Molecular cloning and characterization of a conserved nuclear serine(threonine) protein kinase

Uncovering new insights into the role of the ubiquitin ligase Smurf1 on the regulation of innate immune signaling and resistance to infection

Innate immunity is the body's first line of defense against infections. Innate immune cells express pattern recognition receptors in distinct cellular compartments that are responsible to detect either pathogens-associated molecules or cellular components derived from damaged cells, to trigger intracellular signaling pathways that lead to the activation of inflammatory responses. Inflammation is essential to coordinate immune cell recruitment, pathogen elimination and to keep normal tissue homeostasis. However, uncontrolled, misplaced or aberrant inflammatory responses could lead to tissue damage and drive chronic inflammatory diseases and autoimmunity. In this context, molecular mechanisms that tightly regulate the expression of molecules required for the signaling of innate immune receptors are crucial to prevent pathological immune responses. In this review, we discuss the ubiquitination process and its importance in the regulation of innate immune signaling and inflammation. Then, we summarize the roles of Smurf1, a protein that works on ubiquitination, on the regulation of innate immune signaling and antimicrobial mechanisms, emphasizing its substrates and highlighting its potential as a therapeutic target for infectious and inflammatory conditions.

The Role of Mammalian STK38 in DNA Damage Response and Targeting for Radio-Sensitization

Protein kinases, found in the nucleus and cytoplasm, play essential roles in a multitude of cellular processes, including cell division, proliferation, apoptosis, and signal transduction. STK38 is a member of the protein kinase A (PKA)/PKG/PKC family implicated in regulating cell division and morphogenesis in yeast and C. elegans. However, its function remained largely unknown in mammals. In recent years, advances in research on STK38 and the identification of its substrates has led to a better understanding of its function and role in mammals. This review discusses the structure, expression, and regulation of activity as a kinase, its role in the DNA damage response, cross-talk with other signaling pathways, and its application for radio-sensitization.

Some Insights into the Regulation of Cardiac Physiology and Pathology by the Hippo Pathway

The heart is one of the most fascinating organs in living beings. It beats up to 100,000 times a day throughout the lifespan, without resting. The heart undergoes profound anatomical, biochemical, and functional changes during life, from hypoxemic fetal stages to a completely differentiated four-chambered cardiac muscle. In the middle, many biological events occur after and intersect with each other to regulate development, organ size, and, in some cases, regeneration. Several studies have defined the essential roles of the Hippo pathway in heart physiology through the regulation of apoptosis, autophagy, cell proliferation, and differentiation. This molecular route is composed of multiple components, some of which were recently discovered, and is highly interconnected with multiple known prosurvival pathways. The Hippo cascade is evolutionarily conserved among species, and in addition to its regulatory roles, it is involved in disease by drastically changing the heart phenotype and its function when its components are mutated, absent, or constitutively activated. In this review, we report some insights into the regulation of cardiac physiology and pathology by the Hippo pathway.

The Emerging Roles of NDR1/2 in Infection and Inflammation

The nuclear Dbf2-related (NDR) kinases NDR1 and NDR2 belong to the NDR/LATS (large tumor suppressor) subfamily in the Hippo signaling pathway. They are highly conserved from yeast to humans. It is well-known that NDR1/2 control important cellular processes, such as morphological changes, centrosome duplication, cell proliferation, and apoptosis. Recent studies revealed that NDR1/2 also play important roles in the regulation of infection and inflammation. In this review, we summarized the roles of NDR1/2 in the modulation of inflammation induced by cytokines and innate immune response against the infection of bacteria and viruses, emphasizing on how NDR1/2 regulate signaling transduction through Hippo pathway-dependent and -independent manners.

Mob Family Proteins: Regulatory Partners in Hippo and Hippo-Like Intracellular Signaling Pathways

Studies in yeast first delineated the function of Mob proteins in kinase pathways that regulate cell division and shape; in multicellular eukaryotes Mobs regulate tissue growth and morphogenesis. In animals, Mobs are adaptors in Hippo signaling, an intracellular signal-transduction pathway that restricts growth, impacting the development and homeostasis of animal organs. Central to Hippo signaling are the Nuclear Dbf2-Related (NDR) kinases, Warts and LATS1 and LATS2, in flies and mammals, respectively. A second Hippo-like signaling pathway has been uncovered in animals, which regulates cell and tissue morphogenesis. Central to this emergent pathway are the NDR kinases, Tricornered, STK38, and STK38L. In Hippo signaling, NDR kinase activation is controlled by three activating interactions with a conserved set of proteins. This review focuses on one co-activator family, the highly conserved, non-catalytic Mps1-binder-related (Mob) proteins. In this context, Mobs are allosteric activators of NDR kinases and adaptors that contribute to assembly of multiprotein NDR kinase activation complexes. In multicellular eukaryotes, the Mob family has expanded relative to model unicellular yeasts; accumulating evidence points to Mob functional diversification. A striking example comes from the most sequence-divergent class of Mobs, which are components of the highly conserved Striatin Interacting Phosphatase and Kinase (STRIPAK) complex, that antagonizes Hippo signaling. Mobs stand out for their potential to modulate the output from Hippo and Hippo-like kinases, through their roles both in activating NDR kinases and in antagonizing upstream Hippo or Hippo-like kinase activity. These opposing Mob functions suggest that they coordinate the relative activities of the Tricornered/STK38/STK38L and Warts/LATS kinases, and thus have potential to assemble nodes for pathway signaling output. We survey the different facets of Mob-dependent regulation of Hippo and Hippo-like signaling and highlight open questions that hinge on unresolved aspects of Mob functions.

Mob Family Proteins: Regulatory Partners in Hippo and Hippo-Like Intracellular Signaling Pathways

Studies in yeast first delineated the function of Mob proteins in kinase pathways that regulate cell division and shape; in multicellular eukaryotes Mobs regulate tissue growth and morphogenesis. In animals, Mobs are adaptors in Hippo signaling, an intracellular signal-transduction pathway that restricts growth, impacting the development and homeostasis of animal organs. Central to Hippo signaling are the Nuclear Dbf2-Related (NDR) kinases, Warts and LATS1 and LATS2, in flies and mammals, respectively. A second Hippo-like signaling pathway has been uncovered in animals, which regulates cell and tissue morphogenesis. Central to this emergent pathway are the NDR kinases, Tricornered, STK38, and STK38L. In Hippo signaling, NDR kinase activation is controlled by three activating interactions with a conserved set of proteins. This review focuses on one co-activator family, the highly conserved, non-catalytic Mps1-binder-related (Mob) proteins. In this context, Mobs are allosteric activators of NDR kinases and adaptors that contribute to assembly of multiprotein NDR kinase activation complexes. In multicellular eukaryotes, the Mob family has expanded relative to model unicellular yeasts; accumulating evidence points to Mob functional diversification. A striking example comes from the most sequence-divergent class of Mobs, which are components of the highly conserved Striatin Interacting Phosphatase and Kinase (STRIPAK) complex, that antagonizes Hippo signaling. Mobs stand out for their potential to modulate the output from Hippo and Hippo-like kinases, through their roles both in activating NDR kinases and in antagonizing upstream Hippo or Hippo-like kinase activity. These opposing Mob functions suggest that they coordinate the relative activities of the Tricornered/STK38/STK38L and Warts/LATS kinases, and thus have potential to assemble nodes for pathway signaling output. We survey the different facets of Mob-dependent regulation of Hippo and Hippo-like signaling and highlight open questions that hinge on unresolved aspects of Mob functions.

NDR2 promotes the antiviral immune response via facilitating TRIM25-mediated RIG-I activation in macrophages

Retinoic acid-inducible gene I (RIG-I), a pivotal cytosolic sensor, recognizes viral RNAs to initiate antiviral innate immunity. However, posttranslational regulation of RIG-I signaling is not well understood. We report here that nuclear Dbf2-related kinase 2 (NDR2) functions as a crucial positive regulator of the RIG-I-mediated antiviral immune response. Overexpression of NDR2 or its kinase-inactive mutants potentiates RNA virus-induced production of type I interferons and proinflammatory cytokines and dampens viral replication. NDR2 conditional knockout mice (Lysm+NDR2f/f) show an impaired antiviral immune response. Mechanistically, NDR2 directly associates with RIG-I and TRIM25, thus facilitating the RIG-I/TRIM25 complex and enhancing the TRIM25-mediated K63-linked polyubiquitination of RIG-I, which is required for the RIG-I-mediated antiviral immune response. Furthermore, NDR2 expression is notably down-regulated in peripheral blood from respiratory syncytial virus-infected patients and in virus-infected macrophages. Collectively, these findings provide insights into the function of NDR2 in antiviral immunity and its related clinical significance.

Two Antagonistic Hippo Signaling Circuits Set the Division Plane at the Medial Position in the Ciliate Tetrahymena

In a single cell, ciliates maintain a complex pattern of cortical organelles that are arranged along the anteroposterior and circumferential axes. The underlying molecular mechanisms of intracellular pattern formation in ciliates are largely unknown. Ciliates divide by tandem duplication, a process that remodels the parental cell into two daughters aligned head-to-tail. In the elo1-1 mutant of Tetrahymena thermophila, the segmentation boundary/division plane forms too close to the posterior end of the parental cell, producing a large anterior and a small posterior daughter cell, respectively. We show that ELO1 encodes a Lats/NDR kinase that marks the posterior segment of the cell cortex, where the division plane does not form in the wild-type. Elo1 acts independently of CdaI, a Hippo/Mst kinase that marks the anterior half of the parental cell, and whose loss shifts the division plane anteriorly. We propose that, in Tetrahymena, two antagonistic Hippo circuits focus the segmentation boundary/division plane at the equatorial position, by excluding divisional morphogenesis from the cortical areas that are too close to cell ends.

Downregulated NDR1 protein kinase inhibits innate immune response by initiating an miR146a-STAT1 feedback loop

Interferon (IFN)-stimulated genes (ISGs) play crucial roles in the antiviral immune response; however, IFNs also induce negative regulators that attenuate the antiviral response. Here, we show that both viral and bacterial invasion downregulate Nuclear Dbf2-related kinase 1 (NDR1) expression via the type I IFN signaling pathway. NDR1 promotes the virus-induced production of type I IFN, proinflammatory cytokines and ISGs in a kinase-independent manner. NDR1 deficiency also renders mice more susceptible to viral and bacterial infections. Mechanistically, NDR1 enhances STAT1 translation by directly binding to the intergenic region of miR146a, thereby inhibiting miR146a expression and liberating STAT1 from miR146a-mediated translational inhibition. Furthermore, STAT1 binds to the miR146a promoter, thus decreasing its expression. Together, our results suggest that NDR1 promotion of STAT1 translation is an important event for IFN-dependent antiviral immune response, and suggest that NDR1 has an important role in controlling viral infections.

The authors show that NDR1 promotion of STAT1 translation is an important event for IFN-dependent antiviral immune response. These data suggest that NDR1 has an important role in controlling viral infections.

The Roles of NDR Protein Kinases in Hippo Signalling

The Hippo tumour suppressor pathway has emerged as a critical regulator of tissue growth through controlling cellular processes such as cell proliferation, death, differentiation and stemness. Traditionally, the core cassette of the Hippo pathway includes the MST1/2 protein kinases, the LATS1/2 protein kinases, and the MOB1 scaffold signal transducer, which together regulate the transcriptional co-activator functions of the proto-oncoproteins YAP and TAZ through LATS1/2-mediated phosphorylation of YAP/TAZ. Recent research has identified additional kinases, such as NDR1/2 (also known as STK38/STK38L) and MAP4Ks, which should be considered as novel members of the Hippo core cassette. While these efforts helped to expand our understanding of Hippo core signalling, they also began to provide insights into the complexity and redundancy of the Hippo signalling network. Here, we focus on summarising our current knowledge of the regulation and functions of mammalian NDR kinases, discussing parallels between the NDR pathways in Drosophila and mammals. Initially, we provide a general overview of the cellular functions of NDR kinases in cell cycle progression, centrosome biology, apoptosis, autophagy, DNA damage signalling, immunology and neurobiology. Finally, we put particular emphasis on discussing NDR1/2 as YAP kinases downstream of MST1/2 and MOB1 signalling in Hippo signalling.

STK38 is a critical upstream regulator of MYC's oncogenic activity in human B-cell lymphoma

The MYC protooncogene is associated with the pathogenesis of most human neoplasia. Conversely, its experimental inactivation elicits oncogene addiction. Besides constituting a formidable therapeutic target, MYC also has an essential function in normal physiology, thus creating the need for context-specific targeting strategies. The analysis of post-translational MYC activity modulation yields novel targets for MYC inactivation. Specifically, following regulatory network analysis in human B-cells, we identify a novel role of the STK38 kinase as a regulator of MYC activity and a candidate target for abrogating tumorigenesis in MYC-addicted lymphoma. We found that STK38 regulates MYC protein stability and turnover in a kinase activity-dependent manner. STK38 kinase inactivation abrogates apoptosis following B-cell receptor activation, whereas its silencing significantly decreases MYC levels and increases apoptosis. Moreover, STK38 knockdown suppresses growth of MYC-addicted tumors in vivo, thus providing a novel viable target for treating these malignancies.

The Irish potato famine pathogen Phytophthora infestans translocates the CRN8 kinase into host plant cells

Phytopathogenic oomycetes, such as Phytophthora infestans, secrete an arsenal of effector proteins that modulate plant innate immunity to enable infection. We describe CRN8, a host-translocated effector of P. infestans that has kinase activity in planta. CRN8 is a modular protein of the CRN effector family. The C-terminus of CRN8 localizes to the host nucleus and triggers cell death when the protein is expressed in planta. Cell death induction by CRN8 is dependent on its localization to the plant nucleus, which requires a functional nuclear localization signal (NLS). The C-terminal sequence of CRN8 has similarity to a serine/threonine RD kinase domain. We demonstrated that CRN8 is a functional RD kinase and that its auto-phosphorylation is dependent on an intact catalytic site. Co-immunoprecipitation experiments revealed that CRN8 forms a dimer or multimer. Heterologous expression of CRN8 in planta resulted in enhanced virulence by P. infestans. In contrast, in planta expression of the dominant-negative CRN8(R469A;D470A) resulted in reduced P. infestans infection, further implicating CRN8 in virulence. Overall, our results indicate that similar to animal parasites, plant pathogens also translocate biochemically active kinase effectors inside host cells.

Conserved Orb6 phosphorylation sites are essential for polarized cell growth in Schizosaccharomyces pombe

The Ndr-related Orb6 kinase is a key regulator of polarized cell growth in fission yeast, however the mechanism of Orb6 activation is unclear. Activation of other Ndr kinases involves both autophosphorylation and phosphorylation by an upstream kinase. Previous reports suggest that the Nak1 kinase functions upstream from Orb6. Supporting this model, we show that HA-Orb6 overexpression partially restored cell polarity in nak1 ts cells. We also demonstrated by coimmunoprecipitation and in vitro binding assays that Nak1 and Orb6 physically interact, and that the Nak1 C-terminal region is required for Nak1/Orb6 complex formation in vivo. However, results from in vitro kinase assays did not show phosphorylation of recombinant Orb6 by HA-Nak1, suggesting that Orb6 activation may not involve direct phosphorylation by Nak1. To investigate the role of Orb6 phosphorylation and activity, we substituted Ala at the ATP-binding and conserved phosphorylation sites. Overexpression of kinase-dead HA-Orb6(K122A) in wild-type cells resulted in a loss of cell polarity, suggesting that it has a dominant-negative effect, and it failed to rescue the polarity defect of nak1 or orb6 ts mutants. Recombinant GST-Orb6(S291A) did not autophosphorylate in vitro suggesting that Ser291 is the primary autophosphorylation site. HA-Orb6(S291A) overexpression only partially rescued the orb6 polarity defect and failed to rescue the nak1 defect, suggesting that autophosphorylation is important for Orb6 function. GST-Orb6(T456A) autophosphorylated in vitro, indicating that the conserved phosphorylation site at Thr456 is not essential for kinase activity. However, HA-Orb6(T456A) overexpression had similar effects as overexpressing kinase-dead HA-Orb6(K122A), suggesting that Thr456 is essential for Orb6 function in vivo. Also, we found that both phosphorylation site mutations impaired the ability of Myc-Nak1 to coimmunoprecipitate with HA-Orb6. Together, our results suggest a model whereby autophosphorylation of Ser291 and phosphorylation of Thr456 by an upstream kinase promote Nak1/Orb6 complex formation and Orb6 activation.

The T-loop extension of the tomato protein kinase AvrPto-dependent Pto-interacting protein 3 (Adi3) directs nuclear localization for suppression of plant cell death

In tomato (Solanum lycopersicum), resistance to Pseudomonas syringae pv. tomato is elicited by the interaction of the host Pto kinase with the pathogen effector protein AvrPto, which leads to various immune responses including localized cell death termed the hypersensitive response. The AGC kinase Adi3 functions to suppress host cell death and interacts with Pto only in the presence of AvrPto. The cell death suppression (CDS) activity of Adi3 requires phosphorylation by 3-phosphoinositide-dependent protein kinase 1 (Pdk1) and loss of Adi3 function is associated with the hypersensitive response cell death initiated by the Pto/AvrPto interaction. Here we studied the relationship between Adi3 cellular localization and its CDS activity. Adi3 is a nuclear-localized protein, and this localization is dictated by a nuclear localization signal found in the Adi3 T-loop extension, an approximately 80 amino acid insertion into the T-loop, or activation loop, which is phosphorylated for kinase activation. Nuclear localization of Adi3 is required for its CDS activity and loss of nuclear localization causes elimination of Adi3 CDS activity and induction of cell death. This nuclear localization of Adi3 is dependent on Ser-539 phosphorylation by Pdk1 and non-nuclear Adi3 is found in punctate structures throughout the cell. Our data support a model in which Pdk1 phosphorylation of Adi3 directs nuclear localization for CDS and that disruption of Adi3 nuclear localization may be a mechanism for induction of cell death such as that during the Pto/AvrPto interaction.

Mutations in the C-terminus of the conserved NDR kinase, Cbk1p of Saccharomyces cerevisiae, make the protein independent of upstream activators

In Saccharomyces cerevisiae, the RAM network is involved in cell separation after cytokinesis, cell integrity and cell polarity. The key function of this network is the regulation of the activity of the protein kinase Cbk1p, which is a member of the conserved NDR kinase family. Cbk1p function is controlled by its sub-cellular localization and at least two phosphorylation events: an auto phosphorylation in the kinase domain (S570) and the phosphorylation of a C-terminal hydrophobic motif by an upstream kinase (T743). After a UV mutagenesis, we have isolated 115 independent extragenic suppressors of four ram mutations: tao3, hym1, kic1 and sog2. Over 50% of the suppressors affect a single residue in Cbk1p (S745F), which is close to the phosphorylation site in the hydrophobic motif. Our results show that the CBK1-S745F allele leads to a constitutively active form of Cbk1p that is independent of the upstream RAM network. We hypothesize that the mutant Cbk1-S745Fp mimics the effect of the phosphorylation of T743.

Nuclear Dbf2-related protein kinases (NDRs) in isolated cardiac myocytes and the myocardium: activation by cellular stresses and by phosphoprotein serine-/threonine-phosphatase inhibitors

The nuclear Dbf2-related protein kinases 1 and 2 (NDR1/2) are closely-related AGC family kinases that are strongly conserved through evolution. In mammals, they are activated inter alia by phosphorylation of an hydrophobic domain threonine-residue [NDR1(Thr-444)/NDR2(Thr-442)] by an extrinsic protein kinase followed by autophosphorylation of a catalytic domain serine-residue [NDR1(Ser-281)/NDR2(Ser-282)]. We examined NDR1/2 expression and regulation in primary cultures of neonatal rat cardiac myocytes and in perfused adult rat hearts. In myocytes, transcripts for NDR2, but not NDR1, were induced by the hypertrophic agonist, endothelin-1. NDR1(Thr-444) and NDR2(Thr-442) were rapidly phosphorylated (maximal in 15-30 min) in myocytes exposed to some phosphoprotein Ser-/Thr-phosphatase 1/2 inhibitors (calyculin A, okadaic acid) and, to a lesser extent, by hyperosmotic shock, low concentrations of H(2)O(2), or chelerythrine. In myocytes adenovirally-transduced to express FLAG-NDR2 (which exhibited a mainly-cytoplasmic localisation), the same agents increased FLAG-NDR2 activity as assessed by in vitro protein kinase assays, indicative of FLAG-NDR2(Ser-282/Thr-442) phosphorylation. Calyculin A-induced phosphorylation of NDR1(Thr-444)/NDR2(Thr-442) and activation of FLAG-NDR2 were inhibited by staurosporine, but not by other protein kinase inhibitors tested. In ex vivo rat hearts, NDR1(Thr-444)/NDR2(Thr-442) were phosphorylated in response to ischaemia-reperfusion or calyculin A. From a pathological viewpoint, we conclude that activities of NDR1 and NDR2 are responsive to cytotoxic stresses in heart preparations and this may represent a previously-unidentified response to myocardial ischaemia in vivo.

Phosphoregulation of Cbk1 is critical for RAM network control of transcription and morphogenesis

The budding yeast regulation of Ace2 and morphogenesis (RAM) network integrates cell fate determination and morphogenesis. Its disruption impairs polarized growth and causes mislocalization of the transcription factor Ace2, resulting in failure of daughter cell–specific transcription required for cell separation. We find that phosphoregulation of the conserved AGC family kinase Cbk1 is critical for RAM network function. Intramolecular autophosphorylation of the enzyme's activation loop is critical for kinase activity but is only partially required for cell separation and polarized growth. In marked contrast, phosphorylation of a C-terminal hydrophobic motif is required for Cbk1 function in vivo but not for its kinase activity, suggesting a previously unappreciated level of control for this family of kinases. Phosphorylation of the C-terminal site is regulated over the cell cycle and requires the transcription factor Ace2 as well as all RAM network components. Therefore, Ace2 is not only a downstream target of Cbk1 but also reinforces activation of its upstream regulator.

Structural and functional insights into the regulation of Arabidopsis AGC VIIIa kinases

The AGCVIIIa kinases of Arabidopsis are members of the eukaryotic PKA, PKG, and PKC group of regulatory kinases. One AGCVIIIa kinase, PINOID (PID), plays a fundamental role in the asymmetrical localization of membrane proteins during polar auxin transport. The remaining 16 AGCVIIIa genes have not been associated with single mutant phenotypes, suggesting that the corresponding kinases function redundantly. Consistent with this idea, we find that the genes encoding the Arabidopsis AGCVIIIa kinases have spatially distinct, but overlapping, expression domains. Here we show that the majority of Arabidopsis AGCVIIIa kinases are substrates for the 3-phosphoinositide-dependent kinase 1 (PDK1) and that trans-phosphorylation by PDK1 correlates with activation of substrate AGCVIIIa kinases. Mutational analysis of two conserved regulatory domains was used to demonstrate that sequences located outside of the C-terminal PDK1 interaction (PIF) domain and the activation loop are required for functional interactions between PDK1 and its substrates. A subset of GFP-tagged AGCVIIIa kinases expressed in Saccharomyces cerevisiae and tobacco BY-2 cells were preferentially localized to the cytoplasm (AGC1-7), nucleus (WAG1 and KIPK), and the cell periphery (PID). We present evidence that PID insertion domain sequences are sufficient to direct the observed peripheral localization. We find that PID specifically but non-selectively binds to phosphoinositides and phosphatidic acid, suggesting that PID might directly interact with the plasma membrane through protein-lipid interactions. The initial characterization of the AGCVIIIa kinases presented here provides a framework for elucidating the physiological roles of these kinases in planta.

NDR kinases regulate essential cell processes from yeast to humans

Members of the NDR (nuclear Dbf2-related) protein-kinase family are essential components of pathways that control important cellular processes, such as morphological changes, mitotic exit, cytokinesis, cell proliferation and apoptosis. Recent progress has shed light on the mechanisms that underlie the regulation and function of the NDR family members. Combined data from yeast, worms, flies, mice and human cells now highlight the conserved and important roles of the different NDR kinases in distinct cellular processes.

Depletion of Aspergillus nidulans cotA causes a severe polarity defect which is not suppressed by the nuclear migration mutation nudA2

The Aspergillus nidulans homologue of Neurospora crassa cot-1, cotA, encoding a member of the NDR protein kinase family, has been cloned and expressed under the control of the conditional alcA promoter. Depletion of CotA by repression of the alcA promoter led to a severe growth defect accompanied by loss of polarity. Germlings show greatly enlarged volume of the spores and hyphae, accompanied by an increase in number of nuclei per compartment, though the nucleus/volume ratio is not significantly altered. The depleted CotA phenotype was not suppressed by a nuclear migration mutation nudA2. Double mutants showed an additive, defective phenotype, unlike the suppression of the cot-1 ts mutation by ropy mutations seen in N. crassa, suggesting a different relationship between nuclear migration and the cot signalling pathway in A. nidulans. A functional CotA-GFP fusion protein was found in punctate regions of fluorescence similar to the distribution reported for human NDR2, and as a cap at the hyphal tip.

Regulation of NDR protein kinase by hydrophobic motif phosphorylation mediated by the mammalian Ste20-like kinase MST3

NDR protein kinases are involved in the regulation of cell cycle progression and morphology. NDR1/NDR2 protein kinase is activated by phosphorylation on the activation loop phosphorylation site Ser281/Ser282 and the hydrophobic motif phosphorylation site Thr444/Thr442. Autophosphorylation of NDR is responsible for phosphorylation on Ser281/Ser282, whereas Thr444/Thr442 is targeted by an upstream kinase. Here we show that MST3, a mammalian Ste20-like protein kinase, is able to phosphorylate NDR protein kinase at Thr444/Thr442. In vitro, MST3 selectively phosphorylated Thr442 of NDR2, resulting in a 10-fold stimulation of NDR activity. MOB1A (Mps one binder 1A) protein further increased the activity, leading to a fully active kinase. In vivo, Thr442 phosphorylation after okadaic acid stimulation was potently inhibited by MST3KR, a kinase-dead mutant of MST3. Knockdown of MST3 using short hairpin constructs abolished Thr442 hydrophobic motif phosphorylation of NDR in HEK293F cells. We conclude that activation of NDR is a multistep process involving phosphorylation of the hydrophobic motif site Thr444/2 by MST3, autophosphorylation of Ser281/2, and binding of MOB1A.

Human NDR kinases are rapidly activated by MOB proteins through recruitment to the plasma membrane and phosphorylation

Human nuclear Dbf2-related kinases (NDRs) are up-regulated in certain cancer types, yet their precise function(s) and regulatory mechanism(s) still remain to be defined. Here, we show that active (phosphorylated on Thr444) and inactive human NDRs are both mainly cytoplasmic. Moreover, NDR kinases colocalize at the plasma membrane with human MOBs (hMOBs), which are recently described coactivators of human NDR in vitro. Strikingly, membrane targeting of NDR results in a constitutively active kinase due to phosphorylation on Ser281 and Thr444 that is further activated upon coexpression of hMOBs. Membrane-targeted hMOBs also robustly promoted activation of NDR. We further demonstrate that the in vivo activation of human NDR by membrane-bound hMOBs is dependent on their interaction and occurs solely at the membrane. By using a chimeric molecule of hMOB, which allows inducible membrane translocation, we found that NDR phosphorylation and activation at the membrane occur a few minutes after association of hMOB with membranous structures. We provide insight into a potential in vivo mechanism of NDR activation through rapid recruitment to the plasma membrane mediated by hMOBs.

Drosophila Mob family proteins interact with the related tricornered (Trc) and warts (Wts) kinases

The function of Tricornered (Trc), the Drosophila Ndr (Nuclear Dbf2-related) serine/threonine protein kinase, is required for the normal morphogenesis of a variety of polarized outgrowths including epidermal hairs, bristles, arista laterals, and dendrites. In yeast the Trc homolog Cbk1 needs to bind Mob2 to activate the RAM pathway. In this report, we provide genetic and biochemical data that Drosophila Trc also interacts with and is activated by Drosophila Dmob proteins. In addition, Drosophila Mob proteins appear to interact with the related Warts/Lats kinase, which functions as a tumor suppressor in flies and mammals. Interestingly, the overgrowth tumor phenotype that results from mutations in Dmob1 (mats) was only seen in genetic mosaics and not when the entire animal was mutant. We conclude that unlike in yeast, in Drosophila individual Mob proteins interact with multiple kinases and that individual NDR family kinases interact with multiple Mob proteins. We further provide evidence that Mo25, the Drosophila homolog of the RAM pathway hym1 gene does not function along with Trc.

HIV-1 incorporates and proteolytically processes human NDR1 and NDR2 serine-threonine kinases

Mammalian genomes encode two related serine-threonine kinases, nuclear Dbf2 related (NDR)1 and NDR2, which are homologous to the Saccharomyces cerevisiae Dbf2 kinase. Recently, a yeast genetic screen implicated the Dbf2 kinase in Ty1 retrotransposition. Since several virion-incorporated kinases regulate the infectivity of human immunodeficiency virus type 1 (HIV-1), we speculated that the human NDR1 and NDR2 kinases might play a role in the HIV-1 life cycle. Here we show that the NDR1 and NDR2 kinases were incorporated into HIV-1 particles. Furthermore, NDR1 and NDR2 were cleaved by the HIV-1 protease (PR), both within virions and within producer cells. Truncation at the PR cleavage site altered NDR2 subcellular localization and inhibited NDR1 and NDR2 enzymatic activity. These studies identify two new virion-associated host cell enzymes and suggest a novel mechanism by which HIV-1 alters the intracellular environment of human cells.

Yeast polo-like kinases: functionally conserved multitask mitotic regulators

The polo-like kinases (Plks) are a conserved subfamily of Ser/Thr protein kinases that play pivotal roles in regulating various cellular and biochemical events at multiple stages of M phase. Genetic and biochemical data revealed that both the budding yeast and the fission yeast polo kinase homologs (Cdc5 and Plo1, respectively) bear remarkable functional similarities with those in metazoan organisms, suggesting that the role of Plks is largely conserved throughout evolution. Thus, studies on Plks in genetically amenable lower eucaryotic organisms may yield valuable insights into the function of Plks in higher eucaryotic organisms. In this review, common properties and distinct functions of Cdc5 and Plo1 will be discussed and compared to properties and functions of Plks in higher eucaryotic organisms.

At the interface between signaling and executing anaphase--Cdc14 and the FEAR network

Anaphase is the stage of the cell cycle when the duplicated genome is separated to opposite poles of the cell. The irreversible nature of this event confers a unique burden on the cell and it is therefore not surprising that the regulation of this cell cycle stage is complex. In budding yeast, a signaling network known as the Cdc fourteen early anaphase release (FEAR) network and its effector, the protein phosphatase Cdc14, play a key role in the coordination of the multiple events that occur during anaphase, such as partitioning of the DNA, regulation of spindle stability, activation of microtubule forces, and initiation of mitotic exit. These functions of the FEAR network contribute to genomic stability by coordinating the completion of anaphase and the execution of mitotic exit.

Identification of proteins interacting with the RNAPII FCP1 phosphatase: FCP1 forms a complex with arginine methyltransferase PRMT5 and it is a substrate for PRMT5-mediated methylation

FCP1, a phosphatase specific of the carboxyl-terminal-domain of the large subunit of the RNA polymerase II (RNAPII), stimulates transcription elongation and it is required for general transcription and cell viability. To identify novel interacting proteins of FCP1, we used a human cell line expressing an epitope flagged FCP1 and proteins, which formed complexes with FCP1, were identified by mass spectrometry. We identified four proteins: RPB2 subunit of the RNAPII, the nuclear kinase, NDR1, the methyltransferase PRMT5 and the enhancer of rudimentary homologue (ERH) proteins. Intriguingly, both the PRMT5 and ERH proteins are interacting partners of the SPT5 elongation factor. Interactions of RPB2, ERH, NDR1 and PRMT5 with FCP1 were confirmed by co-immunoprecipitation or in vitro pull-down assays. Interaction between PRMT5 and FCP1 was further confirmed by co-immunoprecipitation of endogenous proteins. We found that FCP1 is a genuine substrate of PRMT5-methylation both in vivo and in vitro, and FCP1-associated PRMT5 can methylate histones H4 in vitro.

Comparison of different isolation techniques prior gene expression profiling of blood derived cells: impact on physiological responses, on overall expression and the role of different cell types

Owing to its clinical accessibility, peripheral blood is probably the best source for the assessment of differences or changes in gene expression associated with disease or drug response and therapy. Gene expression patterns in peripheral blood cells greatly depend on temporal and interindividual variations. However, technical aspects of blood sampling, isolation of cellular components, RNA isolation techniques and clinical aspects such as time to analysis and temperature during processing have been suggested to affect gene expression patterns. We therefore assessed gene expression patterns in peripheral blood from 29 healthy individuals by using Affymetrix microarrays. When RNA isolation was delayed for 20-24 h-a typical situation in clinical studies-gene signatures related to hypoxia were observed, and downregulation of genes associated with metabolism, cell cycle or apoptosis became dominant preventing the assessment of gene signatures of interindividual variation. Similarly, gene expression patterns were strongly dependent on choice of cell and RNA isolation and preparation techniques. We conclude that for large clinical studies, it is crucial to reduce maximally the time to RNA isolation. Furthermore, prior to study initiation, the cell type of interest should already be defined. Our data therefore will help to optimize clinical studies applying gene expression analysis of peripheral blood to exploit drug responses and to better understand changes associated with disease.

The tricornered Ser/Thr protein kinase is regulated by phosphorylation and interacts with furry during Drosophila wing hair development

The Trc/Ndr/Sax1/Cbk1 family of ser/thr kinases plays a key role in the morphogenesis of polarized cell structures in flies, worms, and yeast. Tricornered (Trc), the Drosophila nuclear Dbf2-related (Ndr) serine/threonine protein kinase, is required for the normal morphogenesis of epidermal hairs, bristles, laterals, and dendrites. We obtained in vivo evidence that Trc function was regulated by phosphorylation and that mutations in key regulatory sites resulted in dominant negative alleles. We found that wild-type, but not mutant Trc, is found in growing hairs, and we failed to detect Trc in pupal wing nuclei, implying that in this developmental context Trc functions in the cytoplasm. The furry gene and its homologues in yeast and Caenorhabditis elegans have previously been implicated as being essential for the function of the Ndr kinase family. We found that Drosophila furry (Fry) also is found in growing hairs, that its subcellular localization is dependent on Trc function, and that it can be coimmunoprecipitated with Trc. Our data suggest a feedback mechanism involving Trc activity regulates the accumulation of Fry in developing hairs.

Pneumocystis carinii cell wall biosynthesis kinase gene CBK1 is an environmentally responsive gene that complements cell wall defects of cbk-deficient yeast

Pneumocystis species remain an important cause of life-threatening pneumonia in immunocompromised hosts, including those with AIDS. Responses of the organism to environmental cues both within the lung and elsewhere have been poorly defined. Herein, we report the identification of a cell wall biosynthesis kinase gene (CBK1) homologue in Pneumocystis carinii, isolated by differential display PCR, that is expressed optimally at physiological pH (7 to 8) as opposed to more acidic environments. Expression of Pneumocystis CBK1 was also induced by contact with lung epithelial cells and extracellular matrix. Translation of this gene revealed extensive homology to other fungal CBK1 kinases. Pneumocystis CBK1 expression was equal in the cyst and trophic life forms of the organisms. We further demonstrate that Pneumocystis CBK1 expressed in cbk1 Delta Saccharomyces cerevisiae cells restored defective cell wall separation during proliferation. Consistent with this, Pneumocystis CBK1 expression also stimulated transcription of the CTS1 chitinase in cbk1 Delta mutant yeast cells, an event necessary for cell wall separation. In addition, Pneumocystis CBK1 cDNA supported normal mating projection formation in response to alpha-factor in the cbk1 Delta yeast cells. Site-directed mutations of serine-303 and threonine-494, potential regulatory phosphorylation sites in Pneumocystis CBK1, abolished mating projection formation, indicating a role for these amino acid residues in CBK1 activity. These findings indicate that Pneumocystis CBK1 is an environmentally responsive gene that may function in signaling pathways necessary for cell growth and mating.

Mechanism of Activation of NDR (Nuclear Dbf2-related) Protein Kinase by the hMOB1 Protein

NDR (nuclear Dbf2-related) kinase belongs to a family of kinases that is highly conserved throughout the eukaryotic world. We showed previously that NDR is regulated by phosphorylation and by the Ca(2+)-binding protein, S100B. The budding yeast relatives of Homo sapiens NDR, Cbk1, and Dbf2, were shown to interact with Mob2 (Mps one binder 2) and Mob1, respectively. This interaction is required for the activity and biological function of these kinases. In this study, we show that hMOB1, the closest relative of yeast Mob1 and Mob2, stimulates NDR kinase activity and interacts with NDR both in vivo and in vitro. The point mutations of highly conserved residues within the N-terminal domain of NDR reduced NDR kinase activity as well as human MOB1 binding. A novel feature of NDR kinases is an insert within the catalytic domain between subdomains VII and VIII. The amino acid sequence within this insert shows a high basic amino acid content in all of the kinases of the NDR family known to interact with MOB proteins. We show that this sequence is autoinhibitory, and our data indicate that the binding of human MOB1 to the N-terminal domain of NDR induces the release of this autoinhibition.

Human Mob Proteins Regulate the NDR1 and NDR2 Serine-Threonine Kinases

Human NDR1 (nuclear Dbf2-related) is a widely expressed nuclear serine-threonine kinase that has been implicated in cell proliferation and/or tumor progression. Here we present molecular characterization of the human NDR2 serine-threonine kinase, which shares approximately 87% sequence identity with NDR1. NDR2 is expressed in most human tissues with the highest expression in the thymus. In contrast to NDR1, NDR2 is excluded from the nucleus and exhibits a punctate cytoplasmic distribution. The differential localization of NDR1 and NDR2 suggests that each kinase may serve distinct functions. Thus, to identify proteins that interact with NDR1 or NDR2, epitope-tagged kinases were immunoprecipitated from Jurkat T-cells. Two uncharacterized proteins that are homologous to the Saccharomyces cerevisiae kinase regulators Mob1 and Mob2 were identified. We demonstrate that NDR1 and NDR2 partially colocalize with human Mob2 in HeLa cells and confirm the NDR-Mob interactions in cell extracts. Interestingly, NDR1 and NDR2 form stable complexes with Mob2, and this association dramatically stimulates NDR1 and NDR2 catalytic activity. In summary, this work identifies a unique class of human kinase-activating subunits that may be functionally analagous to cyclins.

Initiation of cytokinesis is controlled through multiple modes of regulation of the Sid2p-Mob1p kinase complex

The Sid2p-Mob1p kinase complex is an important component of the septation initiation network (SIN) in the fission yeast Schizosaccharomyces pombe. However, regulation of this complex is still elusive. Here we show that Mob1p is required not only for the subcellular localization of Sid2p but also for its kinase activity. We identified a region at the amino terminus of Sid2p that is required for Mob1p binding and spindle pole body (SPB) localization. Deletion of this region abolishes Mob1p binding and diminishes SPB localization, whereas this region alone is sufficient to associate with Mob1p and SPBs. We further show that a similar region of the N terminus of the Sid2p-related protein kinase Orb6p binds to the Mob1p-related protein Mob2p, suggesting that this may be a conserved mode of interaction for this family of kinases. Phosphorylation of Ser402 and especially Thr578 is important for Sid2p function. Sid2p with a mutation of Thr578 to Ala (T578A) can no longer rescue sid2-250 mutant cells, and this results in reduction of Mob1p binding. Sid2p mutants mimicking phosphorylation at this site (T578D and T578E) can rescue sid2-250 cells, enhance Sid2p kinase activity, and partially rescue growth defects of upstream sin mutants. Interestingly, Sid2p, but not Mob1p, is self-associated. Our experiments suggest that self-associated Sid2p is inactive. This self-association is mediated by a region that overlaps with Mob1p and SPB binding sites. Overexpression of Mob1p is able to disrupt the self-association of Sid2p. Taken together, our results suggest that Sid2p kinase may utilize multiple modes of regulation including self-association, Mob1p binding, and phosphorylation to achieve its full activity at an appropriate time and place in the cell.

Deletion of a hypoviral-regulated cppk1 gene in a chestnut blight fungus, Cryphonectria parasitica, results in microcolonies

The cppk1 gene encodes a Ser/Thr protein kinase of Cryphonectria parasitica and is transcriptionally up-regulated by the presence of hypovirus CHV1-EP713. A cppk1-null mutant was constructed to determine the function of cppk1. The cppk1-null mutant was initially isolated as a heterokaryotic form containing both wild-type and cppk1-deleted nuclei. The pure cppk1-null homokaryon was obtained by the single spore isolation of the heterokaryon. While the parental heterokaryon appeared normal, the pure cppk1-null mutant exhibited dramatic changes in colony morphology. It showed characteristics of microcolonial growth. The functional complementation, restoration of filamentous growth, of the cppk1-null mutant using a wild-type cppk1 gene indicated that the phenotypic changes were due to the disruption of cppk1. Neither sporulation nor hyphal differentiation into feeding hyphae, a mycelial mat, or aerial hyphae was observed in the cppk1-null mutant. Instead of bright yellow, dark brown pigmentation appeared as the culture grew. Hyphae were shortened and hyperbranched with globose to bulbose cells. Electron microscopy revealed the presence of intrahyphal hyphae, the most striking ultrastructural change. Subtle changes in the expression of CpPK1 resulted in abnormalities in colony morphology and pigmentation, which indicated that cppk1 is important for coordinating growth with development and maintaining cell wall integrity.

Regulation of NDR2 protein kinase by multi-site phosphorylation and the S100B calcium-binding protein

Nuclear Dbf2-related (NDR) protein kinases are a family of AGC group kinases that are involved in the regulation of cell division and cell morphology. We describe the cloning and characterization of the human and mouse NDR2, a second mammalian isoform of NDR protein kinase. NDR1 and NDR2 share 86% amino acid identity and are highly conserved between human and mouse. However, they differ in expression pattern; mouse Ndr1 is expressed mainly in spleen, lung and thymus, whereas mouse Ndr2 shows highest expression in the gastrointestinal tract. NDR2 is potently activated in cells following treatment with the protein phosphatase 2A inhibitor okadaic acid, which also results in phosphorylation on the activation segment residue Ser-282 and the hydrophobic motif residue Thr-442. We show that Ser-282 becomes autophosphorylated in vivo, whereas Thr-442 is targeted by an upstream kinase. This phosphorylation can be mimicked by replacing the hydrophobic motif of NDR2 with a PRK2-derived sequence, resulting in a constitutively active kinase. Similar to NDR1, the autophosphorylation of NDR2 protein kinase was stimulated in vitro by S100B, an EF-hand Ca(2+)-binding protein of the S100 family, suggesting that the two isoforms are regulated by the same mechanisms. Further we show a predominant cytoplasmic localization of ectopically expressed NDR2.

Cloning and characterization of a novel radish protein kinase which is homologous to fungal cot-I like and animal Ndr protein kinases

According to the similarity of the amino acid sequences in their catalytic domains, eukaryotic protein kinases have been classified into the five main groups: 'AGC', 'CaMK', 'CMGC', 'PTK' and 'other'. The AGC group, represented by the cyclic nucleotide-dependent kinases (PKA and PKG), the calcium-phospholipid-dependent kinases (PKC) and the ribosomal S6 protein kinases, are poorly characterized in plants except for a few cases. In this study, in order to gain a better understanding of plant protein kinases in the AGC group, three cDNAs encoding novel protein kinases, RsNdr1 and RsNdr2a/b, were cloned from radish and characterized by molecular and biochemical methods. The deduced amino acid sequences of RsNdr1 and RsNdr2a/b contained all 12 conserved catalytic subdomains which are characteristic of the eukaryotic Ser/Thr protein kinases. A cell lysate from E. coli overexpressing RsNdr1 fusion protein had protein kinase activity toward a conventional protein substrate (myelin basic protein), whereas that from E. coli harboring a fusion plasmid encoding kinase-dead RsNdr1 or RsNdr2 did not show any protein kinase activity. A phylogenetic tree for 17 protein kinases from various organisms showed that the RsNdrs are more closely related to the protein kinases in a particular subgroup of the 'AGC' (fungal cot1-like and animal Ndr kinases) than to the authentic 'AGC' protein kinases, such as PKA, PKC or ribosomal S6 kinase.

Alternative splicing controls myotonic dystrophy protein kinase structure, enzymatic activity, and subcellular localization

Transcripts of the myotonic dystrophy protein kinase (DMPK) gene, a member of the Rho kinase family, are subject to cell-type specific alternative splicing. An imbalance in the splice isoform profile of DMPK may play a role in the pathogenesis of DM1, a severe multisystemic disorder. Here, we report how structural subdomains determine biochemical properties and subcellular distribution of DMPK isoforms. A newly developed kinase assay revealed that DMPK is a Lys/Arg-directed kinase. Individual DMPK isoforms displayed comparable transphosphorylation activity and sequence preference for peptide substrates. However, DMPK autophosphorylation and phosphorylation of MYPT1 (as putative in vivo target of DMPK), were dependent on presence of an alternatively spliced VSGGG motif and the nature of the C terminus. In-gel effects of the VSGGG motif on the migration behavior of full-length kinase provide evidence for a model in which this motif mediates 3-D-conformational changes in DMPK isoforms. Finally, different C termini endow DMPK with the ability to bind to either endoplasmic reticulum or mitochondria or to adopt a cytosolic location. Our results suggest that DMPK isoforms have cell-type and location dependent substrate specificities with a role in organellar and cytoarchitectural dynamics.

Control of cell polarity in fission yeast by association of Orb6p kinase with the highly conserved protein methyltransferase Skb1p

In the fission yeast Schizosaccharomyces pombe, proper establishment and maintenance of cell polarity require Orb6p, a highly conserved serine/threonine kinase involved in regulating both cell morphogenesis and cell cycle control. Orb6p localizes to the cell tips during interphase and to the cell septum during mitosis. To investigate the mechanisms involved in Orb6p function, we conducted a two-hybrid screen to identify proteins that interact with Orb6p. Using this approach, we identified Skb1p, a highly conserved protein methyltransferase that has been implicated previously in cell cycle control, in the coordination of cell cycle progression with morphological changes, and in hyperosmotic stress response. We found that Skb1p associates with Orb6p in S. pombe cells and that the two proteins interact directly in vitro. Loss of Skb1p exacerbates the phenotype of orb6 mutants, suggesting that Skb1p and Orb6p functionally interact in S. pombe cells. Our results suggest that Skb1p affects the intracellular localization of Orb6p and that loss of Skb1p leads to a redistribution of the Orb6p kinase away from the cell tips. Furthermore, we found that Orb6p kinase activity is strongly increased following exposure to salt shock, suggesting that Orb6p has a role in cell response to hyperosmotic stress. Previous studies have shown that Skb1p interacts with the fission yeast p21-activated kinase homologue Pak1p/Shk1p to regulate cell polarity and cell cycle progression. Our findings identify Orb6p as an additional target for Skb1p and suggest a novel function for Skb1p in the control of cell polarity by regulating the subcellular localization of Orb6p.

NDR family of AGC kinases--essential regulators of the cell cycle and morphogenesis

The nuclear Dbf2-related (NDR) family of protein serine/threonine kinases comprises mammalian NDR and large tumor suppressor (LATS) kinases, their orthologs from Drosophila melanogaster and Caenorhabditis elegans, and a number of related kinases from yeast and plants. The members of this family were independently implicated in various aspects of the control of cell division and morphogenesis. They are crucial regulators of the actin and tubulin cytoskeletal organization during polarized growth and cytokinesis in yeast. Furthermore, they are key players in control of proliferation and morphology of many cell types in D. melanogaster and C. elegans. In mammalians, the LATS kinase is a tumor suppressor, negatively regulating the cyclin-dependent kinase CDK1, cell proliferation rate, and modulating cell survival.

Mechanism of Ca2+-mediated regulation of NDR protein kinase through autophosphorylation and phosphorylation by an upstream kinase

NDR1 (nuclear Dbf2-related) is a serine/threonine protein kinase belonging to subfamily of kinases implicated in the regulation of cell division and morphology. Previously, we demonstrated that the activity of NDR1 is controlled by phosphorylation of two regulatory residues, Ser-281 and Thr-444. Moreover, we found that NDR1 becomes activated through a direct interaction with EF-hand Ca(2+)-binding proteins of the S100 family. In this work, we characterize this regulatory mechanism in detail. We found that NDR1 autophosphorylates in vitro predominantly on Ser-281 and to a lesser extent on Thr-74 and Thr-444. All of these residues proved to be crucial also for NDR1 activity in vivo; however, in contrast to Ser-281 and Thr-444, Thr-74 seems to be involved only in binding to S100B rather than directly regulating NDR1 activity per se. When we added Ca(2+)/S100B, we observed an increased autophosphorylation on Ser-281 and Thr-444, resulting in stimulation of NDR1 activity in vitro. Using phosphospecific antibodies, we found that Ser-281 also becomes autophosphorylated in vivo, whereas Thr-444 is targeted predominantly by an as yet unidentified upstream kinase. Significantly, the Ca(2+)-chelating agent BAPTA-AM suppressed the activity and phosphorylation of NDR1 on both Ser-281 and Thr-444, and specifically, these effects were reversed when we added the sarcoplasmic-endoplasmic reticulum Ca(2+) ATPase pump inhibitor thapsigargin.

High-resolution genetic mapping with ordered arrays of Saccharomyces cerevisiae deletion mutants

We present a method for high-resolution genetic mapping that takes advantage of the ordered set of viable gene deletion mutants, which form a set of colinear markers covering almost every centimorgan of the Saccharomyces cerevisiae genome, and of the synthetic genetic array (SGA) system, which automates the construction of double mutants formed by mating and meiotic recombination. The Cbk1 kinase signaling pathway, which consists minimally of CBK1, MOB2, KIC1, HYM1, and TAO3 (PAG1), controls polarized morphogenesis and activation of the Ace2 transcription factor. Deletion mutations in the Cbk1 pathway genes are tolerated differently by common laboratory strains of S. cerevisiae, being viable in the W303 background but dead in the S288C background. Genetic analysis indicated that the lethality of Cbk1 pathway deletions in the S288C background was suppressed by a single allele specific to the W303 background. SGA mapping (SGAM) was used to locate this W303-specific suppressor to the SSD1 locus, which contains a known polymorphism that appears to compromise SSD1 function. This procedure should map any mutation, dominant or recessive, whose phenotype is epistatic to wild type, that is, a phenotype that can be scored from a mixed population of cells obtained by germination of both mutant and wild-type spores. In principle, SGAM should be applicable to the analysis of multigenic traits. Large-scale construction of ordered mutations in other model organisms would broaden the application of this approach.

A protein kinase specifically associated with proliferative forms of Trypanosoma brucei is functionally related to a yeast kinase involved in the co-ordination of cell shape and division

The life cycle of African trypanosomes is characterized by the alternation of proliferative and quiescent stages but the molecular details of this process remain unknown. Here, we describe a new cytoplasmic protein kinase from Trypanosoma brucei, termed TBPK50, that belongs to a family of protein kinases involved in the regulation of the cell cycle, cell shape and proliferation. TBPK50 is expressed only in proliferative forms but is totally absent in quiescent cells despite the fact that the gene is constitutively transcribed at the same level throughout the life cycle. It is probable that TBPK50 has very specific substrate requirements as it was unable to transphosphorylate a range of classical phosphoacceptor substrates in vitro, although an autophosphorylation activity was readily detectable in the same assays. Complementation studies using a fission yeast mutant demonstrated that TBPK50 is a functional homologue of Orb6, a protein kinase involved in the regulation of cellular morphology and cell cycle progression in yeast. These results link the expression of TBPK50 and the growth status of trypanosomes and support the view that this protein kinase is likely to be involved in the control of life cycle progression and cell division of these parasites.

Pag1p, a novel protein associated with protein kinase Cbk1p, is required for cell morphogenesis and proliferation in Saccharomyces cerevisiae

Protein kinases in the Cot-1/Orb6/Ndr/Warts family are important regulators of cell morphogenesis and proliferation. Cbk1p, a member of this family in Saccharomyces cerevisiae, has previously been shown to be required for normal morphogenesis in vegetatively growing cells and in haploid cells responding to mating pheromone. A mutant of PAG1, a novel gene in S. cerevisiae, displayed defects similar to those of cbk1 mutants. pag1 and cbk1 mutants share a common set of suppressors, including the disruption of SSD1, a gene encoding an RNA binding protein, and the overexpression of Sim1p, an extracellular protein. These genetic results suggest that PAG1 and CBK1 act in the same pathway. Furthermore, we found that Pag1p and Cbk1p localize to the same polarized peripheral sites and that they coimmunoprecipitate with each other. Pag1p is a conserved protein. The homologs of Pag1p in other organisms are likely to form complexes with the Cbk1p-related kinases and function with those kinases in the same biological processes.

Lre1 affects chitinase expression, trehalose accumulation and heat resistance through inhibition of the Cbk1 protein kinase in Saccharomyces cerevisiae

The addition of glucose to derepressed cells of the yeast Saccharomyces cerevisiae triggers activation of the cAMP pathway with a rapid drop in stress resistance as a consequence. We have isolated the LRE1 gene as a multicopy suppressor of glucose-induced loss of heat resistance. Overexpression of LRE1 in a wild-type strain causes the same phenotype as observed in strains with reduced activity of the cAMP-PKA pathway: higher heat resistance and enhanced trehalose levels. Deletion of LRE1 results in the opposite phenotypes. Epistasis analysis indicates that these effects are independent of cAMP and PKA, of the protein kinases Yak1, Sch9 and Rim15 and of the transcription factors Msn2 and Msn4. Lre1 has recently been isolated in a two-hybrid screen using the conserved protein kinase Cbk1 as a bait. Cbk1 controls the expression of CTS1 (encoding chitinase) through the transcription factor Ace2. We demonstrate here that overexpression of LRE1 represses CTS1 whereas deletion of LRE1 induces the expression of CTS1. Repression of CTS1 results in deficient cell separation as a result of inefficient degradation of the chitin ring after cytokinesis. Neither deletion nor overexpression of LRE1 has any effect on CTS1 expression in a cbk1Delta mutant, indicating that Lre1 inhibits Cbk1. In addition, we show that increased trehalose accumulation and increased heat resistance caused by overexpression of LRE1 are also the result of inhibition of Cbk1, revealing a novel control pathway for certain targets affected by PKA. The yeast genome contains a homologue of LRE1, YDR528w, which we have called HLR1 (for homologue of Lre1). Deletion and overexpression of HLR1 causes similar but less pronounced effects compared with LRE1.

Programmed translational frameshifting is likely required for expressions of genes encoding putative nuclear protein kinases of the ciliate Euplotes octocarinatus

Three macronuclear genes encoding putative nuclear protein kinases of the ciliate Euplotes octocarinatus syngen 1 were isolated and sequenced. All three deduced gene products share significant properties with a group of recently identified nuclear serine/threonine protein kinases named Ndr. The three predicted proteins contain the twelve conserved catalytic subdomains of protein kinases and 22 near universally-conserved amino acids residues that are characteristic of serine/threonine protein kinases. In addition, there is an approximately 30 amino acid-peptide insertion between subdomains VII and VIII that contains a potential nuclear localization signal. Sequence analysis suggests that expression of the Eondr2 gene requires a + 1 programmed translational frameshift for its translation. Comparison of the deduced EoNdr2 with other known Ndr protein kinases implies that a + 1 ribosomal frameshift occurs at the motif AAATAA.

Thefurrygene ofDrosophilais important for maintaining the integrity of cellular extensions during morphogenesis

The Drosophila imaginal cells that produce epidermal hairs, the shafts of sensory bristles and the lateral extensions of the arista are attractive model systems for studying the morphogenesis of polarized cell extensions. We now report the identification and characterization of furry, an essential Drosophila gene that is involved in maintaining the integrity of these cellular extensions during morphogenesis. Mutations in furry result in the formation of branched arista laterals, branched bristles and a strong multiple hair cell phenotype that consists of clusters of epidermal hairs and branched hairs. By following the morphogenesis of arista laterals in pupae, we have determined that the branched laterals are due to the splitting of individual laterals during elongation. In genetic mosaics furry was found to act cell autonomously in the wing. The phenotypes of double mutant cells argue that furry functions independently of the frizzled planar polarity pathway and that it probably functions in the same pathway as the tricornered gene. We used a P-element insertion allele as a tag to clone the furry gene and found it to be a large and complicated gene that encodes a pair of large conserved proteins of unknown biochemical function.

Protein kinase Cdc15 activates the Dbf2-Mob1 kinase complex

Exit from mitosis in budding yeast requires inactivation of cyclin-dependent kinases through mechanisms triggered by the protein phosphatase Cdc14. Cdc14 activity, in turn, is regulated by a group of proteins, the mitotic exit network (MEN), which includes Lte1, Tem1, Cdc5, Cdc15, Dbf2/Dbf20, and Mob1. The direct biochemical interactions between the components of the MEN remain largely unresolved. Here, we investigate the mechanisms that underlie activation of the protein kinase Dbf2. Dbf2 kinase activity depended on Tem1, Cdc15, and Mob1 in vivo. In vitro, recombinant protein kinase Cdc15 activated recombinant Dbf2, but only when Dbf2 was bound to Mob1. Conserved phosphorylation sites Ser-374 and Thr-544 (present in the human, Caenorhabditis elegans, and Drosophila melanogaster relatives of Dbf2) were required for DBF2 function in vivo, and activation of Dbf2-Mob1 by Cdc15 in vitro. Although Cdc15 phosphorylated Dbf2, Dbf2-Mob1, and Dbf2(S374A/T544A)-Mob1, the pattern of phosphate incorporation into Dbf2 was substantially altered by either the S374A T544A mutations or omission of Mob1. Thus, Cdc15 promotes the exit from mitosis by directly switching on the kinase activity of Dbf2. We propose that Mob1 promotes this activation process by enabling Cdc15 to phosphorylate the critical Ser-374 and Thr-544 phosphoacceptor sites of Dbf2.

The Cbk1p pathway is important for polarized cell growth and cell separation in Saccharomyces cerevisiae

During the early stages of budding, cell wall remodeling and polarized secretion are concentrated at the bud tip (apical growth). The CBK1 gene, encoding a putative serine/threonine protein kinase, was identified in a screen designed to isolate mutations that affect apical growth. Analysis of cbk1Delta cells reveals that Cbk1p is required for efficient apical growth, proper mating projection morphology, bipolar bud site selection in diploid cells, and cell separation. Epitope-tagged Cbk1p localizes to both sides of the bud neck in late anaphase, just prior to cell separation. CBK1 and another gene, HYM1, were previously identified in a screen for genes involved in transcriptional repression and proposed to function in the same pathway. Deletion of HYM1 causes phenotypes similar to those observed in cbk1Delta cells and disrupts the bud neck localization of Cbk1p. Whole-genome transcriptional analysis of cbk1Delta suggests that the kinase regulates the expression of a number of genes with cell wall-related functions, including two genes required for efficient cell separation: the chitinase-encoding gene CTS1 and the glucanase-encoding gene SCW11. The Ace2p transcription factor is required for expression of CTS1 and has been shown to physically interact with Cbk1p. Analysis of ace2Delta cells reveals that Ace2p is required for cell separation but not for polarized growth. Our results suggest that Cbk1p and Hym1p function to regulate two distinct cell morphogenesis pathways: an ACE2-independent pathway that is required for efficient apical growth and mating projection formation and an ACE2-dependent pathway that is required for efficient cell separation following cytokinesis. Cbk1p is most closely related to the Neurospora crassa Cot-1; Schizosaccharomyces pombe Orb6; Caenorhabditis elegans, Drosophila, and human Ndr; and Drosophila and mammalian WARTS/LATS kinases. Many Cbk1-related kinases have been shown to regulate cellular morphology.