A mitotic cascade of NIMA family kinases. Nercc1/Nek9 activates the Nek6 and Nek7 kinases

Nek6 mediates human cancer cell transformation and is a potential cancer therapeutic target

We investigated the role of Nek6, a member of the NIMA-related serine/threonine kinase family, in tumorigenesis. Transcript, protein, and kinase activity levels of Nek6 were highly elevated in the malignant tumors and human cancer cell lines compared with normal tissue and fibroblast cells. Expression of exogenous wild-type Nek6 increased anchorage-independent growth of a variety of human cancer cell lines, whereas overexpression of the kinase-dead Nek6 and RNAi knockdown of endogenous Nek6 suppressed cancer cell transformation and induced apoptosis. Additionally, in in vivo xenograft nude mouse model, knockdown of Nek6 in HeLa cells resulted in reduction of tumor size relative to control siRNA tumors. Most importantly, knocking down endogenous Nek6 levels or exogenous expression of the kinase-dead form did not inhibit cell proliferation, nor did it induce apoptosis in normal fibroblast cells. Taken together, our data indicate a pivotal role for Nek6 in tumorigenesis and establish Nek6 as a potential target for treatment of a variety of human cancers.

An autoinhibitory tyrosine motif in the cell-cycle-regulated Nek7 kinase is released through binding of Nek9

Mitosis is controlled by multiple protein kinases, many of which are abnormally expressed in human cancers. Nek2, Nek6, Nek7, and Nek9 are NIMA-related kinases essential for proper mitotic progression. We determined the atomic structure of Nek7 and discovered an autoinhibited conformation that suggests a regulatory mechanism not previously described in kinases. Additionally, Nek2 adopts the same conformation when bound to a drug-like molecule. In both structures, a tyrosine side chain points into the active site, interacts with the activation loop, and blocks the alphaC helix. Tyrosine mutants of Nek7 and the related kinase Nek6 are constitutively active. The activity of Nek6 and Nek7, but not the tyrosine mutant, is increased by interaction with the Nek9 noncatalytic C-terminal domain, suggesting a mechanism in which the tyrosine is released from its autoinhibitory position. The autoinhibitory conformation is common to three Neks and provides a potential target for selective kinase inhibitors.

An essential role for the Plasmodium Nek-2 Nima-related protein kinase in the sexual development of malaria parasites

The molecular control of cell division and development in malaria parasites is far from understood. We previously showed that a Plasmodium gametocyte-specific NIMA-related protein kinase, nek-4, is required for completion of meiosis in the ookinete, the motile form that develops from the zygote in the mosquito vector. Here, we show that another NIMA-related kinase, Pfnek-2, is also predominantly expressed in gametocytes, and that Pfnek-2 is an active enzyme displaying an in vitro substrate preference distinct from that of Pfnek-4. A functional nek-2 gene is required for transmission of both Plasmodium falciparum and the rodent malaria parasite Plasmodium berghei to the mosquito vector, which is explained by the observation that disruption of the nek-2 gene in P. berghei causes dysregulation of DNA replication during meiosis and blocks ookinete development. This has implications (i) in our understanding of sexual development of malaria parasites and (ii) in the context of control strategies aimed at interfering with malaria transmission.

The NIMA-family kinase Nek3 regulates microtubule acetylation in neurons

NIMA-related kinases (Neks) belong to a large family of Ser/Thr kinases that have critical roles in coordinating microtubule dynamics during ciliogenesis and mitotic progression. The Nek kinases are also expressed in neurons, whose axonal projections are, similarly to cilia, microtubule-abundant structures that extend from the cell body. We therefore investigated whether Nek kinases have additional, non-mitotic roles in neurons. We found that Nek3 influences neuronal morphogenesis and polarity through effects on microtubules. Nek3 is expressed in the cytoplasm and axons of neurons and is phosphorylated at Thr475 located in the C-terminal PEST domain, which regulates its catalytic activity. Although exogenous expression of wild-type or phosphomimic (T475D) Nek3 in cultured neurons has no discernible impact, expression of a phospho-defective mutant (T475A) or PEST-truncated Nek3 leads to distorted neuronal morphology with disturbed polarity and deacetylation of microtubules via HDAC6 in its kinase-dependent manner. Thus, the phosphorylation at Thr475 serves as a regulatory switch that alters Nek3 function. The deacetylation of microtubules in neurons by unphosphorylated Nek3 raises the possibility that it could have a role in disorders where axonal degeneration is an important component.

The Nek6 and Nek7 protein kinases are required for robust mitotic spindle formation and cytokinesis

Nek6 and Nek7 are members of the NIMA-related serine/threonine kinase family. Previous work showed that they contribute to mitotic progression downstream of another NIMA-related kinase, Nek9, although the roles of these different kinases remain to be defined. Here, we carried out a comprehensive analysis of the regulation and function of Nek6 and Nek7 in human cells. By generating specific antibodies, we show that both Nek6 and Nek7 are activated in mitosis and that interfering with their activity by either depletion or expression of reduced-activity mutants leads to mitotic arrest and apoptosis. Interestingly, while completely inactive mutants and small interfering RNA-mediated depletion delay cells at metaphase with fragile mitotic spindles, hypomorphic mutants or RNA interference treatment combined with a spindle assembly checkpoint inhibitor delays cells at cytokinesis. Importantly, depletion of either Nek6 or Nek7 leads to defective mitotic progression, indicating that although highly similar, they are not redundant. Indeed, while both kinases localize to spindle poles, only Nek6 obviously localizes to spindle microtubules in metaphase and anaphase and to the midbody during cytokinesis. Together, these data lead us to propose that Nek6 and Nek7 play independent roles not only in robust mitotic spindle formation but also potentially in cytokinesis.

The NIMA-family kinase Nek6 phosphorylates the kinesin Eg5 at a novel site necessary for mitotic spindle formation

Nek6 and Nercc1 (also known as Nek9) belong to the NIMA family of protein kinases. Nercc1 is activated in mitosis, whereupon it binds, phosphorylates and activates Nek6. Interference with Nek6 or Nercc1 in mammalian cells causes prometaphase-metaphase arrest, and depletion of Nercc1 from Xenopus egg extracts prevents normal spindle assembly. Herein we show that Nek6 is constitutively associated with Eg5 (also known as Kinesin-5 and Kif11), a kinesin that is necessary for spindle bipolarity. Nek6 phosphorylated Eg5 at several sites in vitro and one of these sites, Ser1033, is phosphorylated in vivo during mitosis. Whereas CDK1 phosphorylates nearly all Eg5 at Thr926 during mitosis, Nek6 phosphorylates approximately 3% of Eg5, primarily at the spindle poles. Eg5 depletion caused mitotic arrest, resulting in cells with a monopolar spindle. This arrest could be rescued by wild-type Eg5 but not by Eg5[Thr926Ala]. Despite substantial overexpression, Eg5[Ser1033Ala] rescued 50% of cells compared with wild-type Eg5, whereas an Eg5[Ser1033Asp] mutant was nearly as effective as wild type. Thus, during mitosis Nek6 phosphorylates a subset of Eg5 polypeptides at a conserved site, the phosphorylation of which is crucial for the mitotic function of Eg5.

Integrative approach for differentially overexpressed genes in gastric cancer by combining large-scale gene expression profiling and network analysis

Gene expression profiling is a valuable tool for identifying differentially expressed genes in studies of disease subtype and patient outcome for various cancers. However, it remains difficult to assign biological significance to the vast number of genes. There is an increasing awareness of gene expression profile as an important part of the contextual molecular network at play in complex biological processes such as cancer initiation and progression. This study analysed the transcriptional profiles commonly activated at different stages of gastric cancers using an integrated approach combining gene expression profiling of 222 human tissues and gene regulatory dynamic mapping. We focused on an inferred core network with CDKN1A (p21^WAF1/CIP1) as the hub, and extracted seven candidates for gastric carcinogenesis (MMP7, SPARC, SOD2, INHBA, IGFBP7, NEK6, LUM). They were classified into two groups based on the correlation between expression level and stage. The seven genes were commonly activated and their expression levels tended to increase as disease progressed. NEK6 and INHBA are particularly promising candidate genes overexpressed at the protein level, as confirmed by immunohistochemistry and western blotting. This integrated approach could help to identify candidate players in gastric carcinogenesis and progression. These genes are potential markers of gastric cancer regardless of stage.

Nek6 is involved in G2/M phase cell cycle arrest through DNA damage-induced phosphorylation

Nek6 is a recently identified NIMA-related kinase that is required for mitotic cell cycle progression. In the present study, we examined the role of Nek6 in the DNA damage response. We found that Nek6 is phosphorylated upon IR and UV irradiation through the DNA damage checkpoint in vivo. Nek6 is also directly phosphorylated by the checkpoint kinases Chk1 and Chk2 in vitro. Notably, Nek6 activation during mitosis is completely abolished by IR and UV irradiation. Moreover, the ectopic expression of Nek6 overrides DNA damage-induced G(2)/M arrest. These results suggest that Nek6 is a novel target of the DNA damage checkpoint and that the inhibition of Nek6 activity is required for proper cell cycle arrest in the G(2)/M phase upon DNA damage.

Frequent alterations in the expression of serine/threonine kinases in human cancers

Protein kinases constitute a large family of regulatory enzymes involved in the homeostasis of virtually every cellular process. Subversion of protein kinases has been frequently implicated in malignant transformation. Within the family, serine/threonine kinases (STK) have received comparatively lesser attention, vis-a-vis tyrosine kinases, in terms of their involvement in human cancers. Here, we report a large-scale screening of 125 STK, selected to represent all major subgroups within the subfamily, on nine different types of tumors ( approximately 200 patients), by using in situ hybridization on tissue microarrays. Twenty-one STK displayed altered levels of transcripts in tumors, frequently with a clear tumor type-specific dimension. We identified three patterns of alterations in tumors: (a) overexpression in the absence of expression in the normal tissues (10 kinases), (b) overexpression in the presence of expression by normal tissues (8 kinases), and (c) underexpression (3 kinases). Selected members of the three classes were subjected to in-depth analysis on larger case collections and showed significant correlations between their altered expression and biological and/or clinical variables. Our findings suggest that alteration in the expression of STK is a relatively frequent occurrence in human tumors. Among the overexpressed kinases, 10 were undetectable in normal controls and are therefore ideal candidates for further validation as potential targets of molecular cancer therapy.

Phylogenetic analysis of the Neks reveals early diversification of ciliary-cell cycle kinases

BACKGROUND

NIMA-related kinases (Neks) have been studied in diverse eukaryotes, including the fungus Aspergillus and the ciliate Tetrahymena. In the former, a single Nek plays an essential role in cell cycle regulation; in the latter, which has more than 30 Neks in its genome, multiple Neks regulate ciliary length. Mammalian genomes encode an intermediate number of Neks, several of which are reported to play roles in cell cycle regulation and/or localize to centrosomes. Previously, we reported that organisms with cilia typically have more Neks than organisms without cilia, but were unable to establish the evolutionary history of the gene family.

METHODOLOGY/PRINCIPLE FINDINGS

We have performed a large-scale analysis of the Nek family using Bayesian techniques, including tests of alternate topologies. We find that the Nek family had already expanded in the last common ancestor of eukaryotes, a ciliated cell which likely expressed at least five Neks. We suggest that Neks played an important role in the common ancestor in regulating cilia, centrioles, and centrosomes with respect to mitotic entry, and that this role continues today in organisms with cilia. Organisms that lack cilia generally show a reduction in the number of Nek clades represented, sometimes associated with lineage specific expansion of a single clade, as has occurred in the plants.

CONCLUSION/SIGNIFICANCE

This is the first rigorous phylogenetic analysis of a kinase family across a broad array of phyla. Our findings provide a coherent framework for the study of Neks and their roles in coordinating cilia and cell cycle progression.

Mitotic regulation by NIMA-related kinases

The NIMA-related kinases represent a family of serine/threonine kinases implicated in cell cycle control. The founding member of this family, the NIMA kinase of Aspergillus nidulans, as well as the fission yeast homologue Fin1, contribute to multiple aspects of mitotic progression including the timing of mitotic entry, chromatin condensation, spindle organization and cytokinesis. Mammals contain a large family of eleven NIMA-related kinases, named Nek1 to Nek11. Of these, there is now substantial evidence that Nek2, Nek6, Nek7 and Nek9 also regulate mitotic events. At least three of these kinases, as well as NIMA and Fin1, have been localized to the microtubule organizing centre of their respective species, namely the centrosome or spindle pole body. Here, they have important functions in microtubule organization and mitotic spindle assembly. Other Nek kinases have been proposed to play microtubule-dependent roles in non-dividing cells, most notably in regulating the axonemal microtubules of cilia and flagella. In this review, we discuss the evidence that NIMA-related kinases make a significant contribution to the orchestration of mitotic progression and thereby protect cells from chromosome instability. Furthermore, we highlight their potential as novel chemotherapeutic targets.

Adenovirus E1A proteins direct subcellular redistribution of Nek9, a NimA-related kinase

A monoclonal antibody raised against adenovirus E1A-associated cellular proteins recognized Nek9, a NimA-related protein kinase. Subcellular fractionation and immunofluorescence indicated that Nek9 was primarily cytoplasmic with a small portion located in the nucleus whereas E1A was primarily nuclear. Although co-immunoprecipitation experiments indicated that nuclear Nek9 interacted, directly or indirectly, with E1A, the major effect of E1A was to diminish the amount of Nek9 in the nucleus suggesting that E1A alters the subcellular distribution of Nek9 and that the interaction is transient. A Nek9 deletion mutant lacking a central RCC1-like domain interacted stably with E1A and accumulated in the nucleus in the presence of E1A, possibly representing an intermediate stage of the normally transient Nek9/E1A interaction. The interaction of Nek9 with E1A was dependent on the N-terminal sequences of E1A. Attempts to stably overexpress either Nek9 or the kinase-inactive mutant in various cell lines were unsuccessful; however, the presence of E1A allowed stable overexpression of both proteins. These results suggest that E1A disrupts a nuclear function of Nek9.

Alternative splicing controls nuclear translocation of the cell cycle-regulated Nek2 kinase

Nek2 is a cell cycle-regulated serine/threonine protein kinase that is up-regulated in human cancers. Functionally, it is implicated in control of centrosome separation and bipolar spindle formation in mitotic cells and chromatin condensation in meiotic cells. Two major splice variants have been described in vertebrates, Nek2A and Nek2B, that differ in their non-catalytic C termini. Recently, a third splice variant, Nek2C, was identified that lacks an eight-amino acid internal sequence within the C-terminal domain of Nek2A. This excision occurs at the same position as the Nek2A/Nek2B splice point. As predicted from their high degree of similarity, we show here that Nek2C shares many properties with Nek2A including kinase activity, dimerization, protein phosphatase 1 interaction, mitotic degradation, microtubule binding, and centrosome localization. Unexpectedly, though, the non-centrosomal pool of protein exhibits a marked difference in distribution for the three splice variants. Nek2C is mainly nuclear, Nek2B is mainly cytoplasmic, and Nek2A is evenly distributed within nuclei and cytoplasm. Mutagenesis experiments revealed a functional bipartite nuclear localization sequence (NLS) that spans the splice site leading to Nek2C having a strong NLS, Nek2A having a weak NLS, and Nek2B having no NLS. Finally, we identified a 28-kDa protein in nuclear extracts as a potential novel substrate of Nek2. Thus, alternative splicing provides an unusual mechanism for modulating Nek2 localization, enabling it to have both nuclear and cytoplasmic functions.

Human NIMA-related kinase 6 is one of the Fe65 WW domain binding proteins

The Aspergillus nidulans protein NIMA (never in mitosis, gene A) is a protein kinase required for initiation of mitosis, whereas its inactivation is necessary for mitotic exit. Here, we present evidence that human Nek6 is associated with Fe65. Based on the presence of Fe65 WW domain binding motifs ((267)PPLP(270)) in the Nek6 catalytic domain, we observed that Nek6 interacts physically with Fe65 both in vivo and in vitro, using a pull-down approach. Additionally, we detected co-localization of Nek6 and Fe65 via confocal microscopy. Co-localization of Nek6 and Fe65 was disrupted by mutation of the WW domain binding motifs ((267)PPLP(270)). Finally, when transient transfection assays were performed, interaction of Nek6 (wt) with Fe65 induced substantial cell apoptosis, whereas interaction using the Nek6 pplp mutant ((267)PPLP(270) changes (267)APVA(270)) did not. Thus, our observations indicated that Nek6 binds to Fe65 through its (267)PPLP(270) motif and that the protein-protein interaction between Nek6 and Fe65 regulates their subcellular localization and cell apoptosis.

Large-scale mapping of human protein-protein interactions by mass spectrometry

Mapping protein–protein interactions is an invaluable tool for understanding protein function. Here, we report the first large-scale study of protein–protein interactions in human cells using a mass spectrometry-based approach. The study maps protein interactions for 338 bait proteins that were selected based on known or suspected disease and functional associations. Large-scale immunoprecipitation of Flag-tagged versions of these proteins followed by LC-ESI-MS/MS analysis resulted in the identification of 24 540 potential protein interactions. False positives and redundant hits were filtered out using empirical criteria and a calculated interaction confidence score, producing a data set of 6463 interactions between 2235 distinct proteins. This data set was further cross-validated using previously published and predicted human protein interactions. In-depth mining of the data set shows that it represents a valuable source of novel protein–protein interactions with relevance to human diseases. In addition, via our preliminary analysis, we report many novel protein interactions and pathway associations.

Structure and regulation of the human Nek2 centrosomal kinase

The dimeric Ser/Thr kinase Nek2 regulates centrosome cohesion and separation through phosphorylation of structural components of the centrosome, and aberrant regulation of Nek2 activity can lead to aneuploid defects characteristic of cancer cells. Mutational analysis of autophosphorylation sites within the kinase domain identified by mass spectrometry shows a complex pattern of positive and negative regulatory effects on kinase activity that are correlated with effects on centrosomal splitting efficiency in vivo. The 2.2-A resolution x-ray structure of the Nek2 kinase domain in complex with a pyrrole-indolinone inhibitor reveals an inhibitory helical motif within the activation loop. This helix presents a steric barrier to formation of the active enzyme and generates a surface that may be exploitable in the design of specific inhibitors that selectively target the inactive state. Comparison of this "auto-inhibitory" conformation with similar arrangements in cyclin-dependent kinase 2 and epidermal growth factor receptor kinase suggests a role for dimerization-dependent allosteric regulation that combines with autophosphorylation and protein phosphatase 1c phosphatase activity to generate the precise spatial and temporal control required for Nek2 function in centrosomal maturation.

Nek7 kinase is enriched at the centrosome, and is required for proper spindle assembly and mitotic progression

Members of the NIMA-related kinases (NRK) family are recently emerging as central regulators of various aspects of the cell cycle. However, the cellular roles of the mammalian NRK, Nek7, remain obscure. We show here that the endogenous Nek7 protein is enriched at the centrosome in a microtubule-independent manner. Overexpression of wt or kinase-defective Nek7 resulted in cells of rounder appearance, and higher proportions of multinuclear and apoptotic cells. Down-regulation of Nek7 using a small interfering RNA approach resulted in a significant increase in mitotic cells presenting multipolar spindle phenotype. These results suggest a role for Nek7 in regulating proper spindle assembly and mitotic progression.

Development and comparison of nonradioactive in vitro kinase assays for NIMA-related kinase 2

NIMA (never in mitosis arrest)-related kinase 2 (Nek2) is a serine/threonine kinase required for centrosome splitting and bipolar spindle formation during mitosis. Currently, two in vitro kinase assays are commercially available: (i) a radioactive assay from Upstate Biotechnology and (ii) a nonradioactive fluorescence resonance energy transfer (FRET) assay from Invitrogen. However, due to several limitations such as radioactive waste management and lower sensitivity, a need for more robust nonradioactive assays would be ideal. Accordingly, we have developed four quantitative and sensitive nonradioactive Nek2 in vitro kinase assays: (i) a dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA) using peptides identified from a physiologically relevant protein substrate, (ii) DELFIA using Nek2 itself, (iii) a homogeneous time-resolved FRET assay termed LANCE, and (iv) A method of detecting phosphorylated products by HPLC. The DELFIA and LANCE assays are robust in that they generated more than 10-fold and 20-fold increases in signal-to-noise ratios, respectively, and are amenable to robotic high-throughput screening platforms. Validation of all four assays was confirmed by identifying a panel of small molecule ATP competitive inhibitors from an internal corporate library. The most potent compounds consistently demonstrated less than 100 nM activity regardless of the assay format and therefore were complementary. In summary, the Nek2 in vitro time-resolved FRET kinase assays reported are sensitive, quantitative, reproducible and amenable to high-throughput screening with improved waste management over radioactive assays.

Mitotic kinases: the key to duplication, segregation, and cytokinesis errors, chromosomal instability, and oncogenesis

Chromosomal instability (CIN) and aneuploidy are commonly observed in the vast majority of human solid tumors and in many hematological malignancies. These features are considered defining characteristics of human breast, bladder and kidney cancers since they markedly exceed a 50% aneuploidy frequency. The detection of persistent mitotic kinase over-expression, particularly the Aurora family, and centrosome amplification in precursor/pre-malignant stages, strongly implicate these molecular changes in precipitating the aneuploidy seen in many human neoplasms. Mitotic spindle checkpoint defects may also lead to aneuploid tumors. However, the sustained over-expression and activity of various members of the mitotic kinase families, including Aurora (Aur) (A, B, C), Polo-like (Plk1-4), and Nek (NIMA1-11) in diverse human tumors strongly indicate that these entities are intimately involved in the development of errors in centrosome duplication, chromosome segregation, and cytokinesis. Mitotic kinases have also been implicated in regulating the centrosome cycle, spindle checkpoint and microtubule-kinetochore attachment, spindle assembly, and chromosome condensation. These mitotic kinases are modulated by de-novo synthesis, stability factors, phosphorylation, and ubiquitin-dependent proteolysis. They, in turn, phosphorylate a myriad of centrosomal/mitotic protein substrates, and have the ability to behave as oncogenes (i.e. Aur-A, Plk-1), providing a compelling link between errors in mitosis and oncogenic processes. The recent development of selective small molecule inhibitors of Aurora kinases, in particular, will provide useful tools to ascertain more precisely their role in cancer development. Potent inhibitors of mitotic kinases, when fully developed, have the promise to be effective agents against tumor growth, and possibly, tumor prevention as well.

NIMA-related kinase TbNRKC is involved in basal body separation in Trypanosoma brucei

The NIMA-related kinase 2 (NEK 2) has important cell cycle functions related to centriole integrity and splitting. Trypanosoma brucei does not possess centrioles, however, cytokinesis is coupled to basal body separation events. Here we report the first functional characterisation of a T. brucei basal body-cytoskeletal NIMA-related kinase (NRK) protein, TbNRKC. The TbNRKC kinase domain has high amino acid identity with the human NEK1 kinase domain (50%) but also shares 42% identity with human NEK2. TbNRKC is expressed in bloodstream and procyclic cells and functions as a bona fide kinase in vitro. Remarkably, RNAi knockdown of TbNRKC and overexpression of kinase-dead TbNRKC in procyclic forms induces the accumulation of cells with four basal bodies, whereas overexpression of active protein produces supernumary basal bodies and blocks cytokinesis. TbNRKC is located on mature and immature basal bodies and is the first T. brucei NRK to be found associated with the basal body cytokinesis pathway.

Caught Nek-ing: cilia and centrioles

The Nek family of cell-cycle kinases is widely represented in eukaryotes and includes numerous proteins that were described only recently and remain poorly characterized. Comparing Neks in the context of clades allows us to examine the question of whether microbial eukaryotic Neks, although not strictly orthologs of their vertebrate counterparts, can provide clues to ancestral functions that might be retained in the vertebrate Neks. Relatives of the Nek2/NIMA proteins play important roles at the G2-M transition in nuclear envelope breakdown and centromere separation. Nek6, Nek7 and Nek9 also seem to regulate mitosis. By contrast, Nek1 and Nek8 have been linked with polycystic kidney disease. Results of statistical analysis indicate that the family coevolved with centrioles that function as both microtubule-organizing centers and the basal bodies of cilia. This evolutionary perspective, taken together with functional studies of microbial Neks, provides new insights into the cellular roles of the proteins and disease with which some of them have been linked.

Nek1 shares structural and functional similarities with NIMA kinase

The Aspergillus NIMA serine/threonine kinase plays a pivotal role in controlling entrance into mitosis. A major function attributed to NIMA is the induction of chromatin condensation. We show here that the founder murine NIMA-related kinase, Nek1, is larger than previously reported, and that the full-length protein conserves the structural hallmarks of NIMA. Even though Nek1 bears two classical nuclear localization signals (NLS), the endogenous protein localizes to the cytoplasm. Ectopic overexpression of various Nek1 constructs suggests that the C-terminus of Nek1 bears cytoplasmic localization signal(s). Overexpression of nuclear constructs of Nek1 resulted in abnormal chromatin condensation, with the DNA mainly confined to the periphery of the nucleus. Advanced condensation phenotype was associated with nuclear pore complex dispersal. The condensation was not accompanied by up-regulation of mitotic or apoptotic markers. A similar phenotype has been described following NIMA overexpression, strengthening the notion that the mammalian Nek1 kinase has functional similarity to NIMA.

Interaction of Pin1 with Nek6 and characterization of their expression correlation in Chinese hepatocellular carcinoma patients

The peptidyl-prolyl isomerase Pin1 is prevalently overexpressed in human cancers and is regarded as a new diagnostic and therapeutic target. Pin1 interacts with several proteins involved in cell cycle events in a phosphorylation-dependent manner. Among them, NIMA (never in mitosis, gene A) was first identified to interact with Pin1. In this report, we found that Pin1 could interact with Nek6, one of the human NIMA-related kinases (Neks). This interaction was confirmed by GST pull-down assay, which was further confirmed by immunoprecipitation experiments, as well as immunofluorescence colocalization. We further studied Pin1 and Nek6 mRNA level in 40 pairs of hepatocellular carcinoma cases, finding significant correlations between Nek6 and Pin1 mRNA expression levels in these samples.

Comparative analysis of the kinomes of three pathogenic trypanosomatids: Leishmania major, Trypanosoma brucei and Trypanosoma cruzi

Background

The trypanosomatids Leishmania major, Trypanosoma brucei and Trypanosoma cruzi cause some of the most debilitating diseases of humankind: cutaneous leishmaniasis, African sleeping sickness, and Chagas disease. These protozoa possess complex life cycles that involve development in mammalian and insect hosts, and a tightly coordinated cell cycle ensures propagation of the highly polarized cells. However, the ways in which the parasites respond to their environment and coordinate intracellular processes are poorly understood. As a part of an effort to understand parasite signaling functions, we report the results of a genome-wide analysis of protein kinases (PKs) of these three trypanosomatids.

Results

Bioinformatic searches of the trypanosomatid genomes for eukaryotic PKs (ePKs) and atypical PKs (aPKs) revealed a total of 176 PKs in T. brucei, 190 in T. cruzi and 199 in L. major, most of which are orthologous across the three species. This is approximately 30% of the number in the human host and double that of the malaria parasite, Plasmodium falciparum. The representation of various groups of ePKs differs significantly as compared to humans: trypanosomatids lack receptor-linked tyrosine and tyrosine kinase-like kinases, although they do possess dual-specificity kinases. A relative expansion of the CMGC, STE and NEK groups has occurred. A large number of unique ePKs show no strong affinity to any known group. The trypanosomatids possess few ePKs with predicted transmembrane domains, suggesting that receptor ePKs are rare. Accessory Pfam domains, which are frequently present in human ePKs, are uncommon in trypanosomatid ePKs.

Conclusion

Trypanosomatids possess a large set of PKs, comprising approximately 2% of each genome, suggesting a key role for phosphorylation in parasite biology. Whilst it was possible to place most of the trypanosomatid ePKs into the seven established groups using bioinformatic analyses, it has not been possible to ascribe function based solely on sequence similarity. Hence the connection of stimuli to protein phosphorylation networks remains enigmatic. The presence of numerous PKs with significant sequence similarity to known drug targets, as well as a large number of unusual kinases that might represent novel targets, strongly argue for functional analysis of these molecules.

Active Nercc1 protein kinase concentrates at centrosomes early in mitosis and is necessary for proper spindle assembly

The Nercc1 protein kinase autoactivates in vitro and is activated in vivo during mitosis. Autoactivation in vitro requires phosphorylation of the activation loop at threonine 210. Mitotic activation of Nercc1 in mammalian cells is accompanied by Thr210 phosphorylation and involves a small fraction of total Nercc1. Mammalian Nercc1 coimmunoprecipitates gamma-tubulin and the activated Nercc1 polypeptides localize to the centrosomes and spindle poles during early mitosis, suggesting that active Nercc has important functions at the microtubular organizing center during cell division. To test this hypothesis, we characterized the Xenopus Nercc1 orthologue (XNercc). XNercc endogenous to meiotic egg extracts coprecipitates a multiprotein complex that contains gamma-tubulin and several components of the gamma-tubulin ring complex and localizes to the poles of spindles formed in vitro. Reciprocally, immunoprecipitates of the gamma-tubulin ring complex polypeptide Xgrip109 contain XNercc. Immunodepletion of XNercc from egg extracts results in delayed spindle assembly, fewer bipolar spindles, and the appearance of aberrant microtubule structures, aberrations corrected by addition of purified recombinant XNercc. XNercc immunodepletion also slows aster assembly induced by Ran-GTP, producing Ran-asters of abnormal size and morphology. Thus, Nercc1 contributes to both the centrosomal and the chromatin/Ran pathways that collaborate in the organization of a bipolar spindle.

PDK2: the missing piece in the receptor tyrosine kinase signaling pathway puzzle

Activation of members of the protein kinase AGC (cAMP dependent, cGMP dependent, and protein kinase C) family is regulated primarily by phosphorylation at two sites: a conserved threonine residue in the activation loop and a serine/threonine residue in a hydrophobic motif (HM) near the COOH terminus. Although phosphorylation of these kinases in the activation loop has been found to be mediated by phosphoinositide-dependent protein kinase-1 (PDK1), the kinase(s) that catalyzes AGC kinase phosphorylation in the HM remains uncharacterized. So far, at least 10 kinases have been suggested to function as an HM kinase or the so-called "PDK2," including mitogen-activated protein (MAP) kinase-activated protein kinase-2 (MK2), integrin-linked kinase (ILK), p38 MAP kinase, protein kinase Calpha (PKCalpha), PKCbeta, the NIMA-related kinase-6 (NEK6), the mammalian target of rapamycin (mTOR), the double-stranded DNA-dependent protein kinase (DNK-PK), and the ataxia telangiectasia mutated (ATM) gene product. However, whether any or all of these kinases act as a physiological HM kinase remains to be established. Nonetheless, available data suggest that multiple systems may be used in cells to regulate the activation of the AGC family kinases. It is possible that, unlike activation loop phosphorylation, phosphorylation of the HM site in the different AGC family kinases is mediated by distinct kinases. In addition, phosphorylation of the AGC family kinase at the HM site could be cell type, signaling pathway, and substrate specific. Identification and characterization of the bonafide HM kinase(s) will be essential to verify these hypotheses.

Potential link between the NIMA mitotic kinase and nuclear membrane fission during mitotic exit in Aspergillus nidulans

We have isolated TINC as a NIMA-interacting protein by using the yeast two-hybrid system and have confirmed that TINC interacts with NIMA in Aspergillus nidulans. The TINC-NIMA interaction is stabilized in the absence of phosphatase inhibitors and in the presence of kinase-inactive NIMA, suggesting that the interaction is enhanced when NIMA is not fully activated. TINC is a cytoplasmic protein. TINC homologues and a TINC-like protein (A. nidulans HETC) are conserved in other filamentous fungi. Neither deletion of tinC nor deletion of both tinC and A. nidulans hetC is lethal, but deletion of tinC does produce cold sensitivity as well as osmotic sensitivity. Expression of an amino-terminal-truncated form of TINC (DeltaN-TINC) inhibits colony growth in Aspergillus and localizes to membrane-like structures within the cell. Examination of cell cycle progression in these cells reveals that they progress through multiple defective mitoses. Many cells contain large polyploid single nuclei, while some appear to have separated masses of DNA. Examination of the nuclear envelopes of cells containing more than one DNA mass reveals that both DNA masses are contained within a single nuclear envelope, indicating that nuclear membrane fission is defective. The ability of these cells to separate DNA segregation from nuclear membrane fission suggests that this coordination is normally a regulated process in A. nidulans. Additional experiments demonstrate that expression of DeltaN-TINC results in premature NIMA disappearance in mitotic samples. We propose that TINC's interaction with NIMA and the cell cycle defects produced by DeltaN-TINC expression suggest possible roles for TINC and NIMA during nuclear membrane fission.

Recruitment of NIMA kinase shows that maturation of the S. pombe spindle-pole body occurs over consecutive cell cycles and reveals a role for NIMA in modulating SIN activity

Mitotic exit in Saccharomyces cerevisiae and septation in Schizosaccharomyces pombe are regulated by a conserved signaling network called the mitotic exit and septum initiation networks (SIN), respectively. The network is active on one of the two anaphase B spindle-pole bodies (SPBs). Whereas the inherent asymmetry of growth by budding accounts for elements of the asymmetry in S. cerevisiae, it has been unclear how, or why, the pathway is asymmetric in S. pombe. We show that elements of SPB duplication in S. pombe are conservative, and that the SIN is active on the new SPB. SIN association with the new SPB persists after transient depolymerization of microtubules. The localization of the NIMA-related kinase, Fin1, reveals further complexity in SPB inheritance. Fin1 associates with the SPB bearing the older components in all cells and with the "new" SPB in half of the population. Fin1 only binds the new SPB when this new SPB has arisen from the duplication of an SPB that is two or more cycles old. Thus, each of the four SPBs generated over two consecutive cell cycles are different, because they have distinct fates in the next cell cycle. Fin1 binds the SPB once the SIN is active and the association requires the SIN inhibitors Byr4 and Cdc16. Fin1 physically associates with Byr4. Compromising Fin1 function leads to SIN activation on both anaphase B SPBs and promotes septation, indicating that Fin1 restrains SIN activity on the old SPB.

Nucleolar Nek11 is a novel target of Nek2A in G1/S-arrested cells

We previously reported that Nek11, a member of the NIMA (never-in-mitosis A) family of kinases, is activated in G(1)/S-arrested cells. We provide herein several lines of evidence for a novel interaction between Nek11 and Nek2A. Both Nek11 and Nek2A, but not Nek2B, were detected at nucleoli, and the Nek2A-specific C-terminal end (amino acids 399-445) was responsible for nucleolar localization. Endogenous Nek11 coimmunoprecipitated with endogenous Nek2A, and non-catalytic regions of each kinase were involved in the complex formation. Nek11L interacted with phosphorylated Nek2A but barely with the kinase-inactive Nek2A (K37R) mutant. In addition, both Nek2A autophosphorylation activity and the Nek11L-Nek2A complex formation increased in G(1)/S-arrested cells. These results indicate that autophosphorylation of Nek2A could stimulate its interaction with Nek11L at the nucleolus. Moreover, Nek2 directly phosphorylated Nek11 in the C-terminal non-catalytic region and elevated Nek11 kinase activity. The non-catalytic region of Nek11 showed autoinhibitory activity through intramolecular interaction with its N-terminal catalytic domain. Nek2 dissociated this autoinhibitory interaction. Altogether, our studies demonstrate a unique mechanism of Nek11 activation by Nek2A in G(1)/S-arrested cells and suggest a novel possibility for nucleolar function of the NIMA family.

Nore1 inhibits tumor cell growth independent of Ras or the MST1/2 kinases

Nore1, a noncatalytic protein identified by its ability to bind selectively to active Ras, is most closely related in amino-acid sequence to the tumor suppressor RASSF1. Both are expressed predominantly as a longer (Nore1A/RASSF1A) and/or shorter (Nore1B/RASSF1C) polypeptide; all four polypeptides contain a Ras-association domain and bind, through their conserved carboxytermini, the proapoptotic protein kinases MST1 and MST2. Moreover, the expression of the longer polypeptide is downregulated in human tumor cell lines through promoter methylation (frequently for RASSF1A, less regularly for Nore1A). Forced expression of RASSF1A in several such lines (including the NSCLC line A549) has been shown to suppress tumorigenicity; herein we inquire whether Nore has growth inhibitory activity. Four tumor cell lines were tested, selected for their low expression of both Nore1A and Nore1B; the two NSCLC lines, A549 and NCI-H460, each have a mutant active Ras oncogene, whereas the two melanoma lines G361 and M14 each contain the constitutively active BRaf(V599E) oncogene and wild-type Ras. The expression of Nore1A or Nore1B suppresses colony formation by the A549 and G361 lines, as effectively in A549 as does RASSF1A; colony formation in the NCI-H460 and M14 lines is unaffected. Nore1A inhibits anchorage-independent growth by A549 cells and delays A549 progression through G1 without evidence of increased apoptosis. The growth suppressive action of Nore1A is largely unaffected by deletion of both the MST- and Ras-binding domains, as well as by mutation of the Nore1A zinc finger. Thus, Nore1 suppresses the growth of some tumor cell lines through as yet unidentified effectors, independent of Ras-like proteins or MST1/2.