PSD composition independently of DLiprin-α.

Convergent use of RhoGAP toxins by eukaryotic parasites and bacterial pathogens

Inactivation of host Rho GTPases is a widespread strategy employed by bacterial pathogens to manipulate mammalian cellular functions and avoid immune defenses. Some bacterial toxins mimic eukaryotic Rho GTPase-activating proteins (GAPs) to inactivate mammalian GTPases, probably as a result of evolutionary convergence. An intriguing question remains whether eukaryotic pathogens or parasites may use endogenous GAPs as immune-suppressive toxins to target the same key genes as bacterial pathogens. Interestingly, a RhoGAP domain-containing protein, LbGAP, was recently characterized from the parasitoid wasp Leptopilina boulardi, and shown to protect parasitoid eggs from the immune response of Drosophila host larvae. We demonstrate here that LbGAP has structural characteristics of eukaryotic RhoGAPs but that it acts similarly to bacterial RhoGAP toxins in mammals. First, we show by immunocytochemistry that LbGAP enters Drosophila immune cells, plasmatocytes and lamellocytes, and that morphological changes in lamellocytes are correlated with the quantity of LbGAP they contain. Demonstration that LbGAP displays a GAP activity and specifically interacts with the active, GTP-bound form of the two Drosophila Rho GTPases Rac1 and Rac2, both required for successful encapsulation of Leptopilina eggs, was then achieved using biochemical tests, yeast two-hybrid analysis, and GST pull-down assays. In addition, we show that the overall structure of LbGAP is similar to that of eukaryotic RhoGAP domains, and we identify distinct residues involved in its interaction with Rac GTPases. Altogether, these results show that eukaryotic parasites can use endogenous RhoGAPs as virulence factors and that despite their differences in sequence and structure, eukaryotic and bacterial RhoGAP toxins are similarly used to target the same immune pathways in insects and mammals.

Drosophila female sterile mutation spoonbill interferes with multiple pathways in oogenesis

spoonbill is a Drosophila female-sterile mutation, which displays a range of eggshell and egg chamber patterning defects. Previous analysis has shown that the mutation interfered with the function of two major signaling pathways, GRK/EGFR and DPP. In this report, the nature of spoonbill was further investigated to examine whether it was associated with additional pathways in oogenesis. Clonal analysis, presented here, demonstrated that most of the aberrant phenotypes associated with spoonbill were dependent on a mutant germline. Nevertheless, SPOONBILL may function also in the soma to ensure proper polarization and migration of the border-cell-cluster. Further, genetic interaction studies implicated spoonbill in additional unrelated pathways such as the one(s) involved in actin polymerization/depolymerization. Based on the previous data and the results presented here, it is anticipated that spoonbill may encode a multifunctional protein that perhaps coordinately regulated the activity of multiple signaling pathways during oogenesis.

Molecular insights into the self-assembly mechanism of dystrophia myotonica kinase

Self-assembly via coiled-coil domains (CC) is crucial for the regulation of the dystrophia myotonica kinase (DMPK) -related family of kinases. These CC domains are thought to form dimeric arrangements and thus to mediate dimerization in these enzymes. Using size exclusion chromatography combined with multiangle static light scattering, we analyzed the oligomeric state of DMPK as well as that of a truncated variant lacking the CC fraction. Remarkably, both forms were found to assemble into robust dimers. In contrast, the CC domain in isolation yielded trimeric assemblies, indicating that the oligomerization properties of CC domains from this kinase family are more diversified than anticipated. The crystal structure of this CC has been elucidated to 1.6 angstroms resolution and its properties in solution established using sedimentation equilibrium and thermal denaturation. These data show that, contrary to expectations, the self-assembly of DMPK is not dictated by the association properties of its CC domain. Instead, it appears to be driven by sequence segments flanking both N and C termini of the catalytic kinase fraction, as suggested by models of head-to-head dimers based on small angle X-ray scattering data. Our findings support a shared pattern of assembly across DMPK, ROCKs, and MRCK members of this family.

Characterization of Drosophila mini-me, a gene required for cell proliferation and survival

In the developing Drosophila eye, the morphogenetic furrow is a developmental organizing center for patterning and cell proliferation. The furrow acts both to limit eye size and to coordinate the number of cells to the number of facets. Here we report the molecular and functional characterization of Drosophila mini-me (mnm), a potential regulator of cell proliferation and survival in the developing eye. We first identified mnm as a dominant modifier of hedgehog loss-of-function in the developing eye. We report that mnm encodes a conserved protein with zinc knuckle and RING finger domains. We show that mnm is dispensable for patterning of the eye disc, but required in the eye for normal cell proliferation and survival. We also show that mnm null mutant cells exhibit altered cell cycle profiles and contain excess nucleic acid. Moreover, mnm overexpression can induce cells to proliferate and incorporate BrdU. Thus, our data implicate mnm as a regulator of mitotic progression during the proliferative phase of eye development, possibly through the control of nucleic acid metabolism.

Nuclear movement regulated by Cdc42, MRCK, myosin, and actin flow establishes MTOC polarization in migrating cells

The microtubule-organizing center (MTOC) is reoriented between the nucleus and the leading edge in many migrating cells and contributes to directional migration. Models suggest that the MTOC is moved to its position during reorientation. By direct imaging of wound-edge fibroblasts after triggering MTOC reorientation with soluble factors, we found instead that the nucleus moved away from the leading edge to reorient the MTOC, while the MTOC remained stationary. Rearward nuclear movement was coupled with actin retrograde flow and was regulated by a pathway involving Cdc42, MRCK, myosin, and actin. Nuclear movement was unaffected by the inhibition of dynein, Par6, or PKCzeta, yet these components were essential for MTOC reorientation, as they maintained the MTOC at the cell centroid. These results show that nuclear repositioning is an initial polarizing event in migrating cells and that the positions of the nucleus and the MTOC are established by separate regulatory pathways.

PAK and other Rho-associated kinases--effectors with surprisingly diverse mechanisms of regulation

The Rho GTPases are a family of molecular switches that are critical regulators of signal transduction pathways in eukaryotic cells. They are known principally for their role in regulating the cytoskeleton, and do so by recruiting a variety of downstream effector proteins. Kinases form an important class of Rho effector, and part of the biological complexity brought about by switching on a single GTPase results from downstream phosphorylation cascades. Here we focus on our current understanding of the way in which different Rho-associated serine/threonine kinases, denoted PAK (p21-activated kinase), MLK (mixed-lineage kinase), ROK (Rho-kinase), MRCK (myotonin-related Cdc42-binding kinase), CRIK (citron kinase) and PKN (protein kinase novel), interact with and are regulated by their partner GTPases. All of these kinases have in common an ability to dimerize, and in most cases interact with a variety of other proteins that are important for their function. A diversity of known structures underpin the Rho GTPase-kinase interaction, but only in the case of PAK do we have a good molecular understanding of kinase regulation. The ability of Rho GTPases to co-ordinate spatial and temporal phosphorylation events explains in part their prominent role in eukaryotic cell biology.

Citron kinase is an essential effector of the Pbl-activated Rho signalling pathway in Drosophila melanogaster

Pebble (Pbl)-activated RhoA signalling is essential for cytokinesis in Drosophila melanogaster. Here we report that the Drosophila citron gene encodes an essential effector kinase of Pbl-RhoA signalling in vivo. Drosophila citron is expressed in proliferating tissues but is downregulated in differentiating tissues. We find that Citron can bind RhoA and that localisation of Citron to the contractile ring is dependent on the cytokinesis-specific Pbl-RhoA signalling. Phenotypic analysis of mutants showed that citron is required for cytokinesis in every tissue examined, with mutant cells exhibiting multinucleate and hyperploid phenotypes. Strong genetic interactions were observed between citron and pbl alleles and constructs. Vertebrate studies implicate at least two Rho effector kinases, Citron and Rok, in cytokinesis. By contrast, we failed to find evidence for a role for the Drosophila ortholog of Rok in cell division. We conclude that Citron plays an essential, non-redundant role in the Rho signalling pathway during Drosophila cytokinesis.

Genome-wide survey of protein kinases required for cell cycle progression

Cycles of protein phosphorylation are fundamental in regulating the progression of the eukaryotic cell through its division cycle. Here we test the complement of Drosophila protein kinases (kinome) for cell cycle functions after gene silencing by RNA-mediated interference. We observed cell cycle dysfunction upon downregulation of 80 out of 228 protein kinases, including most kinases that are known to regulate the division cycle. We find new enzymes with cell cycle functions; some of these have family members already known to phosphorylate microtubules, actin or their associated proteins. Additionally, depletion of several signalling kinases leads to specific mitotic aberrations, suggesting novel roles for familiar enzymes. The survey reveals the inter-digitation of systems that monitor cellular physiology, cell size, cellular stress and signalling processes with the basic cell cycle regulatory machinery.

Drosophila citron kinase is required for the final steps of cytokinesis

The mechanisms underlying completion of cytokinesis are still poorly understood. Here, we show that the Drosophila orthologue of mammalian Citron kinases is essential for the final events of the cytokinetic process. Flies bearing mutations in the Drosophila citron kinase (dck) gene were defective in both neuroblast and spermatocyte cytokinesis. In both cell types, early cytokinetic events such as central spindle assembly and contractile ring formation were completely normal. Moreover, cytokinetic rings constricted normally, leading to complete furrow ingression. However late telophases of both cell types displayed persistent midbodies associated with disorganized F actin and anillin structures. Similar defects were observed in dck RNA interference (RNAi) telophases, which, in addition to abnormal F actin and anillin rings, also displayed aberrant membrane protrusions at the cleavage site. Together, these results indicate that mutations in the dck gene result in morphologically abnormal intercellular bridges and in delayed resolution of these structures, suggesting that the wild-type function of dck is required for abscission at the end of cytokinesis. The phenotype of Dck-depleted cells is different from those observed in most Drosophila cytokinesis mutants but extraordinarily similar to that caused by anillin RNAi, suggesting that Dck and anillin are in the same pathway for completion of cytokinesis.

Expression of the human myotonic dystrophy kinase-related Cdc42-binding kinase gamma is regulated by promoter DNA methylation and Sp1 binding

Myotonic dystrophy kinase-related Cdc42 binding kinases (MRCKs) are family members most related to the myotonic dystrophy kinase (DMPK), RhoA-binding kinase (ROK), and citron kinase. Two highly conserved members, MRCKalpha and -beta, have been previously identified and characterized. We now describe a novel isoform, MRCKgamma, which is functionally and structurally related to members of this kinase family. We show these kinases to have marked similarities in their genomic organization, substrate phosphorylation, and catalytic autoinhibition. Unlike MRCKalpha and -beta, which are expressed ubiquitously, MRCKgamma mRNA was only expressed in heart and skeletal muscle. In cultured cells, MRCKgamma showed differential expression with high levels of expression only in certain cell lines. DNA analysis showed that lack of expression is correlated with promoter DNA methylation. We have mapped the methylation sites in the MRCKgamma promoter. Significantly, agents that suppressed DNA methylation caused increases in the expression of the kinase in low-expressing cells, further supporting the notion that promoter DNA methylation plays an important role in the expression of MRCKgamma. Analysis of the MRCKgamma promoter has also revealed two proximal Sp1 sites that are essential for transcriptional activity. We conclude that both promoter DNA methylation and Sp1 binding are important regulators for MRCKgamma expression.

Homodimerization through coiled-coil regions enhances activity of the myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) is the protein product of the human DM-1 locus on chromosome 19q13.1 and has been implicated in the cardiac and behavioral dysfunctions of the disorder. DMPK contains four distinct regions: a leucine-rich repeat (L), a serine-threonine protein kinase catalytic domain (PK), an alpha-helical coiled-coil region (H), and a putative transmembrane-spanning tail (T). Multiple protein kinases that participate in cytoskeletal and cell cycle functions share homology with DMPK in the PK and H regions. Here we show that the LPKH and PKH subfragments of DMPK formed dimers of 140000 molecular weight, whereas the LPK subfragment remained a monomer of 62000 apparent molecular weight. The H domain thus appeared to be required for dimerization of DMPK subfragments. Caspase 1 cleaved LPKH between the PK and H regions. After cleavage, LPKH dimers became LPK-like monomers, consistent with the H domain mediating dimerization. The V(max) and k(cat)/K(m) of LPKH with a synthetic peptide kinase substrate were over 10-fold greater than either LPK or caspase-cleaved LPKH. The K(m) of dimeric LPKH was over three-fold greater than those of the monomeric proteins. Dimerization appeared to significantly affect the catalytic efficiency and substrate binding of DMPK. These interactions are likely to be functionally significant in other members of the myotonic dystrophy family of protein kinases with extensive coiled-coil domains.

Isolation of a novel cDNA enriched in the undifferentiated chick retina and lens

By using a differential screening strategy, we have identified a previously uncharacterised gene that is preferentially expressed in chick retinal precursor cells as well as in the anterior epithelial cells of the lens at early stages of development. The EURL transcript has an open reading frame of 293 amino acids and a 3' untranslated region of >850 nucleotides. The only recognizable feature in the EURL (early undifferentiated retina and lens) amino acid sequence is a putative coiled-coil domain located at the C-terminus of the protein. The human EURL orthologue maps to chromosome 21q21. Northern blot analysis, in situ hybridization of tissue sections, and whole-mount hybridization indicate elevated levels of EURL mRNA in the embryonic dorsal retina from stage 14 (day 2) to stage 18 (day 3). At day 3.5, when >90% of the retinal cells are in the proliferative stage, EURL transcripts are found primarily in the peripheral dorsal retina, i.e., the most undifferentiated part of the dorsal retina. EURL transcripts are also detected in the lens at stage 18 and remain abundant in the proliferating epithelial cells of the lens until at least day 11. The distribution pattern of EURL in the developing retina and lens suggest a role before the events leading to cell determination and differentiation.

Wg/Wnt signal can be transmitted through arrow/LRP5,6 and Axin independently of Zw3/Gsk3beta activity

Activation of the Wnt signaling cascade provides key signals during development and in disease. Here we provide evidence, by designing a Wnt receptor with ligand-independent signaling activity, that physical proximity of Arrow (LRP) to the Wnt receptor Frizzled-2 triggers the intracellular signaling cascade. We have uncovered a branch of the Wnt pathway in which Armadillo activity is regulated concomitantly with the levels of Axin protein. The intracellular pathway bypasses Gsk3beta/Zw3, the kinase normally required for controlling beta-catenin/Armadillo levels, suggesting that modulated degradation of Armadillo is not required for Wnt signaling. We propose that Arrow (LRP) recruits Axin to the membrane, and that this interaction leads to Axin degradation. As a consequence, Armadillo is no longer bound by Axin, resulting in nuclear signaling by Armadillo.

Genomic organization of human myotonic dystrophy kinase-related Cdc42-binding kinase alpha reveals multiple alternative splicing and functional diversity

Myotonic dystrophy kinase-related Cdc42-binding kinase alpha (MRCKalpha) is a Cdc42/Rac interactive binding-containing serine/threonine kinase with multiple functional domains. Its roles in the regulation of peripheral actin reorganization in HeLa cells and NGF-induced neurite outgrowth in PC12 cells have been documented. Here we report the characterization of the genomic structure and alternative splicing of the human counterpart. Human MRCKalpha gene is located on chromosome 1q42.1, spanning a genomic region of 250-300 kb and is composed of 41 exons. Four exons in the internal variable region and six in the 3' end were found to undergo extensive alternative splicing, giving rise to 96 possible transcripts of different combinations. The region of the internal splice site that defines a variable region in between two functional domains of opposite regulatory effects on MRCKalpha catalytic activity, and the 3' end splice site that generates variants with differential GTPase binding activity suggest a role for these alternative splicing events in MRCKalpha regulation.

Diabetic flies? Using Drosophila melanogaster to understand the causes of monogenic and genetically complex diseases

Approximately three-quarters of human disease loci have counterparts in the fruit fly Drosophila melanogaster. This model organism is therefore extremely valuable for using to understand the role of these loci in normal development, and for unravelling genetic pathways in which these loci take part. Important advantages for Drosophila in such studies are its completed genome, the unparalleled collection of mutations already in existence, the relative ease in which new mutations can be generated, the existence of convenient techniques for inactivating or overexpressing genes in dispensable tissues that are easily observed and measured, and the ability to readily carry out second-site modifier genetics. Recent work in Drosophila on the insulin-signaling pathway, a pathway of profound clinical importance, is reviewed as an illustration of how such research can provide fundamental insights into the functions of this pathway in regulating growth and development. Moreover, Drosophila research is now identifying heretofore unknown regulators of insulin signaling, as well as indicating novel functions for this pathway in suppressing benign tumor formation and regulating life span.

Analysis of small GTPase signaling pathways using p21-activated kinase mutants that selectively couple to Cdc42

p21-activated kinase 1 (Pak1) is an effector for the small GTPases Cdc42 and Rac. Because Pak1 binds to and is activated by both these GTPases, it has been difficult to precisely delineate the signaling pathways that link extracellular stimuli to Pak1 activation. To separate activation of Pak1 by Cdc42 versus activation by Rac, we devised a genetic screen in yeast that enabled us to create and identify Pak1 mutants that selectively couple to Cdc42 but not Rac1. We recovered several such Pak1 mutants and found that the residues most often affected lie within the p21 binding domain, a region previously known to mediate Pak1 binding to GTPases, but that several mutations also map outside the borders of the p21 binding domain. Pak1 mutants that associate with Cdc42 but not Rac1 were also activated by Cdc42 but not Rac1. In rat 3Y1 cells expressing oncogenic Ha-Ras, the Pak1 mutants defective in Rac1 binding are not activated, suggesting that Ras signals through a GTPase other than Cdc42 to activate Pakl. Similar results were obtained when epidermal growth factor was used to activate Pak1. However, Pak1 mutants that are unable to bind Rac are nonetheless well activated by calf serum, implying that this stimulus may induce Pak activation independent of Rac.

The Rac GTPase-activating protein RotundRacGAP interferes with Drac1 and Dcdc42 signalling in Drosophila melanogaster

RhoGTPases are negatively regulated by GTPase-activating proteins (GAPs). Here we demonstrate that Drosophila RotundRacGAP is active in vitro on Drac1 and Dcdc42 but not Drho1. Similarly, in yeast, RotundRacGAP interacts specifically with Drac1 and Dcdc42, as well as with their activated V12 forms, showing a particularly strong interaction with Dcdc42V12. In the fly, lowering RotundRacGAP dosage specifically modifies eye defects induced by expressing Drac1 or Dcdc42 but not Drho1, confirming that Drac1 and Dcdc42 are indeed in vivo targets of RotundRacGAP. Furthermore, embryonic-directed expression of either RotundRacGAP, or dominant negative Drac1N17, transgenes induces similar defects in dorsal closure and inhibits Drac1-dependent cytoskeleton assembly at the leading edge. Expression of truncated forms of RotundRacGAP shows that the GAP domain of RotundRacGAP is essential for its function. Unexpectedly, transgenes encoding Drac1N17, Dcdc42N17, or RotundRacGAP do not affect the c-Jun N-terminal kinase-dependent gene expression of decapentaplegic and puckered, indicating that another Drac1-independent signal redundantly activates this pathway. Finally, in a situation where Drac1 is constitutively activated, RotundRacGAP greatly reduces the ectopic expression of decapentaplegic, possibly by negatively regulating Dcdc42.

Drosophila Rho-associated kinase (Drok) links Frizzled-mediated planar cell polarity signaling to the actin cytoskeleton

Frizzled (Fz) and Dishevelled (Dsh) are components of an evolutionarily conserved signaling pathway that regulates planar cell polarity. How this signaling pathway directs asymmetric cytoskeletal reorganization and polarized cell morphology remains unknown. Here, we show that Drosophila Rho-associated kinase (Drok) works downstream of Fz/Dsh to mediate a branch of the planar polarity pathway involved in ommatidial rotation in the eye and in restricting actin bundle formation to a single site in developing wing cells. The primary output of Drok signaling is regulating the phosphorylation of nonmuscle myosin regulatory light chain, and hence the activity of myosin II. Drosophila myosin VIIA, the homolog of the human Usher Syndrome 1B gene, also functions in conjunction with this newly defined portion of the Fz/Dsh signaling pathway to regulate the actin cytoskeleton.

Intermolecular and intramolecular interactions regulate catalytic activity of myotonic dystrophy kinase-related Cdc42-binding kinase alpha

Myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK) is a Cdc42-binding serine/threonine kinase with multiple functional domains. We had previously shown MRCKalpha to be implicated in Cdc42-mediated peripheral actin formation and neurite outgrowth in HeLa and PC12 cells, respectively. Here we demonstrate that native MRCK exists in high-molecular-weight complexes. We further show that the three independent coiled-coil (CC) domains and the N-terminal region preceding the kinase domain are responsible for intermolecular interactions leading to MRCKalpha multimerization. N terminus-mediated dimerization and consequent transautophosphorylation are critical processes regulating MRCKalpha catalytic activities. A region containing the two distal CC domains (CC2 and CC3; residues 658 to 930) was found to interact intramolecularly with the kinase domain and negatively regulates its activity. Its deletion also resulted in an active kinase, confirming a negative autoregulatory role. We provide evidence that the N terminus-mediated dimerization and activation of MRCK and the negative autoregulatory kinase-distal CC interaction are two mutually exclusive events that tightly regulate the catalytic state of the kinase. Disruption of this interaction by a mutant kinase domain resulted in increased kinase activity. MRCK kinase activity was also elevated when cells were treated with phorbol ester, which can interact directly with a cysteine-rich domain next to the distal CC domain. We therefore suggest that binding of phorbol ester to MRCK releases its autoinhibition, allowing N-terminal dimerization and subsequent kinase activation.

enok encodes a Drosophila putative histone acetyltransferase required for mushroom body neuroblast proliferation

Mushroom bodies in the Drosophila brain are centers for olfactory learning and memory. We have previously shown that the mushroom bodies comprise three types of neurons with distinct axonal projections. These three types of neurons are generated sequentially from common neuroblasts. We report here the identification of a gene that we have named enoki mushroom (enok), which when it is mutated gives rise to mushroom bodies with reduced axonal structures. enok encodes a putative histone acetyltransferase (HAT) of the MYST family, members of which have been implicated as important modulators of transcriptional activity. A single amino acid change in the zinc finger motif of the putative catalytic HAT domain gives the same phenotype as a null allele, and this finding indicates the importance of HAT activity to Enok's function. Further phenotypic analysis demonstrates that the mushroom body defect is due to an arrest in neuroblast proliferation rather than a failure of either cell fate switching or axon branching. Clonal analyses in the wing discs and the ovaries suggest that enok is essential for normal cell proliferation in some, but not all, tissues. Our results provide in vivo evidence for essential functions of a histone acetyltransferase in the construction of the Drosophila brain.

Embryonic morphogenesis in Caenorhabditis elegans integrates the activity of LET-502 Rho-binding kinase, MEL-11 myosin phosphatase, DAF-2 insulin receptor and FEM-2 PP2c phosphatase

let-502 rho-binding kinase and mel-11 myosin phosphatase regulate Caenorhabditis elegans embryonic morphogenesis. Genetic analysis presented here establishes the following modes of let-502 action: (i) loss of only maternal let-502 results in abnormal early cleavages, (ii) loss of both zygotic and maternal let-502 causes elongation defects, and (iii) loss of only zygotic let-502 results in sterility. The morphogenetic function of let-502 and mel-11 is apparently redundant with another pathway since elimination of these two genes resulted in progeny that underwent near-normal elongation. Triple mutant analysis indicated that unc-73 (Rho/Rac guanine exchange factor) and mlc-4 (myosin light chain) act in parallel to or downstream of let-502/mel-11. In contrast mig-2 (Rho/Rac), daf-2 (insulin receptor), and age-1 (PI3 kinase) act within the let-502/mel-11 pathway. Mutations in the sex-determination gene fem-2, which encodes a PP2c phosphatase (unrelated to the MEL-11 phosphatase), enhanced mutations of let-502 and suppressed those of mel-11. fem-2's elongation function appears to be independent of its role in sexual identity since the sex-determination genes fem-1, fem-3, tra-1, and tra-3 had no effect on mel-11 or let-502. By itself, fem-2 affects morphogenesis with low penetrance. fem-2 blocked the near-normal elongation of let-502; mel-11 indicating that fem-2 acts in a parallel elongation pathway. The action of two redundant pathways likely ensures accurate elongation of the C. elegans embryo.

The tricornered gene, which is required for the integrity of epidermal cell extensions, encodes the Drosophila nuclear DBF2-related kinase

During their differentiation epidermal cells of Drosophila form a rich variety of polarized structures. These include the epidermal hairs that decorate much of the adult cuticular surface, the shafts of the bristle sense organs, the lateral extensions of the arista, and the larval denticles. These cuticular structures are produced by cytoskeletal-mediated outgrowths of epidermal cells. Mutations in the tricornered gene result in the splitting or branching of all of these structures. Thus, tricornered function appears to be important for maintaining the integrity of the outgrowths. tricornered mutations however do not have major effects on the growth or shape of these cellular extensions. Inhibiting actin polymerization in differentiating cells by cytochalasin D or latrunculin A treatment also induces the splitting of hairs and bristles, suggesting that the actin cytoskeleton might be a target of tricornered. However, the drugs also result in short, fat, and occasionally malformed hairs and bristles. The data suggest that the function of the actin cytoskeleton is important for maintaining the integrity of cellular extensions as well as their growth and shape. Thus, if tricornered causes the splitting of cellular extensions by interacting with the actin cytoskeleton it likely does so in a subtle way. Consistent with this possibility we found that a weak tricornered mutant is hypersensitive to cytochalasin D. We have cloned the tricornered gene and found that it encodes the Drosophila NDR kinase. This is a conserved ser/thr protein kinase found in Caenorhabditis elegans and humans that is related to a number of kinases that have been found to be important in controlling cell structure and proliferation.

Rho GTPases in neuronal morphogenesis

The Rho family of small GTPases act as intracellular molecular switches that transduce signals from extracellular stimuli to the actin cytoskeleton and the nucleus. Recent evidence implicates Rho GTPases in the regulation of neuronal morphogenesis, including migration, polarity, axon growth and guidance, dendrite elaboration and plasticity, and synapse formation. Signalling pathways from membrane receptors to Rho GTPases and from Rho GTPases to the actin cytoskeleton are beginning to be discovered. Mutations in these signalling pathways have been reported in human neurological diseases, which underscores their importance in the development and function of the nervous system.

Neuronal cell shape and neurite initiation are regulated by the Ndr kinase SAX-1, a member of the Orb6/COT-1/warts serine/threonine kinase family

The Caenorhabditis elegans sax-1 gene regulates several aspects of neuronal cell shape. sax-1 mutants have expanded cell bodies and ectopic neurites in many classes of neurons, suggesting that SAX-1 functions to restrict cell and neurite growth. The ectopic neurites in sensory neurons of sax-1 mutants resemble the defects caused by decreased sensory activity. However, the activity-dependent pathway, mediated in part by the UNC-43 calcium/calmodulin-dependent kinase II, functions in parallel with SAX-1 to suppress neurite initiation. sax-1 encodes a serine/threonine kinase in the Ndr family that is related to the Orb6 (Schizosaccharomyces pombe), Warts/Lats (Drosophila), and COT-1 (Neurospora) kinases that function in cell shape regulation. These kinases have similarity to Rho kinases but lack consensus Rho-binding domains. Dominant negative mutations in the C. elegans RhoA GTPase cause neuronal cell shape defects similar to those of sax-1 mutants, and genetic interactions between rhoA and sax-1 suggest shared functions. These results suggest that SAX-1/Ndr kinases are endogenous inhibitors of neurite initiation and cell spreading.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Regulation of the cytoskeleton and cell adhesion by the Rho family GTPases in mammalian cells

Members of the Rho family of small Ras-like GTPases--including RhoA, -B, and -C, Rac1 and -2, and Cdc42--exhibit guanine nucleotide-binding activity and function as molecular switches, cycling between an inactive GDP-bound state and an active GTP-bound state. The Rho family GTPases participate in regulation of the actin cytoskeleton and cell adhesion through specific targets. Identification and characterization of these targets have begun to clarify how the Rho family GTPases act to regulate cytoskeletal structure and cell-cell and cell-substratum contacts in mammalian cells. The Rho family GTPases are also involved in regulation of smooth muscle contraction, cell morphology, cell motility, neurite retraction, and cytokinesis. However, the molecular mechanisms by which the Rho family GTPases participate in the regulation of such processes are not well established.

Rac-1 and Raf-1 kinases, components of distinct signaling pathways, activate myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) is a serine-threonine protein kinase encoded by the myotonic dystrophy (DM) locus on human chromosome 19q13.3. It is a close relative of other kinases that interact with members of the Rho family of small GTPases. We show here that the actin cytoskeleton-linked GTPase Rac-1 binds to DMPK, and coexpression of Rac-1 and DMPK activates its transphosphorylation activity in a GTP-sensitive manner. DMPK can also bind Raf-1 kinase, the Ras-activated molecule of the MAP kinase pathway. Purified Raf-1 kinase phosphorylates and activates DMPK. The interaction of DMPK with these distinct signals suggests that it may play a role as a nexus for cross-talk between their respective pathways and may partially explain the remarkable pleiotropy of DM.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Citron, a Rho target that affects contractility during cytokinesis

The small GTPase Rho, which regulates cell shape, is thought to contribute to cytokinesis. Recently, Citron was characterized as a Rho target. This large protein contains a Ser/Thr kinase domain related to that of ROCK, another Rho effector. Both endogenous Citron and recombinant Citron localize to the cleavage furrow in dividing cells and to the midbody in post-mitotic cells. Moreover, overexpression of Citron deleted from its C-terminal sequence caused abnormal contractions specifically during cytokinesis, resulting in the formation of multinucleated cells. Cell shape, F-actin, intermediate filaments, and microtubules appeared essentially normal in these cells during interphase. Thus, Citron is a Rho effector that appears to function during cytokinesis, modulating its contractile process. In brain, however, Citron is highly expressed in a subset of neurons as a brain-specific isoform that lacks a kinase domain, Citron-N. This protein accumulates in synapses and associates to the NMDA receptor via interaction with the adaptor protein PSD95, suggesting that the function of Citron is specialized in the neurons.

Myotonic dystrophy protein kinase (DMPK) induces actin cytoskeletal reorganization and apoptotic-like blebbing in lens cells

DMPK, the product of the DM locus, is a member of the same family of serine-threonine protein kinases as the Rho-associated enzymes. In DM, membrane inclusions accumulate in lens fiber cells producing cataracts. Overexpression of DMPK in cultured lens epithelial cells led to apoptotic-like blebbing of the plasma membrane and reorganization of the actin cytoskeleton. Enzymatically active DMPK was necessary for both effects; inactive mutant DMPK protein did not produce either effect. Active RhoA but not constitutive GDP-state mutant protein produced similar effects as DMPK. The similar actions of DMPK and RhoA suggest that they may function in the same regulatory network. The observed effects of DMPK may be relevant to the removal of membrane organelles during normal lens differentiation and the retention of intracellular membranes in DM lenses.

Identification and characterization of Drosophila homolog of Rho-kinase

The Rho family of small GTPases and their associated regulators and targets are essential mediators of diverse morphogenetic events in development. Mammalian Rho-kinase/ROK alpha, one of the targets of Rho, has been shown to bind to Rho in GTP-bound form and to phosphorylate the myosin light chain (MLC) and the myosin-binding subunit (MBS) of myosin phosphatase, resulting in the activation of myosin. Thus, Rho-kinase/ROK alpha has been suggested to play essential roles in the formation of stress fibers and focal adhesions. We have identified the Drosophila homolog of Rho-kinase/ROK alpha, DRho-kinase, which has conserved the basic structural feature of Rho-kinase/ROK alpha consisting of the N-terminal kinase, central coiled-coil and C-terminal pleckstrin homology (PH) domains. A two-hybrid analysis demonstrated that DRho-kinase interacts with the GTP-bound form of the Drosophila Rho. Drho1, at the conserved Rho-binding site. DRho-kinase can phosphorylate MLC and MBS, preferable substrates for bovine Rho-kinase, in vitro. DRho-kinase is ubiquitously expressed throughout development, in a pattern essentially identical to that of Drho1. These results suggest that DRho-kinase is an effector of Drho1.

MSE55, a Cdc42 effector protein, induces long cellular extensions in fibroblasts

Cdc42 is a member of the Rho GTPase family that regulates multiple cellular activities, including actin polymerization, kinase-signaling activation, and cell polarization. MSE55 is a nonkinase CRIB (Cdc42/Rac interactive-binding) domain-containing molecule of unknown function. Using glutathione S-transferase-capture experiments, we show that MSE55 binds to Cdc42 in a GTP-dependent manner. MSE55 binding to Cdc42 required an intact CRIB domain, because a MSE55 CRIB domain mutant no longer interacted with Cdc42. To study the function of MSE55 we transfected either wild-type MSE55 or a MSE55 CRIB mutant into mammalian cells. In Cos-7 cells, wild-type MSE55 localized at membrane ruffles and increased membrane actin polymerization, whereas expression of the MSE55 CRIB mutant showed fewer membrane ruffles. In contrast to these results, MSE55 induced the formation of long, actin-based protrusions in NIH 3T3 cells as detected by immunofluorescence and live-cell video microscopy. MSE55-induced protrusion formation was blocked by expression of dominant-negative N17Cdc42, but not by expression of dominant-negative N17Rac. These findings indicate that MSE55 is a Cdc42 effector protein that mediates actin cytoskeleton reorganization at the plasma membrane.

The myotonic dystrophy kinase-related Cdc42-binding kinase is involved in the regulation of neurite outgrowth in PC12 cells

The myotonic dystrophy kinase-related Cdc42-binding kinase (MRCKalpha) has been implicated in the morphological activities of Cdc42 in nonneural cells. Both MRCKalpha and the kinase-related Rho-binding kinase (ROKalpha) are involved in nonmuscle myosin light-chain phosphorylation and associated actin cytoskeleton reorganization. We now show that in PC12 cells, overexpression of the kinase domain of MRCKalpha and ROKalpha resulted in retraction of neurites formed on nerve growth factor (NGF) treatment, as observed with RhoA. However, introduction of kinase-dead MRCKalpha did not result in NGF-independent neurite outgrowth as observed with dominant negative kinase-dead ROKalpha or the Rho inhibitor C3. Neurite outgrowth induced by NGF or kinase-dead ROKalpha was inhibited by dominant negative Cdc42(N17), Rac1(N17), and the Src homology 3 domain of c-Crk, indicating the participation of common downstream components. Neurite outgrowth induced by either agent was blocked by kinase-dead MRCKalpha lacking the p21-binding domain or by a minimal C-terminal regulatory region consisting of the cysteine-rich domain/pleckstrin homology domain plus a region with homology to citron. The latter region alone was an effective blocker of NGF-induced outgrowth. These results suggest that although ROKalpha is involved in neurite retraction promoted by RhoA, the related MRCKalpha is conversely involved in neurite outgrowth promoted by Cdc42 and Rac.

Cloning and chromosomal localization of human Cdc42-binding protein kinase beta

The p21 GTPases, Rho and Cdc42, regulate numerous cellular functions by binding to members of a serine/threonine protein kinase subfamily. These functions include the remodeling of the cell cytoskeleton that is a feature of cell growth and differentiation. Two of these p21 GTPase-regulated kinases, the myotonic dystrophy protein kinase-related Cdc42-binding kinases (MRCKalpha and beta), have been recently characterized in rat. Both of these proteins phosphorylate nonmuscle myosin light chain, a prerequisite for the activation of actin-myosin contractility. Here we report the cDNA cloning of the human homologue of MRCKbeta, CDC42BPB, which was found by Northern blot analysis to be expressed in a wide range of tissues. The human CDC42BPB gene maps to cytogenetic band 14q32.3 by FISH analysis.

Inducible recruitment of Cdc42 or WASP to a cell-surface receptor triggers actin polymerization and filopodium formation

BACKGROUND

Cdc42, a GTP-binding protein of the Rho family, controls actin cytoskeletal organization and helps to generate actin-based protruding structures, such as filopodia. In vitro, Cdc42 regulates actin polymerization by facilitating the creation of free barbed ends - the more rapidly growing ends of actin filaments - and subsequent elongation at these ends. The Wiskott- Aldrich syndrome protein, WASP, which has a pleckstrin-homology domain and a Cdc42/Rac-binding motif, has been implicated in cell signaling and cytoskeleton reorganization. We have investigated the consequences of local recruitment of activated Cdc42 or WASP to the plasma membrane.

RESULTS

We used an activated Cdc42 protein that could be recruited to an engineered membrane receptor by adding rapamycin as a bridge, and added antibody-coupled beads to aggregate these receptors. Inducible recruitment of Cdc42 to clusters of receptors stimulated actin polymerization, resulting in the formation of membrane protrusions. Cdc42-induced protrusions were enriched in the vasodilator-stimulated phosphoprotein VASP and the focal-adhesion-associated proteins zyxin and ezrin. The Cdc42 effector WASP could also induce the formation of protrusions, albeit of different morphology.

CONCLUSIONS

This is the first demonstration that the local recruitment of activated Cdc42 or its downstream effector, WASP, to a membrane receptor in whole cells is sufficient to trigger actin polymerization that results in the formation of membrane protrusions. Our data suggest that Cdc42-induced actin-based protrusions result from the local and serial recruitment of cytoskeletal proteins including zyxin, VASP, and ezrin.