Properties and regulation of the cell cycle-specific NIMA protein kinase of Aspergillus nidulans

NEK5 interacts with topoisomerase IIβ and is involved in the DNA damage response induced by etoposide

Cells are daily submitted to high levels of DNA lesions that trigger complex pathways and cellular responses by cell cycle arrest, apoptosis, alterations in transcriptional response, and the onset of DNA repair. Members of the NIMA-related kinase (NEK) family have been related to DNA damage response and repair and the first insight about NEK5 in this context is related to its role in centrosome separation resulting in defects in chromosome integrity. Here we investigate the potential correlation between NEK5 and the DNA damage repair index. The effect of NEK5 in double-strand breaks caused by etoposide was accessed by alkaline comet assay and revealed that NEK5-silenced cells are more sensitive to etoposide treatment. Topoisomerase IIβ (TOPIIβ) is a target of etoposide that leads to the production of DNA breaks. We demonstrate that NEK5 interacts with TOPIIβ, and the dynamics of this interaction is evaluated by proximity ligation assay. The complex NEK5/TOPIIβ is formed immediately after etoposide treatment. Taken together, the results of our study reveal that NEK5 depletion increases DNA damage and impairs proper DNA damage response, pointing out NEK5 as a potential kinase contributor to genomic stability.

Epigenetic and Posttranslational Modifications in Regulating the Biology of Aspergillus Species

Epigenetic and posttranslational modifications have been proved to participate in multiple cellular processes and suggested to be an important regulatory mechanism on transcription of genes in eukaryotes. However, our knowledge about epigenetic and posttranslational modifications mainly comes from the studies of yeasts, plants, and animals. Recently, epigenetic and posttranslational modifications have also raised concern for the relevance of regulating fungal biology in Aspergillus. Emerging evidence indicates that these modifications could be a connection between genetic elements and environmental factors, and their combined effects may finally lead to fungal phenotypical changes. This article describes the advances in typical DNA and protein modifications in the genus Aspergillus, focusing on methylation, acetylation, phosphorylation, ubiquitination, sumoylation, and neddylation.

The observation of plcA mutation and localization in Aspergillus nidulans

To know the function of the plcA gene, which encodes a putative phosphoinositide-specific phospholipase C, in a model filamentous fungus Aspergillus nidulans, it was disrupted thorough homologous recombination and examined. The germination rate of ΔplcA was reduced by approximately 65% and germination of ΔplcA at a lower temperature (25°C) was much slower than germination under normal conditions (37°C), suggesting the plcA is responsible for cold-sensitivity. The hyphal growth of ΔplcA was slightly reduced at 37°C and conspicuously reduced at 25°C. While germinating ΔplcA formed giant swollen spores, and generated short and thick hyphae. The results of the nuclear examination of ΔplcA showed nuclear division with missegregation, and the rate of nuclear division was lower than that of wild type at both 25°C and 37°C. The results of this study showed that plcA is localized to the nucleus through intracellular calcium signaling in A. nidulans. The abnormal nuclear division, resulting from plcA gene deletion, affects conidiation in asexual development. Taken together, these results suggested that plcA is required for normal vegetative growth, morphogenesis, conidiation, and nuclear division in A. nidulans.

On the molecular mechanisms of mitotic kinase activation

During mitosis, human cells exhibit a peak of protein phosphorylation that alters the behaviour of a significant proportion of proteins, driving a dramatic transformation in the cell's shape, intracellular structures and biochemistry. These mitotic phosphorylation events are catalysed by several families of protein kinases, including Auroras, Cdks, Plks, Neks, Bubs, Haspin and Mps1/TTK. The catalytic activities of these kinases are activated by phosphorylation and through protein–protein interactions. In this review, we summarize the current state of knowledge of the structural basis of mitotic kinase activation mechanisms. This review aims to provide a clear and comprehensive primer on these mechanisms to a broad community of researchers, bringing together the common themes, and highlighting specific differences. Along the way, we have uncovered some features of these proteins that have previously gone unreported, and identified unexplored questions for future work. The dysregulation of mitotic kinases is associated with proliferative disorders such as cancer, and structural biology will continue to play a critical role in the development of chemical probes used to interrogate disease biology and applied to the treatment of patients.

The Toxoplasma gondii centrosome is the platform for internal daughter budding as revealed by a Nek1 kinase mutant

The pathology and severity of toxoplasmosis results from the rapid replication cycle of the apicomplexan parasite Toxoplasma gondii. The tachyzoites divide asexually through endodyogeny, wherein two daughter cells bud inside the mother cell. Before mitosis is completed, the daughter buds form around the duplicated centrosomes and subsequently elongate to serve as the scaffold for organellogenesis and organelle partitioning. The molecular control mechanism of this process is poorly understood. Here, we characterized a T. gondii NIMA-related kinase (Nek) ortholog that was identified in a chemical mutagenesis screen. A temperature-sensitive mutant, V-A15, possesses a Cys316Arg mutation in TgNek1 (a novel mutant allele in Neks), which is responsible for growth defects at the restrictive temperature. Phenotypic analysis of V-A15 indicated that TgNek1 is essential for centrosome splitting, proper formation of daughter cells and faithful segregation of genetic material. In vitro kinase assays showed that the mutation abolishes the kinase activity of TgNek1. TgNek1 is recruited to the centrosome prior to its duplication and localizes on the duplicated centrosomes facing the spindle poles in a cell-cycle-dependent manner. Mutational analysis of the activation loop suggests that localization and activity are spatio-temporally regulated by differential phosphorylation. Collectively, our results identified a novel temperature-sensitive allele for a Nek kinase and highlight its essential function in centrosome splitting in Toxoplasma. Moreover, these results conclusively show for the first time that Toxoplasma bud assembly is facilitated by the centrosome because defective centrosome splitting results in single daughter cell budding.

Phosphorylation of Nup98 by multiple kinases is crucial for NPC disassembly during mitotic entry

Disassembly of nuclear pore complexes (NPCs) is a decisive event during mitotic entry in cells undergoing open mitosis, yet the molecular mechanisms underlying NPC disassembly are unknown. Using chemical inhibition and depletion experiments we show that NPC disassembly is a phosphorylation-driven process, dependent on CDK1 activity and supported by members of the NIMA-related kinase (Nek) family. We identify phosphorylation of the GLFG-repeat nucleoporin Nup98 as an important step in mitotic NPC disassembly. Mitotic hyperphosphorylation of Nup98 is accomplished by multiple kinases, including CDK1 and Neks. Nuclei carrying a phosphodeficient mutant of Nup98 undergo nuclear envelope breakdown slowly, such that both the dissociation of Nup98 from NPCs and the permeabilization of the nuclear envelope are delayed. Together, our data provide evidence for a phosphorylation-dependent mechanism underlying disintegration of NPCs during prophase. Moreover, we identify mitotic phosphorylation of Nup98 as a rate-limiting step in mitotic NPC disassembly.

Prolyl isomerase Pin1 regulates transcription factor LSF (TFCP2) by facilitating dephosphorylation at two serine-proline motifs

Transcription factor LSF is essential for cell cycle progression, being required for activating expression of the thymidylate synthase (Tyms) gene at the G1/S transition. We previously established that phosphorylation of LSF in early G1 at Ser-291 and Ser-309 inhibits its transcriptional activity and that dephosphorylation later in G1 is required for its reactivation. Here we reveal the role of prolyl cis-trans isomerase Pin1 in activating LSF, by facilitating dephosphorylation at both Ser-291 and Ser-309. We demonstrate that Pin1 binds LSF both in vitro and in vivo. Using coimmunoprecipitation assays, we identify three SP/TP motifs in LSF (at residues Ser-291, Ser-309, and Thr-329) that are required and sufficient for association with Pin1. Co-expression of Pin1 enhances LSF transactivation potential in reporter assays. The Pin1-dependent enhancement of LSF activity requires residue Thr-329 in LSF, requires both the WW and PPiase domains of Pin1, and correlates with hypophosphorylation of LSF at Ser-291 and Ser-309. These findings support a model in which the binding of Pin1 at the Thr-329-Pro-330 motif in LSF permits isomerization by Pin1 of the peptide bonds at the nearby phosphorylated SP motifs (Ser-291 and Ser-309) to the trans configuration, thereby facilitating their dephosphorylation.

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.

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.

PINA Is Essential for Growth and Positively Influences NIMA Function in Aspergillus nidulans

The phospho-Ser/Thr-directed prolyl-isomerase Pin1 was originally identified in vertebrate systems as a negative regulator of NIMA, a Ser/Thr protein kinase that regulates the G(2)/M transition in Aspergillus nidulans. Here we explore the physiological roles of the Pin1 orthologue, PINA, in A. nidulans and evaluate the relevance of the interaction of PINA with NIMA in this fungus. We find pinA to be an essential gene in A. nidulans. In addition, when PINA levels are reduced 50-fold the cells grow at a reduced rate. Upon germination under conditions that repress PINA expression, the cells are delayed in the interphase activation of NIMX(cdc2), whereas they traverse the other phases of the cell cycle at a similar rate to controls. These results indicate that a marked reduction of PINA results in a lengthening of G(1). Additionally, PINA repression increases the rate at which the cells enter mitosis following release from a hydroxyurea arrest without altering the sensitivity of the fungus to agents that activate the replication or DNA damage checkpoints. In contrast to predictions based on Pin1, the physical interaction between PINA and NIMA is primarily dependent upon the prolylisomerase domain of PINA and the C-terminal 303 amino acids of NIMA. Finally, reduction of PINA levels exacerbates the nimA5 temperature-sensitive mutant, whereas overexpression of PINA decreases the severity of this mutation, results that are consistent with a positive genetic interaction between PINA and NIMA. Thus, although PINA is essential and positively regulates NIMA function, A. nidulans is most sensitive to a reduction in PINA concentration in G(1) rather than in G(2)/M.

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

The Nek family of protein kinases in humans is composed of 11 members that share an amino-terminal catalytic domain related to NIMA, an Aspergillus kinase involved in the control of several aspects of mitosis, and divergent carboxyl-terminal tails of varying length. Nek6 (314AA) and Nek7 (303AA), 76% identical, have little noncatalytic sequence but bind to the carboxyl-terminal noncatalytic tail of Nercc1/Nek9, a NIMA family protein kinase that is activated in mitosis. Microinjection of anti-Nercc1 antibodies leads to spindle abnormalities and prometaphase arrest or chromosome missegregation. Herein we show that Nek6 is increased in abundance and activity during mitosis; activation requires the phosphorylation of Ser206 on the Nek6 activation loop. This phosphorylation and the activity of recombinant Nek6 is stimulated by coexpression with an activated mutant of Nercc1. Moreover, Nercc1 catalyzes the direct phosphorylation of prokaryotic recombinant Nek6 at Ser206 in vitro concomitant with 20-25-fold activation of Nek6 activity; Nercc1 activates Nek7 in vitro in a similar manner. Nercc1/Nek9 is likely to be responsible for the activation of Nek6 during mitosis and probably participates in the regulation of Nek7 as well. These findings support the conclusion that Nercc1/Nek9 and Nek6 represent a novel cascade of mitotic NIMA family protein kinases whose combined function is important for mitotic progression.

Identification of CfNek, a novel member of the NIMA family of cell cycle regulators, as a polypeptide copurifying with tubulin polyglutamylation activity in Crithidia

Post-translational glutamylation of tubulin plays an important role in regulating the interaction between microtubules and associated proteins, but so far the enzymes involved in this process have not been cloned from any cellular source. Using a modified purification scheme that employs a hydroxyapaptite chromatography as the final step we identified a 54 kDa band as the major polypeptide copurifying with tubulin polyglutamylation activity from the trypanosomatid Crithidia fasciculata. Based on peptide sequence information we have cloned the corresponding cDNA and identify Crithidia p54 as a novel member (termed CfNek) of the NIMA family of putative cell cycle regulators. CfNek is a protein of 479 amino acids that contains an unusual protein kinase domain that lacks the glycine-rich loop in subdomain I. The protein also harbours a PEST sequence and a pleckstrin homology domain. The tubulin polyglutamylase preparation displays the beta-casein phosphorylation activity typical for NIMA related kinases. Recombinant His-tagged CfNek expressed in Crithidia localises to the flagellar attachment zone/basal body of the parasite. After purification on a Ni(2+)-column the recombinant enzyme preparation displays ATP-dependent tubulin polyglutamylation activity as well as casein-phosphorylation activity.

Nek11, a new member of the NIMA family of kinases, involved in DNA replication and genotoxic stress responses

DNA replication and genotoxic stresses activate various checkpoint-associated protein kinases, and checkpoint dysfunction often leads to cell lethality. Here, we have identified new members of the mammalian NIMA family of kinases, termed Nek11L and Nek11S (NIMA-related kinase 11 Long and Short isoform) as novel DNA replication/damage stresses-responsive kinases. Molecular cloning and biochemical studies showed that the catalytic domain of Nek11 is most similar to Nek4 and Nek3, and substrate specificity of Nek11L is distinguishable from those of NIMA and Nek2. The expression of nek11L mRNA increased through S to G(2)/M phase, and subcellular localization of Nek11 protein altered between interphase and prometaphase, suggesting multiple roles of Nek11. We found an activation of Nek11 kinase activity when cells were treated with various DNA-damaging agents and replication inhibitors, and this activation of Nek11 was suppressed by caffeine in HeLaS3 cells. The transient expression of wild-type Nek11L enhanced the aphidicolin-induced S-phase arrest, whereas the aphidicolin-induced S-phase arrest was reduced in the U2OS cell lines expressing kinase-negative Nek11L (K61R), and these cells were more sensitive to aphidicolin-induced cell lethality. Collectively, these results suggest that Nek11 has a role in the S-phase checkpoint downstream of the caffeine-sensitive pathway.

Purification, cloning, and characterization of Nek8, a novel NIMA-related kinase, and its candidate substrate Bicd2

We describe the isolation, cloning, and characterization of human Nek8, a new mammalian NIMA-related kinase, and its candidate substrate Bicd2. Nek8 was isolated as a beta-casein kinase activity in rabbit lung and has an N-terminal catalytic domain homologous to the Nek family of protein kinases. Nek8 also contains a central domain with homology to RCC1, a guanine nucleotide exchange factor for the GTPase Ran, and a C-terminal coiled-coil domain. Like Nek2, Nek8 prefers beta-casein over other exogenous substrates, has shared biochemical requirements for kinase activity, and is capable of autophosphorylation and oligomerization. Nek8 activity is not cell cycle regulated, but like Nek3, levels are consistently higher in G(0)-arrested cells. During the purification of Nek8 a second protein co-chromatographed with Nek8 activity. This protein, Bicd2, is a human homolog of the Drosophila protein Bicaudal D, a coiled-coil protein. Bicd2 is phosphorylated by Nek8 in vitro, and the endogenous proteins associate in vivo. Bicd2 localizes to cytoskeletal structures, and its subcellular localization is dependent on microtubule morphology. Treatment of cells with nocodazole leads to dramatic reorganization of Bicd2, and correlates with Nek8 phosphorylation. This may be indicative of a role for Nek8 and Bicd2 associated with cell cycle independent microtubule dynamics.

Pfnek-1, a NIMA-related kinase from the human malaria parasite Plasmodium falciparum Biochemical properties and possible involvement in MAPK regulation

We have cloned Pfnek-1, a gene encoding a novel protein kinase from the human malaria parasite Plasmodium falciparum. This enzyme displays maximal homology to the never-in-mitosis/Aspergillus (NIMA)/NIMA-like kinase (Nek) family of protein kinases, whose members are involved in eukaryotic cell division processes. Similar to other P. falciparum protein kinases and many enzymes of the NIMA/Nek family, Pfnek-1 possesses a large C-terminal extension in addition to the catalytic domain. Bacterially expressed recombinant Pfnek-1 protein is able to autophosphorylate and phosphorylate a panel of protein substrates with a specificity that is similar to that displayed by other members of the NIMA/Nek family. However, the FXXT motif usually found in NIMA/Nek protein kinases is substituted in Pfnek-1 by a SMAHS motif, which is reminiscent of a MAP/ERK kinase (MEK) activation site. Mutational analysis indicates that only one of the serine residues in this motif is essential for Pfnek-1 kinase activity in vitro. We show (a) that recombinant Pfnek-1 is able to specifically phosphorylate Pfmap-2, an atypical P. falciparum MAPK homologue, in vitro, and (b) that coincubation of Pfnek-1 and Pfmap-2 results in a synergistic increase in exogenous substrate labelling. This suggests that Pfnek-1 may be involved in the modulation of MAPK pathway output in malaria parasites. Finally, we demonstrate that recombinant Pfnek-1 can be used in inhibition assays to monitor the effect of kinase inhibitors, which opens the way to the screening of chemical libraries aimed at identifying potential new antimalarials.

Mitotic histone H3 phosphorylation by the NIMA kinase in Aspergillus nidulans

Phosphorylation of histone H3 serine 10 correlates with chromosome condensation and is required for normal chromosome segregation in Tetrahymena. This phosphorylation is dependent upon activation of the NIMA kinase in Aspergillus nidulans. NIMA expression also induces Ser-10 phosphorylation inappropriately in S phase-arrested cells and in the absence of NIMX(cdc2) activity. At mitosis, NIMA becomes enriched on chromatin and subsequently localizes to the mitotic spindle and spindle pole bodies. The chromatin-like localization of NIMA early in mitosis is tightly correlated with histone H3 phosphorylation. Finally, NIMA can phosphorylate histone H3 Ser-10 in vitro, suggesting that NIMA is a mitotic histone H3 kinase, perhaps helping to explain how NIMA promotes chromatin condensation in A. nidulans and when expressed in other eukaryotes.

Phosphorylation-dependent prolyl isomerization: a novel cell cycle regulatory mechanism

Protein phosphorylation by proline-directed protein kinases plays an essential role in triggering a programmed set of cell cycle events. We have recently isolated an essential and conserved mitotic regulator, Pin1. Pin1 is a phosphorylation-dependent prolyl isomerase that specifically isomerizes the phosphorylated serine/threonine-proline bond. Pin1 also binds and regulates the function of a conserved set of mitosis-specific phosphoproteins. These results suggest phosphorylation-dependent prolyl isomerization to be a novel cell cycle regulatory mechanism. This new post-translational regulation may allow the general increase in protein phosphorylation to be converted into the organised and programmed set of structural modifications that occur during mitosis. In addition, since inhibition of Pin1 induces mitotic arrest and apoptosis, Pin1 may be a potential new drug target.

Activity and substrate specificity of the murine STK2 Serine/Threonine kinase that is structurally related to the mitotic regulator protein NIMA of Aspergillus nidulans

We isolated a murine STK2 (mSTK2) cDNA that is homologous to murine Nek1 serine/threonine kinase, a family member related to the cell cycle regulator kinase NIMA of Aspergillus nidulans. Structural comparison demonstrated that the kinase domain of mSTK2 is highly similar to NIMA/Nek family but the C-terminal region is not similar to any proteins except for human STK2 (hSTK2). Similarly to Nek1, mSTK2 is expressed ubiquitously among various organs and is upregulated in the testis. The expression and localization of mSTK2 are not associated with the cell cycle progression of mitogen-activated lymphocyte and DNA-transfected fibroblast. The substrate specificity of mSTK2 is similar to NIMA, but the phosphorylation is observed exclusively upon threonine residues rather than serine. The mSTK2 is shown to be a new member of the NIMA/Nek family with similar substrate specificity, which might participate in a different role from NIMA kinase involved in the cell cycle regulation.

Modulation of Xenopus oocyte-expressed phospholemman-induced ion currents by co-expression of protein kinases

Phospholemman (PLM), the major sarcolemmal substrate for phosphorylation by cAMP-dependent kinase (PKA) protein kinase C (PKC) and NIMA kinase in muscle, induces hyperpolarization-activated anion currents in Xenopus oocytes, most probably by enhancing endogenous oocyte currents. PLM peptides from the cytoplasmic tail are phosphorylated by PKA at S68, by NIMA kinase at S63, and by PKC at both S63 and S68. We have confirmed the phosphorylation sites in the intact protein, and we have investigated the role of phosphorylation in the regulatory activity of PLM using oocyte expression experiments. We found: (1) the cytoplasmic domain is not essential for inducing currents in oocytes; (2) co-expression of PKA increased the amplitude of oocyte currents and the amount of PLM in the oocyte membrane largely, but not exclusively, through phosphorylation of S68; (3) co-expression of PKA had no effect on a PLM mutant in which all putative phosphorylation sites had been inactivated by serine to alanine mutation (SSST 62, 63, 68, 69 AAAA); (4) co-expression of PKC had no effect in this system; (5) co-expression of NIMA kinase increased current amplitude and membrane protein level, but did not require PLM phosphorylation. These findings point to a role for phosphorylation in the function of PLM.

Activity of the human centrosomal kinase, Nek2, depends on an unusual leucine zipper dimerization motif

Nek2 is a human cell cycle-regulated kinase that is structurally related to the mitotic regulator, NIMA, of Aspergillus nidulans. Localization studies have shown that Nek2 is a core component of the centrosome, the microtubule organizing center of the cell, and functional approaches suggest a possible role for Nek2 in centrosome separation at the G2/M transition. Here, we have investigated the importance of an unusual leucine zipper coiled-coil motif present in the C-terminal noncatalytic domain of the Nek2 kinase. Glycerol gradient centrifugation indicated that endogenous Nek2 is present in HeLa cells as a salt-resistant 6 S complex, the predicted size of a Nek2 homodimer. Recombinant Nek2 overexpressed in insect cells also formed a 6 S complex, whereas a Nek2 mutant specifically lacking the leucine zipper motif was monomeric. Using yeast two-hybrid interaction analyses and coprecipitation assays, we found that Nek2 can indeed form homodimers both in vivo and in vitro and that this dimerization specifically required the leucine zipper motif. Moreover, deletion of the leucine zipper prevented a trans-autophosphorylation reaction on the C-terminal domain of Nek2 and strongly reduced Nek2 kinase activity on exogenous substrates. Finally, we emphasize that the Nek2 leucine zipper described here differs from classical leucine zippers in that it exhibits a radically different arrangement of hydrophobic and charged amino acids. Thus, this study reveals not only an important mechanism for the regulation of the Nek2 kinase but, furthermore, highlights an unusual organization of a leucine zipper dimerization motif.

Cloning and characterization of the murine Nek3 protein kinase, a novel member of the NIMA family of putative cell cycle regulators

We have cloned and characterized murine Nek3 (NIMA-related kinase 3), a novel mammalian gene product structurally related to the cell cycle-regulatory kinase NIMA of Aspergillus nidulans. By RNase protection, low levels of Nek3 expression could be detected in all organs examined, regardless of proliferative index. In contrast to Nek1 and Nek2, Nek3 levels were not particularly elevated in either the male or the female germ line. Nek3 levels showed at most marginal variations through the cell cycle, but they were elevated in G0-arrested, quiescent fibroblasts. Furthermore, no cell cycle-dependent changes in Nek3 activity could be detected, and no effects upon cell cycle progression could be observed upon antibody microinjection or overexpression of either wild-type or catalytically inactive Nek3. Finally, Nek3 was found to be a predominantly cytoplasmic enzyme. These data indicate that Nek3 differs from previously characterized Neks with regard to all parameters investigated, including organ specificity of expression, cell cycle dependence of expression and activity, and subcellular localization. Hence, the structural similarity between mammalian Neks may not necessarily be indicative of a common function, and it is possible that some members of this kinase family may perform functions that are not directly related to cell cycle control.

Function of WW domains as phosphoserine- or phosphothreonine-binding modules

Protein-interacting modules help determine the specificity of signal transduction events, and protein phosphorylation can modulate the assembly of such modules into specific signaling complexes. Although phosphotyrosine-binding modules have been well-characterized, phosphoserine- or phosphothreonine-binding modules have not been described. WW domains are small protein modules found in various proteins that participate in cell signaling or regulation. WW domains of the essential mitotic prolyl isomerase Pin1 and the ubiquitin ligase Nedd4 bound to phosphoproteins, including physiological substrates of enzymes, in a phosphorylation-dependent manner. The Pin1 WW domain functioned as a phosphoserine- or phosphothreonine-binding module, with properties similar to those of SRC homology 2 domains. Phosphoserine- or phosphothreonine-binding activity was required for Pin1 to interact with its substrates in vitro and to perform its essential function in vivo.

A role for NIMA in the nuclear localization of cyclin B in Aspergillus nidulans

NIMA promotes entry into mitosis in late G2 by some mechanism that is after activation of the Aspergillus nidulans G2 cyclin-dependent kinase, NIMXCDC2/NIMECyclin B. Here we present two independent lines of evidence which indicate that this mechanism involves control of NIMXCDC2/NIMECyclin B localization. First, we found that NIMECyclin B localized to the nucleus and the nucleus-associated organelle, the spindle pole body, in a NIMA-dependent manner. Analysis of cells from asynchronous cultures, synchronous cultures, and cultures arrested in S or G2 showed that NIMECyclin B was predominantly nuclear during interphase, with maximal nuclear accumulation in late G2. NIMXCDC2 colocalized with NIMECyclin B in G2 cells. Although inactivation of NIMA using either the nimA1 or nimA5 temperature-sensitive mutations blocked cells in G2, NIMXCDC2/NIMECyclin B localization was predominantly cytoplasmic rather than nuclear. Second, we found that nimA interacts genetically with sonA, which is a homologue of the yeast nucleocytoplasmic transporter GLE2/RAE1. Mutations in sonA were identified as allele-specific suppressors of nimA1. The sonA1 suppressor alleviated the nuclear division and NIMECyclin B localization defects of nimA1 cells without markedly increasing NIMXCDC2 or NIMA kinase activity. These results indicate that NIMA promotes the nuclear localization of the NIMXCDC2/ NIMECyclin B complex, by a process involving SONA. This mechanism may be involved in coordinating the functions of NIMXCDC2 and NIMA in the regulation of mitosis.

BIMAAPC3, a component of the Aspergillus anaphase promoting complex/cyclosome, is required for a G2 checkpoint blocking entry into mitosis in the absence of NIMA function

Temperature sensitive (ts) nimA mutants of Aspergillus nidulans arrest at a unique point in G2 which is post activation of CDC2. Here we show that this G2 arrest is due to loss of nimA function and that it is dependent on BIMAAPC3, a component of the anaphase promoting complex/cyclosome (APC/C). Whereas nimA single mutants arrested in G2 with decondensed chromatin and interphase microtubule arrays, nimA, bimAAPC3 double mutants arrested growth with condensed chromatin and aster-like microtubule arrays. nimA, bimAAPC3 double mutants entered mitosis with kinetics similar to bimAAPC3 single mutants and wild-type cells, indicating a checkpoint-like role for BIMAAPC3 in G2. Even cells which had been depleted for NIMA protein and which contained insignificant levels of NIMA kinase activity entered mitosis on inactivation of bimAAPC3. BIMAAPC3 was present in a >25S complex containing BIMEAPC1, and bimAAPC3 mutants were sensitive to elevated CYCLIN B expression, consistent with BIMAAPC3 being a component of the APC/C. Inactivation of bimAAPC3 had little affect on the steady state levels of the B-type cyclin, NIMECyclin B. Our results indicate that BIMAAPC3, and most likely the APC/C itself, is activated in G2 in nimA mutants. We propose that APC/C activation is part of a novel, late G2 checkpoint, which responds to a defective process or structure in nimA mutants, and which prevents inappropriate entry into mitosis.

Inhibitors of calcineurin block expression of cyclins A and E induced by fibroblast growth factor in Swiss 3T3 fibroblasts

In Swiss 3T3 fibroblasts, growth factor-stimulated progression from G1 to S phase involves activation of the Ca2+/calmodulin-dependent serine/threonine-specific protein phosphatase 2B (calcineurin). Here we report that both cobalt and the calcium chelator EGTA, inhibitors of calcium uptake, as well as cyclosporin A and FK-506, specific inhibitors of calcineurin function, abolished fibroblast growth factor (FGF)-induced expression of cyclins A and E, but not cyclin D1. At 0.1 microM concentration cyclosporin A completely blocked FGF-induced expression of cyclins E and A and it inhibited FGF-stimulated DNA synthesis by 40%; full inhibition of DNA synthesis required 10 microM cyclosporin A. PD 98059, an inhibitor of mitogen-activated protein (MAP) kinase kinase, and hemicholinium-3, an inhibitor of FGF-induced MAP kinase activity, did not inhibit the stimulatory effect of FGF on the expression of cyclin E. On the other hand, the inhibitory effect of 0.1 microM cyclosporin A on FGF-stimulated DNA synthesis was additive with that of hemicholinium-3, suggesting that the two inhibitors acted by different mechanisms. The inhibitors of calcineurin and calcium uptake also completely blocked the stimulatory effects of lysophosphatidic acid on the expression of cyclins E and A, but not cyclin D1. The results suggest that FGF- or lysophosphatidic acid-induced transcription of cyclin A and cyclin E genes is mediated by calcineurin involving a MAP kinase-independent mechanism and that increased expression of cyclins A and E is required for the maximal stimulatory effects of these mitogens on DNA synthesis.

Regulation of p34cdc2/cyclinB H1 and NIMA kinases during the G2/M transition and checkpoint responses in Aspergillus nidulans

In A. nidulans, activation of both p34cdc2/cyclinB H1 and NIMA kinases is required to initiate mitosis. These two kinases are regulated at several levels during interphase and are activated independently as protein kinases during G2. They are also targeted for negative regulation, to prevent mitosis by mitotic entry checkpoint controls, when DNA is not replicated or is damaged. Then, to initiate mitosis, they promote each other's mitotic functions to coordinately promote mitosis upon completion of interphase events. In addition, inactivation of both kinases by mitotic specific proteolysis is also required for progression through mitosis into G1.

Role of Ca++/calmodulin binding proteins in Aspergillus nidulans cell cycle regulation

The goal of this review is to summarise the current knowledge concerning the targets of Ca++/calmodulin that are essential for cell cycle progression in lower eukaryotes. Emphasis is placed on Aspergillus nidulans since this is the only organism to date shown to posses essential Ca++ dependent calmodulin activated enzymes. Two such enzymes are the calmodulin activated protein phosphatase, calcineurin and the calmodulin dependent protein kinase. These proteins, each the product of a unique gene, are required for progression of quiescent spores into the proliferative cycle and also for execution of the nuclear division cycle in exponentially growing germlings.

The NIMA kinase: a mitotic regulator in Aspergillus nidulans and vertebrate cells

CDC2 has been shown to regulate entry into mitosis in eukaryotic cells. However, in Aspergillus nidulans, activation of CDC2 itself is not sufficient to trigger mitosis if another mitotic protein kinase, NIMA, is not activated. Superficially, NIMA and CDC2 have analogous functions and are regulated in a similar manner. NIMA activity is tightly regulated during the cell cycle. Overexpression of NIMA induces germinal vesicle breakdown in Xenopus oocytes and promotes premature entry into mitosis in all eukaryotic cells examined, whereas dominant-negative mutant NIMA causes a specific G2 arrest in Aspergillus nidulans and human cells, as is the case for CDC2. However, NIMA and CDC2 have quite distinct primary sequence substrate specificities. Furthermore, the regulatory mechanisms that govern the cell cycle-dependent abundance, activity and localization are largely intramolecular for NIMA but intermolecular for CDC2. More importantly, a NIMA-like pathway is also required for the G2/M transition in vertebrate cells. Thus, NIMA may represent a new essential eukaryotic cell cycle regulator, although its homologues in other species are yet to be identified.

Ubiquitin-dependent degradation of cyclin B is accelerated in polyploid megakaryocytes

During the endomitotic cell cycle of megakaryocytic cell lines, the levels of cyclin B1 and the activity of cyclin B1-dependent Cdc2 kinase, although detectable, are reduced as compared with megakaryocytes undergoing a mitotic cell cycle. The levels of cyclin A, however, are comparable during both cell cycles. The expression of cyclin B1 mRNA is also equivalent in proliferating and polyploidizing cells. In the current study, we found that the rate of cyclin B1 protein degradation is enhanced in polyploidizing megakaryocytes. This finding has led us to further investigate whether the ubiquitin-proteosome pathway responsible for cyclin B degradation is accelerated in these cells. Our data indicate that polyploidizing megakaryocytic cell lines nad primary bone marrow cells treated with the megakaryocyte proliferation- and ploidy-promoting factor, the c-Mpl ligand, display increased activities of the ubiquitin-proteosome pathway, which degrades cyclin B, as compared with proliferating megakaryocytic cell lines or diploid bone marrow cells, respectively. This degradation has all the hallmarks of a ubiquitin pathway, including the dependence on ATP, the appearance of high molecular weight conjugated forms of cyclin B, and inhibition of the proteolytic process by a mutated form of the ubiquitin-conjugating enzyme Ubc4. Our studies also indicate that the ability to degrade cyclin A is equivalent in both the mitotic and endomitotic cell cycles. The increased potential of polyploid megakaryocytes to degrade cyclin B may be part of the cellular programming that leads to aborted mitosis.

Structural and functional analysis of the mitotic rotamase Pin1 suggests substrate recognition is phosphorylation dependent

The human rotamase or peptidyl-prolyl cis-trans isomerase Pin1 is a conserved mitotic regulator essential for the G2/M transition of the eukaryotic cell cycle. We report the 1.35 A crystal structure of Pin1 complexed with an AlaPro dipeptide and the initial characterization of Pin1's functional properties. The crystallographic structure as well as pH titration studies and mutagenesis of an active site cysteine suggest a catalytic mechanism that includes general acid-base and covalent catalysis during peptide bond isomerization. Pin1 displays a preference for an acidic residue N-terminal to the isomerized proline bond due to interaction of this acidic side chain with a basic cluster. This raises the possibility of phosphorylation-mediated control of Pin1-substrate interactions in cell cycle regulation.

The NIMA-related kinase 2, Nek2, is expressed in specific stages of the meiotic cell cycle and associates with meiotic chromosomes

The Aspergillus nimA gene encodes a Ser/Thr protein kinase which is required for mitosis, in addition to Cdc2, and which has been suggested to have a role in chromosomal condensation. In this study, we isolated a potential murine homologue of nimA, Nek2, which was shown to be expressed most abundantly in the testis of the adult tissues examined. Its expression in the testis was restricted to the germ cells, with highest levels detected in spermatocytes at pachytene and diplotene stages. Immunohistochemical analysis revealed that Nek2 localized to nuclei, exhibiting a non-uniform distribution within the nucleus. Nek2 appeared to be associated with meiotic chromosomes, an association that was better defined by immunolocalization to hypotonically dispersed meiotic chromosomes. This localization was more apparent in regions of dense chromatin, including the sex vesicle, and was also obvious at some of the chromosome ends. The presence of Nek2 protein was not unique to male germ cells, as it was found in meiotic pachytene stage oocytes as well. Furthermore, in an in vitro experimental setting in which meiotic chromosome condensation was induced with okadaic acid, a concomitant induction of Nek2 kinase activity was observed. The expression of Nek2 in meiotic prophase is consistent with the hypothesis that in vivo, Nek2 is involved in the G2/M phase transition of the cell cycle. Our results further provide evidence that in vivo, mouse Nek2 is involved in events of meiosis, including but not limited to chromosomal condensation.

Expression of a constitutively active Ca2+/calmodulin-dependent kinase in Aspergillus nidulans spores prevents germination and entry into the cell cycle

The unique gene for Ca2+/calmodulin-dependent protein kinase (CaMK) has been shown to be essential in Aspergillus nidulans. Disruption of the gene prevents entry of spores into the nuclear division cycle. Here we show that expression of a constitutively active form of CaMK also prevents spores from entering the first S phase in response to a germinating stimulus. Expression of the constitutively active kinase induces premature activation of NIMEcyclin B/NIMXcdc2 in G0/G1. As NIMXcdc2 is present in spores, the elevation of maturation promotion factor activity may be secondary to the early production of NIMEcyclin B or post-translation modification of maturation promotion factor. The expression of the constitutively active CaMK also results in the appearance of NIMA kinase activity within 1 h of the germinating signal. These results support the contention that the activities of maturation promotion factor and NIMA are coincidentally regulated in A. nidulans and suggest that the unscheduled appearance of one or both of these activities may be sufficient to prevent A. nidulans spores from entering into DNA synthesis.

A human peptidyl-prolyl isomerase essential for regulation of mitosis

The NIMA kinase is essential for progression through mitosis in Aspergillus nidulans, and there is evidence for a similar pathway in other eukaryotic cells. Here we describe the human protein Pin1, a peptidyl-prolyl cis/trans isomerase (PPIase) that interacts with NIMA. PPIases are important in protein folding, assembly and/or transport, but none has so far been shown to be required for cell viability. Pin1 is nuclear PPIase containing a WW protein interaction domain, and is structurally and functionally related to Ess1/Ptf1, an essential protein in budding yeast. PPIase activity is necessary for Ess1/Pin1 function in yeast. Depletion of Pin1/Ess1 from yeast or HeLa cells induces mitotic arrest, whereas HeLa cells overexpressing Pin1 arrest in the G2 phase of the cell cycle. Pin1 is thus an essential PPIase that regulates mitosis presumably by interacting with NIMA and attenuating its mitosis-promoting activity.

Maitotoxin, a calcium channel activator, inhibits cell cycle progression through the G1/S and G2/M transitions and prevents CDC2 kinase activation in GH4C1 cells

Calcium regulates progression through several checkpoints in the cell cycle, including the G1/S-phase transition, G2/M-phase transition, and exit from mitosis. In the GH4C1 rat pituitary cell line, calcium mobilizing polypeptides and calcium channel activation inhibit cell proliferation. This report examines the effects of maitotoxin (MTX), an activator of type L voltage-dependent calcium channels (L-VDCC), on calcium influx and cell cycle progression in GH4C1 cells. MTX causes both a block from G1 to S-phase and a concentration-dependent accumulation of cells in G2+M. MTX does not increase the mitotic index; thus, sustained calcium channel activation by MTX results in an accumulation of cells in G2. In order to temporally localize the MTX-induced G2 block relative to cell cycle regulatory events at the G2/M transition, we assessed the relative activity of two cell cycle regulatory protein kinases, CDC2 and CDK2, in MTX-treated cells. CDC2-specific histone kinase activity in MTX-treated cells is lower than either in cells blocked in mitosis with the microtubule destabilizing agent demecolcine or in randomly cycling cells. In contrast, the activity of CDK2 is highest in MTX-treated cells, consistent with a G2 block prior to CDC2 activation. Together, these results implicate with a G2 block prior to CDC2 activation. Together, these results implicate calcium as an intracellular signal required for progression through G2 phase of the cell cycle prior to CDC2 kinase activation.

PkpA, a novel Phycomyces blakesleeanus serine/threonine protein kinase

This work reports the cloning and sequencing of pkpA, a gene of the filamentous fungus Phycomyces blakesleeanus, whose expression seems to be coupled to vegetative growth. This gene encodes a putative serine/threonine-specific protein kinase, whose sequence is related to that of the yeast protein STE20, involved in pheromone-response pathways, and to a number of MAPK kinase proteins. However, detailed analysis of the kinase sequence suggests that PkpA is a novel serine/threonine protein kinase that probably participates as an intermediate in an intracellular system controlling nuclear proliferation in P. blakesleeanus.

Isolation of a functional homolog of the cell cycle-specific NIMA protein kinase of Aspergillus nidulans and functional analysis of conserved residues

To investigate the degree of conservation of the cell cycle-specific NIMA protein kinase of Aspergillus nidulans, and to help direct its functional analysis, we cloned a homolog (designated nim-1) from Neurospora crassa. Over the catalytic domain NIM-1 is 75% identical to NIMA, but overall the identity drops to 52%. nim-1 was able to functionally complement nimA5 in A. nidulans. Mutational analysis of potential activating phosphorylation sites found in NIMA, NIM-1, and related protein kinases was performed on NIMA. Mutation of threonine 199 (conserved in all NIMA-related kinases) inhibited NIMA beta-casein kinase activity and abolished its in vivo function. This site conforms to a minimal consensus phosphorylation site for NIMA (FXXT) and is analogous to the autophosphorylation site of cyclic-AMP-dependent protein kinases. However, mutation of a unique cysteine residue found only in the catalytic site of NIMA and NIM-1 had no effect on NIMA kinase activity or function. Three temperature-sensitive alleles of nimA that cause arrest in G2 were sequenced and shown to generate three different amino acid substitutions. None of the mutations prevented accumulation of NIMA protein during G2 arrest, but all prevented the p34cdc2/cyclin B-dependent phosphorylation of NIMA normally seen during mitotic initiation even though p34cdc2/cyclin B H1 kinase activity was fully activated.

Substrate specificity and cell cycle regulation of the Nek2 protein kinase, a potential human homolog of the mitotic regulator NIMA of Aspergillus nidulans

The human Nek2 protein kinase is the closest known mammalian relative of the mitotic regulator NIMA of Aspergillus nidulans. The two kinases share 47% sequence identity over their catalytic domains and display a similar cell cycle-dependent expression peaking at the G2 to M phase transition. Hence, it is attractive to speculate that human Nek2 and fungal NIMA may carry out similar functions at the onset of mitosis. To study the biochemical properties and substrate specificity of human Nek2 and compare them to those reported previously for other NIMA-related protein kinases, we have expressed Nek2 in insect cells. We show that recombinant Nek2 is active as a serine/threonine-specific protein kinase and may undergo autophosphorylation. Both human Nek2 and fungal NIMA phosphorylate a similar, albeit not identical, set of proteins and synthetic peptides, and beta-casein was found to be a suitable substrate for assaying Nek2 in vitro. By exploiting these findings, we have studied the cell cycle regulation of Nek2 activity in HeLa cells. We show that Nek2 activity parallels its abundance, being low during M and G1 but high during S and G2 phase. Taken together, our results suggest that human Nek2 resembles fungal NIMA in its primary structure, cell cycle regulation of expression, and substrate specificity, but that Nek2 may function earlier in the cell cycle than NIMA.

A microtubule-associated protein (MAP2) kinase restores microtubule motility in embryonic brain

Motility driven by the microtubule motors, kinesin and cytoplasmic dynein, is inhibited by MAP2 (López, L. A., and Sheetz, M. P. (1993) Cell Motil. Cytoskeleton 24, 1-16). The MAP2 inhibition is reversed by a kinase that is co-purified with chicken embryonic MAP2, completely releasing MAP2 from the microtubules. We have identified this activity with a kinase, embryonic MAP2 kinase (M(r) = 100,000), which phosphorylates MAP2 at serine amino acid residues. This kinase is c-AMP independent and inhibited by potassium fluoride and glycerol 2-phosphate. Only the phosphorylation produced by embryonic MAP2 kinase can change the affinity of MAP2 by microtubules. Bovine MAP2 kinase, Cdc2 kinase, mitogenic activated protein kinase, and the NIMA kinase are able to phosphorylate MAP2 but do not change the affinity for microtubules. In vivo, embryonic MAP2 kinase could play a major role in the regulation of motility and positioning of membranous organelles within the cells even at substoichiometric levels.

Evidence for a NIMA-like mitotic pathway in vertebrate cells

NIMA is essential for entry into mitosis in Aspergillus nidulans. To examine whether there is a NIMA-like pathway in other eukaryotic cell cycles, we expressed NIMA and its dominant negative mutants in two different eukaryotic systems. In Xenopus oocytes, NIMA induced germinal vesicle breakdown without activating Mos, CDC2, or MAP kinase. In HeLa cells, NIMA induced premature mitotic events without activating CDC2, whereas the mutants caused a specific G2 arrest but did not block mutant CDC2T14AY15F-induced premature entry into mitosis. A sequence essential for both these phenotypes was mapped to a region of approximately 100 amino acids lying just after the catalytic domain of NIMA that shows a significant similarity to protein interaction domains in other proteins. These results provide evidence for the existence of a NIMA-like mitotic pathway in vertebrate cells.

Calmodulin-dependent protein kinase II is required for G1/S progression in HeLa cells

Calmodulin (CaM) has been previously shown to be essential for cell cycle progression in eukaryotic cells, being required at the G1/S, G2/M, and metaphase-anaphase transitions. Little is known about the the specific CaM-dependent enzymes that mediate Ca2+/CaM signaling to affect cell proliferation. In this study we show that inhibition of calmodulin kinase II (CaMKII) in HeLa cells using the CaMKII inhibitor KN-93 causes cell cycle arrest, demonstrating that CaMKII is required for cell cycle progression. Detailed analysis of arrest cells suggests that CaMKII is required for the initiation of DNA synthesis. Cells treated with KN-93 arrest with a G1 DNA content, but with elevated cyclin-dependent histone H1 kinase activity, suggesting that CaMKII may act at a point very close to the onset of DNA synthesis in mammalian cells.

Calcium, calmodulin and cell cycle regulation

Calcium and its ubiquitous intracellular receptor calmodulin are required for cell proliferation. Studies in a variety of model systems are beginning to identify components of the calcium/calmodulin cascade required for movement of quiescent cells into the cell cycle as well as for proliferating cells to move from G1 to S, G2 to M and through mitosis. Two calcium/calmodulin-dependent enzymes, the multifunctional calcium/calmodulin-dependent protein kinase and the protein phosphatase 2B (calcineurin) as well as a spindle pole body protein that binds calmodulin in the absence of calcium have been shown to be essential at specific phases of the cell cycle. In addition, the status of the intracellular calcium pools is critical for normal traverse of the cell cycle.