Spk1, a new kinase from Saccharomyces cerevisiae, phosphorylates proteins on serine, threonine, and tyrosine

The DNA damage checkpoint: A tale from budding yeast

Studies performed in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe have led the way in defining the DNA damage checkpoint and in identifying most of the proteins involved in this regulatory network, which turned out to have structural and functional equivalents in humans. Subsequent experiments revealed that the checkpoint is an elaborate signal transduction pathway that has the ability to sense and signal the presence of damaged DNA and transduce this information to influence a multifaceted cellular response that is essential for cancer avoidance. This review focuses on the work that was done in Saccharomyces cerevisiae to articulate the checkpoint concept, to identify its players and the mechanisms of activation and deactivation.

Mass Spectrometry-Based Identification of Phospho-Tyr in Plant Proteomics

O-Phosphorylation (phosphorylation of the hydroxyl-group of S, T, and Y residues) is among the first described and most thoroughly studied posttranslational modification (PTM). Y-Phosphorylation, catalyzed by Y-kinases, is a key step in both signal transduction and regulation of enzymatic activity in mammalian systems. Canonical Y-kinase sequences are absent from plant genomes/kinomes, often leading to the assumption that plant cells lack O-phospho-l-tyrosine (pY). However, recent improvements in sample preparation, coupled with advances in instrument sensitivity and accessibility, have led to results that unequivocally disproved this assumption. Identification of hundreds of pY-peptides/proteins, followed by validation using genomic, molecular, and biochemical approaches, implies previously unappreciated roles for this "animal PTM" in plants. Herein, we review extant results from studies of pY in plants and propose a strategy for preparation and analysis of pY-peptides that will allow a depth of coverage of the plant pY-proteome comparable to that achieved in mammalian systems.

From yeast to humans: Understanding the biology of DNA Damage Response (DDR) kinases

The DNA Damage Response (DDR) is a complex network of biological processes that protect cells from accumulating aberrant DNA structures, thereby maintaining genomic stability and, as a consequence, preventing the development of cancer and other diseases. The DDR pathway is coordinated by a signaling cascade mediated by the PI3K-like kinases (PIKK) ATM and ATR and by their downstream kinases CHK2 and CHK1, respectively. Together, these kinases regulate several aspects of the cellular program in response to genomic stress. Much of our understanding of these kinases came from studies performed in the 1990s using yeast as a model organism. The purpose of this review is to present a historical perspective on the discovery of the DDR kinases in yeast and the importance of this model for the identification and functional understanding of their mammalian orthologues.

Advances in kinome research of parasitic worms - implications for fundamental research and applied biotechnological outcomes

Protein kinases are enzymes that play essential roles in the regulation of many cellular processes. Despite expansions in the fields of genomics, transcriptomics and bioinformatics, there is limited information on the kinase complements (kinomes) of most eukaryotic organisms, including parasitic worms that cause serious diseases of humans and animals. The biological uniqueness of these worms and the draft status of their genomes pose challenges for the identification and classification of protein kinases using established tools. In this article, we provide an account of kinase biology, the roles of kinases in diseases and their importance as drug targets, and drug discovery efforts in key socioeconomically important parasitic worms. In this context, we summarise methods and resources commonly used for the curation, identification, classification and functional annotation of protein kinase sequences from draft genomes; review recent advances made in the characterisation of the worm kinomes; and discuss the implications of these advances for investigating kinase signalling and developing small-molecule inhibitors as new anti-parasitic drugs.

The Pivotal Role of Protein Phosphorylation in the Control of Yeast Central Metabolism

Protein phosphorylation is the most frequent eukaryotic post-translational modification and can act as either a molecular switch or rheostat for protein functions. The deliberate manipulation of protein phosphorylation has great potential for regulating specific protein functions with surgical precision, rather than the gross effects gained by the over/underexpression or complete deletion of a protein-encoding gene. In order to assess the impact of phosphorylation on central metabolism, and thus its potential for biotechnological and medical exploitation, a compendium of highly confident protein phosphorylation sites (p-sites) for the model organism Saccharomyces cerevisiae has been analyzed together with two more datasets from the fungal pathogen Candida albicans. Our analysis highlights the global properties of the regulation of yeast central metabolism by protein phosphorylation, where almost half of the enzymes involved are subject to this sort of post-translational modification. These phosphorylated enzymes, compared to the nonphosphorylated ones, are more abundant, regulate more reactions, have more protein–protein interactions, and a higher fraction of them are ubiquitinated. The p-sites of metabolic enzymes are also more conserved than the background p-sites, and hundreds of them have the potential for regulating metabolite production. All this integrated information has allowed us to prioritize thousands of p-sites in terms of their potential phenotypic impact. This multi-source compendium should enable the design of future high-throughput (HTP) mutation studies to identify key molecular switches/rheostats for the manipulation of not only the metabolism of yeast, but also that of many other biotechnologically and medically important fungi and eukaryotes.

Cytokine-dependent activation of JAK-STAT pathway in Saccharomyces cerevisiae

Protein phosphorylation is an important post-translational modification for intracellular signaling molecules, mostly found in serine and threonine residues. Tyrosine phosphorylations are very few events (less than 0.1% to phosphorylated serine/threonine residues), but capable of governing cell fate decisions involved in proliferation, differentiation, apoptosis, and oncogenic transformation. Hence, it is important for drug discovery and system biology to measure the intracellular level of phosphotyrosine. Although mammalian cells have been conventionally utilized for this purpose, accurate determination of phosphotyrosine level often suffers from high background due to the unexpected crosstalk among endogenous signaling molecules. This situation led us firstly to establish the ligand-induced activation of homomeric receptor tyrosine kinase (i.e., epidermal growth factor receptor) in Saccharomyces cerevisiae, a lower eukaryote possessing organelles similar to higher eukaryote but not showing substantial level of tyrosine kinase activity. In this study, we expressed heteromeric receptor tyrosine kinase (i.e., a complex of interleukin-5 receptor (IL5R) α chain, common β chain, and JAK2 tyrosine kinase) in yeast. When coexpressed with a cell wall-anchored form of IL5, the yeast exerted the autophosphorylation of JAK2, followed by the phosphorylation of transcription factor STAT5a and subsequent nuclear accumulation of phosphorylated STAT5a. Taken together, yeast could be an ideal host for sensitive detection of phosphotyrosine generated by a wide variety of tyrosine kinases. Biotechnol. Bioeng. 2016;113: 1796-1804. © 2016 Wiley Periodicals, Inc.

Tyrosine Phosphorylation Based Homo-dimerization of Arabidopsis RACK1A Proteins Regulates Oxidative Stress Signaling Pathways in Yeast

Scaffold proteins are known as important cellular regulators that can interact with multiple proteins to modulate diverse signal transduction pathways. RACK1 (Receptor for Activated C Kinase 1) is a WD-40 type scaffold protein, conserved in eukaryotes, from Chlamydymonas to plants and humans, plays regulatory roles in diverse signal transduction and stress response pathways. RACK1 in humans has been implicated in myriads of neuropathological diseases including Alzheimer and alcohol addictions. Model plant Arabidopsis thaliana genome maintains three different RACK1 genes termed RACK1A, RACK1B, and RACK1C with a very high (85-93%) sequence identity among them. Loss of function mutation in Arabidopsis indicates that RACK1 proteins regulate diverse environmental stress signaling pathways including drought and salt stress resistance pathway. Recently deduced crystal structure of Arabidopsis RACK1A- very first among all of the RACK1 proteins, indicates that it can potentially be regulated by post-translational modifications, like tyrosine phosphorylations and sumoylation at key residues. Here we show evidence that RACK1A proteins, depending on diverse environmental stresses, are tyrosine phosphorylated. Utilizing site-directed mutagenesis of key tyrosine residues, it is found that tyrosine phosphorylation can potentially dictate the homo-dimerization of RACK1A proteins. The homo-dimerized RACK1A proteins play a role in providing UV-B induced oxidative stress resistance. It is proposed that RACK1A proteins ability to function as scaffold protein may potentially be regulated by the homo-dimerized RACK1A proteins to mediate diverse stress signaling pathways.

Structural basis of Rad53 kinase activation by dimerization and activation segment exchange

The protein kinase Rad53 is a key regulator of the DNA damage checkpoint in budding yeast. Its human ortholog, CHEK2, is mutated in familial breast cancer and mediates apoptosis in response to genotoxic stress. Autophosphorylation of Rad53 at residue Thr354 located in the kinase activation segment is essential for Rad53 activation. In this study, we assessed the requirement of kinase domain dimerization and the exchange of its activation segment during the Rad53 activation process. We solved the crystal structure of Rad53 in its dimeric form and found that disruption of the observed head-to-tail, face-to-face dimer structure decreased Rad53 autophosphorylation on Thr354 in vitro and impaired Rad53 function in vivo. Moreover, we provide critical functional evidence that Rad53 trans-autophosphorylation may involve the interkinase domain exchange of helix αEF via an invariant salt bridge. These findings suggest a mechanism of autophosphorylation that may be broadly applicable to other protein kinases.

Patterns of structural dynamics in RACK1 protein retained throughout evolution: a hydrogen-deuterium exchange study of three orthologs

RACK1 is a member of the WD repeat family of proteins and is involved in multiple fundamental cellular processes. An intriguing feature of RACK1 is its ability to interact with at least 80 different protein partners. Thus, the structural features enabling such interactomic flexibility are of great interest. Several previous studies of the crystal structures of RACK1 orthologs described its detailed architecture and confirmed predictions that RACK1 adopts a seven-bladed β-propeller fold. However, this did not explain its ability to bind to multiple partners. We performed hydrogen-deuterium (H-D) exchange mass spectrometry on three orthologs of RACK1 (human, yeast, and plant) to obtain insights into the dynamic properties of RACK1 in solution. All three variants retained similar patterns of deuterium uptake, with some pronounced differences that can be attributed to RACK1's divergent biological functions. In all cases, the most rigid structural elements were confined to B-C turns and, to some extent, strands B and C, while the remaining regions retained much flexibility. We also compared the average rate constants for H-D exchange in different regions of RACK1 and found that amide protons in some regions exchanged at least 1000-fold faster than in others. We conclude that its evolutionarily retained structural architecture might have allowed RACK1 to accommodate multiple molecular partners. This was exemplified by our additional analysis of yeast RACK1 dimer, which showed stabilization, as well as destabilization, of several interface regions upon dimer formation.

Histone tyrosine phosphorylation comes of age

Histones were discovered over a century ago and have since been found to be the most extensively posttranslationally modified proteins, although tyrosine phosphorylation of histones had remained elusive until recently. The year 2009 proved to be a landmark year for histone tyrosine (Y) phosphorylation as five research groups independently discovered this modification. Three groups describe phosphorylation of Y142 in the variant histone H2A.X, where it may be involved in the cellular decision making process to either undergo DNA repair or apoptosis in response to DNA damage. Further, one group suggests that phosphorylation of histone H3 on Y99 is crucial for its regulated proteolysis in yeast, while another found that Y41 phosphorylation modulates chromatin architecture and oncogenesis in mammalian cells. These pioneering studies provide the initial conceptual framework for further analyses of the diverse roles of tyrosine phosphorylation on different histones, with far reaching implications for human health and disease.

Histone levels are regulated by phosphorylation and ubiquitylation-dependent proteolysis

Histone levels are tightly regulated to prevent harmful effects such as genomic instability and hypersensitivity to DNA damaging agents due to the accumulation of these highly basic proteins when DNA replication slows down or stops. Although chromosomal histones are stable, excess (non-chromatin bound) histones are rapidly degraded in a Rad53 kinase dependent manner in Saccharomyces cerevisiae. Here we demonstrate that excess histones associate with Rad53 in vivo, appear to undergo modifications such as tyrosine phosphorylation and polyubiquitylation, before their proteolysis by the proteasome. We have identified the tyrosine 99 residue of histone H3 as being critical for the efficient ubiquitylation and degradation of this histone. We have also identified the E2 proteins Ubc4 and Ubc5, as well as the E3 ubiquitin ligase Tom1, as enzymes involved in the ubiquitylation of excess histones. Regulated histone proteolysis has major implications for the maintenance of epigenetic marks on chromatin, genomic stability and the packaging of sperm DNA.

Damped-dynamics flexible fitting

In fitting atomic structures into EM maps, it often happens that the map corresponds to a different conformation of the structure. We have developed a new methodology to handle these situations that preserves the covalent geometry of the structure and allows the modeling of large deformations. The first goal is achieved by working in generalized coordinates (positional and internal coordinates), and the second by avoiding harmonic potentials. Instead, we use dampers (shock absorbers) between every pair of atoms, combined with a force field that attracts the atomic structure toward incompletely occupied regions of the EM map. The trajectory obtained by integrating the resulting equations of motion converges to a conformation that, in our validation cases, was very close to the target atomic structure. Compared to current methods, our approach is more efficient and robust against wrong solutions and to overfitting, and does not require user intervention or subjective decisions. Applications to the computation of transition pathways between known conformers, homology and loop modeling, as well as protein docking, are also discussed.

Growth factor-stimulated phosphorylation cascades: activation of growth factor-stimulated MAP kinase

Protein phosphorylation is an important mechanism in the response of cells to growth factors by which signals can be conveyed from cell surface receptors to intracellular targets. In addition to stimulation of protein tyrosine phosphorylation, activation of growth factor receptors having protein tyrosine kinase activity leads to dramatic alterations in the levels of protein serine/threonine phosphorylation. Several growth factor-stimulated serine/threonine-specific kinases have been identified as potential mediators of such signalling. MAP (microtubule-associated protein) kinase has emerged as a very interesting member of this group, because it activates a separate kinase, pp90rsk, which is also growth factor-stimulated. MAP kinase itself appears to be regulated by protein phosphorylation, because it can be inactivated by protein phosphatases. We have identified two 60 kDa proteins that promote the phosphorylation and full activation of MAP kinase in a manner paralleling its activation by growth factors in intact cells. These 'MAP kinase activators' are themselves stimulated by growth factors, suggesting that they function as intermediates between the MAP kinase and cell surface receptors in a growth factor-stimulated kinase cascade. Identification of the components of this protein kinase cascade reveals a mechanism by which at least some of the effects of receptor tyrosine kinases can be mediated through serine/threonine phosphorylation.

Evidence for a minimal eukaryotic phosphoproteome?

BACKGROUND

Reversible phosphorylation catalysed by kinases is probably the most important regulatory mechanism in eukaryotes.

METHODOLOGY/PRINCIPAL FINDINGS

We studied the in vitro phosphorylation of peptide arrays exhibiting the majority of PhosphoBase-deposited protein sequences, by factors in cell lysates from representatives of various branches of the eukaryotic species. We derived a set of substrates from the PhosphoBase whose phosphorylation by cellular extracts is common to the divergent members of different kingdoms and thus may be considered a minimal eukaryotic phosphoproteome. The protein kinases (or kinome) responsible for phosphorylation of these substrates are involved in a variety of processes such as transcription, translation, and cytoskeletal reorganisation.

CONCLUSIONS/SIGNIFICANCE

These results indicate that the divergence in eukaryotic kinases is not reflected at the level of substrate phosphorylation, revealing the presence of a limited common substrate space for kinases in eukaryotes and suggests the presence of a set of kinase substrates and regulatory mechanisms in an ancestral eukaryote that has since remained constant in eukaryotic life.

GINS Is a DNA Polymerase ϵ Accessory Factor during Chromosomal DNA Replication in Budding Yeast

GINS is a protein complex found in eukaryotic cells that is composed of Sld5p, Psf1p, Psf2p, and Psf3p. GINS polypeptides are highly conserved in eukaryotes, and the GINS complex is required for chromosomal DNA replication in yeasts and Xenopus egg. This study reports purification and biochemical characterization of GINS from Saccharomyces cerevisiae. The results presented here demonstrate that GINS forms a 1:1 complex with DNA polymerase epsilon (Pol epsilon) holoenzyme and greatly stimulates its catalytic activity in vitro. In the presence of GINS, Pol epsilon is more processive and dissociates more readily from replicated DNA, while under identical conditions, proliferating cell nuclear antigen slightly stimulates Pol epsilon in vitro. These results strongly suggest that GINS is a Pol epsilon accessory protein during chromosomal DNA replication in budding yeast. Based on these results, we propose a model for molecular dynamics at eukaryotic chromosomal replication fork.

Protein kinases associated with the yeast phosphoproteome

Background

Protein phosphorylation is an extremely important mechanism of cellular regulation. A large-scale study of phosphoproteins in a whole-cell lysate of Saccharomyces cerevisiae has previously identified 383 phosphorylation sites in 216 peptide sequences. However, the protein kinases responsible for the phosphorylation of the identified proteins have not previously been assigned.

Results

We used Predikin in combination with other bioinformatic tools, to predict which of 116 unique protein kinases in yeast phosphorylates each experimentally determined site in the phosphoproteome. The prediction was based on the match between the phosphorylated 7-residue sequence and the predicted substrate specificity of each kinase, with the highest weight applied to the residues or positions that contribute most to the substrate specificity. We estimated the reliability of the predictions by performing a parallel prediction on phosphopeptides for which the kinase has been experimentally determined.

Conclusion

The results reveal that the functions of the protein kinases and their predicted phosphoprotein substrates are often correlated, for example in endocytosis, cytokinesis, transcription, replication, carbohydrate metabolism and stress response. The predictions link phosphoproteins of unknown function with protein kinases with known functions and vice versa, suggesting functions for the uncharacterized proteins. The study indicates that the phosphoproteins and the associated protein kinases represented in our dataset have housekeeping cellular roles; certain kinases are not represented because they may only be activated during specific cellular responses. Our results demonstrate the utility of our previously reported protein kinase substrate prediction approach (Predikin) as a tool for establishing links between kinases and phosphoproteins that can subsequently be tested experimentally.

Signal Transduction: How Rad53 Kinase Is Activated

Recent studies have elucidated the activation mechanism of the Rad53 checkpoint kinase and the role of Rad9-like adaptor proteins in mediating signal transduction from PIKK sensor kinases that detect DNA damage to the effector kinases that play a part in mending that damage.

Comparative proteome bioinformatics: identification of a whole complement of putative protein tyrosine kinases in the model flowering plant Arabidopsis thaliana

Phosphorylation by protein tyrosine kinases is crucial to the control of growth and development of multicellular eukaryotes, including humans, and it also seems to play an important role in multicellular prokaryotes. A plant tyrosine-specific kinase has not been identified yet; hence, plants have been suggested to share with unicellular eukaryote yeast a tyrosine phosphorylation system where a limited number of stress proteins are tyrosyl-phosphorylated only by a few dual-specificity (serine/threonine and tyrosine) kinases. However, preliminary evidence obtained so far suggests that tyrosine phosphorylation in plants depends on the developmental conditions. Since sequencing of the genome of the model flowering plant Arabidopsis thaliana has been recently completed, we have performed a bioinformatic screening of the whole Arabidopsis proteome to identify a model complement of bona fide protein tyrosine kinases. In silico analyses suggest that < 4% of Arabidopsis kinases are tyrosine-specific kinases, whose gene expression has been assessed by a preliminary polymerase chain reaction screening of an Arabidopsis cDNA library. Finally, immunological evidence confirms that the number of Arabidopsis proteins specifically phosphorylated on tyrosine residues is much higher than in yeast.

Developmentally regulated dual-specificity kinase from peanut that is induced by abiotic stresses

Tyrosine (Tyr) phosphorylation represents an important biochemical mechanism to regulate many cellular processes. No Tyr kinase has been cloned so far in plants. Dual-specificity kinases are reported in plants and the function of these kinases remains unknown. A 1.7-kb cDNA that encodes serine/threonine/Tyr (STY) kinase was isolated by screening peanut (Arachis hypogaea) expression library using the anti-phospho-Tyr antibody. The histidine-tagged recombinant kinase histidine-6-STY predominantly autophosphorylated on Tyr and phosphorylated the histone primarily on threonine. Genomic DNA gel-blot analysis revealed that STY kinase is a member of a small multigene family. The transcript of STY kinase is accumulated in the mid-maturation stage of seed development, suggesting a role in the signaling of storage of seed reserves. The STY kinase mRNA expression, as well as kinase activity, markedly increased in response to cold and salt treatments; however, no change in the protein level was observed, suggesting a posttranslational activation mechanism. The activation of the STY kinase is detected after 12 to 48 h of cold and salt treatments, which indicates that the kinase may not participate in the initial response to abiotic stresses, but may play a possible role in the adaptive process to adverse conditions. The transcript levels and kinase activity were unaltered with abscisic acid treatment, suggesting an abscisic acid-independent cold and salt signaling pathway. Here, we report the first identification of a non-MAP kinase cascade dual-specificity kinase involved in abiotic stress and seed development.

Physical interactions of Dmnk with Orb: implications in the regulated localization of Orb by Dmnk during oogenesis and embryogenesis

The Dmnk (Drosophila maternal nuclear kinase) gene, encoding a nuclear protein serine/threonine kinase, is expressed predominantly in the germline cells during embryogenesis, suggesting its possible role in the establishment of germ cells. We report here that Dmnk interacts physically with Drosophila RNA binding protein Orb, which plays crucial roles in the establishment of Drosophila oocyte by regulating the distribution and translation of several maternal mRNAs. Considering similar spatiotemporal expression pattern of Dmnk and orb during oogenesis and early embryogenesis, it is suggested that Dmnk plays a role in establishment of germ cells by interacting with Orb. Although there are two forms of Dmnk proteins, Dmnk-L (long) and Dmnk-S (short) via the developmentally regulated alternative splicing, Orb can associate with both forms of Dmnk proteins when expressed in culture cells. However, immunohistochemical analysis revealed that Dmnk-S, but not Dmnk-L, can affect the subcellular localization of Orb in a kinase activity-dependent manner, suggesting differential functions of Dmnk-S and Dmnk-L in the regulation of Orb.

Regulation of chromosome replication

The initiation of DNA replication in eukaryotic cells is tightly controlled to ensure that the genome is faithfully duplicated once each cell cycle. Genetic and biochemical studies in several model systems indicate that initiation is mediated by a common set of proteins, present in all eukaryotic species, and that the activities of these proteins are regulated during the cell cycle by specific protein kinases. Here we review the properties of the initiation proteins, their interactions with each other, and with origins of DNA replication. We also describe recent advances in understanding how the regulatory protein kinases control the progress of the initiation reaction. Finally, we describe the checkpoint mechanisms that function to preserve the integrity of the genome when the normal course of genome duplication is perturbed by factors that damage the DNA or inhibit DNA synthesis.

Using a phage display library to identify basic residues in A-Raf required to mediate binding to the Src homology 2 domains of the p85 subunit of phosphatidylinositol 3'-kinase

Src homology 2 (SH2) domains are found in a variety of cytoplasmic proteins involved in mediating signals from cell surface receptors to various intracellular pathways. They fold as modular units and are capable of recognizing and binding to short linear peptide sequences containing a phosphorylated tyrosine residue. Here we show that each of the SH2 domains of the p85 subunit of phosphatidylinositol 3-kinase selects phage displayed peptide sequences containing the core (L/I)-A-(R/K)-I-R. The serine/threonine kinase A-Raf, containing the sequence LQRIRS, is associated with the p85 protein in both quiescent and growth factor stimulated cells. This suggests that p85 and A-Raf exist in a protein complex in cells and that complex formation does not require growth factor stimulation. We also show that p85 and A-Raf can bind directly to each other in vitro and that this interaction is mediated in part by the p85 SH2 domains. Further, the p85 SH2 domains require at least one of four distinct basic-X-basic sequence motifs within A-Raf for binding. This is the first description of a phosphotyrosine-independent SH2 domain interaction that requires basic residues on the SH2 ligand.

Analysis of yeast protein kinases using protein chips

We have developed a novel protein chip technology that allows the high-throughput analysis of biochemical activities, and used this approach to analyse nearly all of the protein kinases from Saccharomyces cerevisiae. Protein chips are disposable arrays of microwells in silicone elastomer sheets placed on top of microscope slides. The high density and small size of the wells allows for high-throughput batch processing and simultaneous analysis of many individual samples. Only small amounts of protein are required. Of 122 known and predicted yeast protein kinases, 119 were overexpressed and analysed using 17 different substrates and protein chips. We found many novel activities and that a large number of protein kinases are capable of phosphorylating tyrosine. The tyrosine phosphorylating enzymes often share common amino acid residues that lie near the catalytic region. Thus, our study identified a number of novel features of protein kinases and demonstrates that protein chip technology is useful for high-throughput screening of protein biochemical activity.

Tyrosine versus serine/threonine phosphorylation by protein kinase casein kinase-2. A study with peptide substrates derived from immunophilin Fpr3

Protein kinase casein kinase-2 (CK2) is a spontaneously active, ubiquitous, and pleiotropic enzyme that phosphorylates seryl/threonyl residues specified by multiple negatively charged side chains, the one at position n + 3 being of crucial importance (minimum consensus S/T-x-x-E/D/S(P)/T(P). Recently CK2 has been reported to catalyze phosphorylation of the yeast nucleolar immunophilin Fpr3 at a tyrosyl residue (Tyr(184)) fulfilling the consensus sequence of Ser/Thr substrates (Wilson, L.K., Dhillon, N., Thorner, J., and Martin, G.S. (1997) J. Biol. Chem. 272, 12961-12967). Here we show that, by contrast to other tyrosyl peptides fulfilling the consensus sequence for CK2, a peptide reproducing the sequence around Fpr3 Tyr(184) (DEDADIY(184)DEEDYDL) is phosphorylated by CK2, albeit with much higher K(m) (384 versus 4. 3 microM) and lower V(max) (8.4 versus 1,132 nmol.min(-1).mg(-1)) than its derivative with Tyr(184) replaced by serine. The replacement of Asp at position n + 1 with alanine and, to a lesser extent, of Ile at n - 1 with Asp are especially detrimental to tyrosine phosphorylation as compared with serine phosphorylation, which is actually stimulated by the Ile to Asp modification. In contrast the replacement of Glu at n + 3 with alanine almost suppresses serine phosphorylation but not tyrosine phosphorylation. It can be concluded that CK2 is capable to phosphorylate, under special circumstances, tyrosyl residues, which are specified by structural features partially different from those that optimize Ser/Thr phosphorylation.

Caffeine can override the S-M checkpoint in fission yeast

The replication checkpoint (or ‘S-M checkpoint’) control prevents progression into mitosis when DNA replication is incomplete. Caffeine has been known for some time to have the capacity to override the S-M checkpoint in animal cells. We show here that caffeine also disrupts the S-M checkpoint in the fission yeast Schizosaccharomyces pombe. By contrast, no comparable effects of caffeine on the S. pombe DNA damage checkpoint were seen. S. pombe cells arrested in early S phase and then exposed to caffeine lost viability rapidly as they attempted to enter mitosis, which was accompanied by tyrosine dephosphorylation of Cdc2. Despite this, the caffeine-induced loss of viability was not blocked in a temperature-sensitive cdc2 mutant incubated at the restrictive temperature, although catastrophic mitosis was prevented under these conditions. This suggests that, in addition to S-M checkpoint control, a caffeine-sensitive function may be important for maintenance of cell viability during S phase arrest. The lethality of a combination of caffeine with the DNA replication inhibitor hydroxyurea was suppressed by overexpression of Cds1 or Chk1, protein kinases previously implicated in S-M checkpoint control and recovery from S phase arrest. In addition, the same combination of drugs was specifically tolerated in cells overexpressing either of two novel S. pombe genes isolated in a cDNA library screen. These findings should allow further molecular investigation of the regulation of S phase arrest, and may provide a useful system with which to identify novel drugs that specifically abrogate the checkpoint control.

hRAD17, a structural homolog of the Schizosaccharomyces pombe RAD17 cell cycle checkpoint gene, stimulates p53 accumulation

The RAD17 gene product of S. Pombe is an essential component of the checkpoint control pathway which responds to both DNA damage and disruption of replication. We have identified a human cDNA that encodes a polypeptide which is structurally conserved with the S. Pombe Rad17 protein. The human gene, designated hRAD17, predicts an encoded protein of 590 amino acids and a molecular weight of 69 kD. Amino acid sequence alignment revealed that hRadl7 has 28.3% and 52.5% similarity with the S. Pombe Rad17 protein, and 21.8% identity and 45.8% similarity to the budding yeast cell cycle checkpoint protein, Rad 24. When introduced into the S. Pombe rad17 mutant, hRAD17 was able to partially revert its hydroxyurea and ionizing radiation hypersensitivity, but not its UV hypersensitivity. Permanent overexpression of the hRAD17 gene in human fibrosarcoma cells resulted in p53 activation and a significant reduction of S- and G2/M-phase cells accompanied by an accumulation of the G1-phase population, suggesting that hRAD17 may have a role in cell cycle checkpoint control. Immunostaining of HT-1080 cells transiently transfected with a hRAD17 construct confirmed the nuclear accumulation of p53, which mimics the induction caused by DNA damage. Using FISH analysis, we have mapped the hRAD17 locus to human chromosome 5q11.2.

RAD53 regulates DBF4 independently of checkpoint function in Saccharomyces cerevisiae

The Cdc7p and Dbf4p proteins form an active kinase complex in Saccharomyces cerevisiae that is essential for the initiation of DNA replication. A genetic screen for mutations that are lethal in combination with cdc7-1 led to the isolation of seven lsd (lethal with seven defect) complementation groups. The lsd7 complementation group contained two temperature-sensitive dbf4 alleles. The lsd1 complementation group contained a new allele of RAD53, which was designated rad53-31. RAD53 encodes an essential protein kinase that is required for the activation of DNA damage and DNA replication checkpoint pathways, and that is implicated as a positive regulator of S phase. Unlike other RAD53 alleles, we demonstrate that the rad53-31 allele retains an intact checkpoint function. Thus, the checkpoint function and the DNA replication function of RAD53 can be functionally separated. The activation of DNA replication through RAD53 most likely occurs through DBF4. Two-hybrid analysis indicates that the Rad53p protein binds to Dbf4p. Furthermore, the steady-state level of DBF4 message and Dbf4p protein is reduced in several rad53 mutant strains, indicating that RAD53 positively regulates DBF4. These results suggest that two different functions of the cell cycle, initiation of DNA replication and the checkpoint function, can be coordinately regulated through the common intermediate RAD53.

p53 inhibits entry into mitosis when DNA synthesis is blocked

Human and mouse fibroblasts with normal p53 fail to enter mitosis when DNA synthesis is blocked by aphidicolin or hydroxyurea. Isogenic p53-null fibroblasts do enter mitosis with incompletely replicated DNA, revealing that p53 contributes to a checkpoint that ensures that mitosis does not occur until DNA synthesis is complete. When treated with N-(phosphonacetyl)-L-aspartate (PALA), which inhibits pyrimidine nucleotide synthesis, leading to synthesis of damaged DNA from highly unbalanced dNTP pools, p53-null cells enter mitosis after they have completed DNA replication, but cells with wild-type p53 do not, revealing that p53 also mediates a checkpoint that monitors the quality of newly replicated DNA.

Tyrosine phosphorylation regulates cell cycle-dependent nuclear localization of Cdc48p

Cdc48p from Saccharomyces cerevisiae and its highly conserved mammalian homologue VCP (valosin-containing protein) are ATPases with essential functions in cell division and homotypic fusion of endoplasmic reticulum vesicles. Both are mainly attached to the endoplasmic reticulum, but relocalize in a cell cycle-dependent manner: Cdc48p enters the nucleus during late G1; VCP aggregates at the centrosome during mitosis. The nuclear import signal sequence of Cdc48p was localized near the amino terminus and its function demonstrated by mutagenesis. The nuclear import is regulated by a cell cycle-dependent phosphorylation of a tyrosine residue near the carboxy terminus. Two-hybrid studies indicate that the phosphorylation results in a conformational change of the protein, exposing the nuclear import signal sequence previously masked by a stretch of acidic residues.

Characterization of a 54-kilodalton human protein kinase recognized by an antiserum raised against the myotonin kinase

We characterized a 54-kDa human protein kinase recognized by an antiserum raised against the human myotonin protein kinase. This protein kinase displays a serine/threonine kinase activity in the heart and a tyrosine kinase activity in the skeletal muscle. Both kinase activities were attributed to the same 54-kDa protein based on the identity of one-dimensional peptide maps. We showed that the tyrosine kinase activity observed in the skeletal muscle results from a phosphorylation of this protein kinase on tyrosine residues by a tyrosine kinase specifically expressed in this tissue. The tyrosine dephosphorylation of the skeletal muscle 54-kDa protein kinase allowed it to phosphorylate with the highest activity the same peptide substrates as those phosphorylated by the human recombinant myotonin kinase. These results show that a muscle-specific tyrosine phosphorylation event converts a serine/threonine kinase to a tyrosine kinase. They also suggest that the 54-kDa protein kinase is a member of the myotonin kinase family.

Block of the cell cycle of the yeast Saccharomyces cerevisiae by tyrphostin, an inhibitor of protein tyrosine kinase

Tyrphostins are inhibitors of the epidermal growth factor receptor tyrosine kinase. To elucidate the biological function of protein tyrosine kinases in yeast cells, a mutant hypersensitive to tyrphostin was isolated and investigated for its response to the drug. The mutation was recessive and was designated tpt1 for tyrphostin hypersensitive. A tpt1 strain cannot grow in the presence of tyrphostin, implying that a biological process sensitive to tyrphostin is essential for cell growth. Microscopic observation indicated that large-budded cells were accumulated in the presence of the inhibitor. The results suggest the involvement of protein tyrosine phosphorylation in the cell cycle progression of Saccharomyces cerevisiae.

Characterization of the checkpoint gene RAD53/MEC2 in Saccharomyces cerevisiae

Saccharomyces cerevisiae cells carrying mutations in RAD53/MEC2 fail to arrest in the S phase when DNA replication is blocked (the S/M checkpoint) or in the G2 phase when DNA is damaged (the G2/M checkpoint). We isolated and determined the DNA sequence of RAD53 and found that it is identical to the SPK1 gene previously identified by Stern et al. (1991). In addition to its checkpoint functions, we show here that RAD53 is essential for cell viability because null mutants are inviable. Weak genomic suppressors of the essential function do arise frequently, though they do not suppress the checkpoint defects of the null mutant. This genetically separates the essential and checkpoint functions. We show genetically that the protein kinase domain is essential for all RAD53-dependent functions tested because a site-specific mutation that inactivates the protein kinase activity results in a mutant phenotype indistinguishable from that of a null mutant. Overexpression of RAD53, or its kinase domain alone, resulted in a delay in cell-cycle progression that required the intact kinase function. The cell-cycle delay did not require any of the checkpoint genes tested (e.g. rad9 or mecl), indicating that the cell-cycle delay is either unrelated to the checkpoint responses, or that it occurs constitutively because RAD53 acts further downstream of the checkpoint genes tested. Finally, elimination of sequences in the promoter region of RAD53 revealed complex regulatory elements.

S-phase and DNA-damage checkpoints: a tale of two yeasts

Many genes required for the S-phase and DNA-damage checkpoints have been identified in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This year many checkpoint genes have been sequenced, providing new information about the mechanism of checkpoint control. Several of these genes are conserved between the two yeasts but others are species-specific.

Enrichment of yeast protein tyrosine kinase activity by substrate affinity chromatography

The direct biochemical analysis of protein tyrosine kinases from yeast has been difficult due to their very low activity in crude cell lysates. Here we present a procedure for the enrichment and partial purification of protein tyrosine kinases from Saccharomyces cerevisiae based on single-step substrate affinity chromatography using a synthetic random co-polymer of glutamic acid and tyrosine. Fractionation of cell lysates on a poly-glutamic acid:tyrosine (4:1)-Sepharose affinity column resulted in a 4000-fold increase in tyrosine kinase activity. Active fractions contain at least six potential protein kinases as judged by in situ phosphorylation assay and Western blot analysis using anti-phosphotyrosine. We propose that this protocol may also be useful for the initial identification and purification of tyrosine kinases from other organisms exhibiting low levels of this enzymatic activity in cell lysates.

The REC1 gene of Ustilago maydis, which encodes a 3'-->5' exonuclease, couples DNA repair and completion of DNA synthesis to a mitotic checkpoint

Mutation in the REC1 gene of Ustilago maydis results in extreme sensitivity to killing by ultraviolet light. The lethality of the rec1-1 mutant was found to be partially suppressed if irradiated cells were held artificially in G2-phase by addition of a microtubule inhibitor. This mutant was also found to be sensitive to killing when DNA synthesis was inhibited by external means through addition of hydroxyurea or by genetic control in a temperature-sensitive mutant strain defective in DNA synthesis. Flow cytometric analysis of exponentially growing cultures indicated that wild-type cells accumulated in G2 after UV irradiation, while rec1-1 cells appeared to exit from G2 and accumulate in G1/S. Analysis of mRNA levels in synchronized cells indicated that the REC1 gene is periodically expressed with the cell cycle and reaches maximal levels at G1/S. The results are interpreted to mean that a G2-M checkpoint is disabled in the rec1-1 mutant. It is proposed that the REC1 gene product functions in a surveillance system operating during S-phase and G2 to find and repair stretches of DNA with compromised integrity and to communicate with the cell cycle apparatus.

In vivo association of v-Abl with Shc mediated by a non-phosphotyrosine-dependent SH2 interaction

A necessary downstream element of Abelson murine leukemia virus (Ab-MLV)-mediated transformation is Ras, which can be activated by the phosphotyrosine-dependent association of Shc with the Grb2-mSos complex. Here we show that Shc is tyrosine-phosphorylated and associates with Grb2 in v-Abl-transformed cells, whereas Shc in NIH3T3 cells is phosphorylated solely on serine and is not Grb2-associated. In addition, Shc coprecipitates with P120 v-Abl and P70 v-Abl, which lacks the carboxyl terminus. Surprisingly, a kinase-defective mutant of P120 also binds Shc, demonstrating that Shc/v-Abl association is a phosphotyrosine-independent interaction. Glutathione S-transferase fusion proteins were used to map the interacting domains and showed that Shc from both NIH3T3 and v-Abl-transformed cells binds to the Abl SH2 domain and that P120 v-Abl binds to a region in the amino terminus of Shc. Consistent with these data, a v-Abl mutant encoding only the Gag and SH2 regions was able to bind Shc in vivo. The unique non-phosphotyrosine-mediated binding of Shc may allow direct tyrosine phosphorylation of Shc by v-Abl and subsequent activation of the Ras pathway through assembly of a signaling complex with Grb2-mSos.

The yeast immunophilin Fpr3 is a physiological substrate of the tyrosine-specific phosphoprotein phosphatase Ptp1

The tyrosine-specific phosphoprotein phosphatase encoded by the Saccharomyces cerevisiae PTP1 gene dephosphorylates artificial substrates in vitro, but little is known about its functions and substrates in vivo. The presence of Ptp1 resulted in dephosphorylation of multiple tyrosine-phosphorylated proteins in yeast expressing a heterologous tyrosine-specific protein kinase, indicating that Ptp1 can dephosphorylate a broad range of substrates in vivo. Correspondingly, several proteins phosphorylated at tyrosine by endogenous protein kinases exhibited a marked increase in tyrosine phosphorylation in ptp1 mutant cells. One of these phosphotyrosyl proteins (p70) was also dephosphorylated in vitro when incubated with recombinant Ptp1. p70 was purified to homogeneity; analysis of four tryptic peptides revealed that p70 is identical to the recently described FPR3 gene product, a nucleolarly localized proline rotamase of the FK506- and rapamycin-binding family. The identity of p70 with Fpr3 was confirmed in the demonstration that the abundance of tyrosine-phosphorylated p70 in ptp1 mutants was strictly correlated with the level of FPR3 expression; immobilized phosphotyrosyl Fpr3 was directly dephosphorylated by recombinant Ptp1. Site-directed mutagenesis demonstrated that the site of tyrosine phosphorylation is Tyr-184, which resides within the nucleolin-like amino-terminal domain of Fpr3. Protein kinase activities from yeast cell extracts can bind to and phosphorylate the immobilized amino-terminal domain of Fpr3 on serine, threonine, and tyrosine. Fpr3 represents the first phosphotyrosyl protein identified in S. cerevisiae that is not itself a protein kinase and is as yet the only known physiological substrate of Ptp1.

Posttranscriptional and posttranslational control of enolase expression in the facultative Crassulacean acid metabolism plant Mesembryanthemum Crystallinum L

During the induction of Crassulacean acid and metabolism by environmental stresses in the common ice plant (Mesembryanthemum crystallinum L.), enzyme activities involved in glycolysis and gluconeogenesis, including enolase (2-phospho-D-glycerate hydrolase), increase significantly. In this study, we describe two nearly identical cDNA clones (Pgh1a and Pgh1b) encoding enolase from the common ice plant. This cytoplasmically localized enzyme is encoded by a gene family of at least two members. The polypeptides encoded by these cDNAs share a high degree of amino acid sequence identity (86.7-88.3%) with other higher plant enolases. Enolase activity increased more than 4-fold in leaves during salt stress. This increase was accompanied by a dramatic increase in Pgh1 transcription rate and the accumulation of enolase transcripts in leaves. Pgh1 transcript levels also increased in leaves in response to low temperature, drought, and anaerobic stress conditions and upon treatment of unstressed plants with the plant growth regulators abscisic acid and 6-benzylaminopurine. In roots, enolase transcripts increased in abundance in response to salt, low and high temperature, and anaerobic stresses. Surprisingly, we observed no increase in enolase protein levels, despite the increased levels of mRNA and enzyme activity during salt stress. The stress-induced increase in enolase activity is therefore due to posttranslational regulation of steady-state enzyme pools. Our results demonstrate that the stress-induced shift to Crassulacean acid metabolism in the ice plant involves complex regulatory control mechanisms that operate at the transcriptional, posttranscriptional, and postranslational levels.

A kinase from fission yeast responsible for blocking mitosis in S phase

In virtually all eukaryotes, mitosis starts after the completion of DNA synthesis. This orderly process is ensured by the checkpoint mechanism that blocks the onset of mitosis while DNA is being synthesized or is damaged. In the fission yeast Schizosaccharomyces pombe, this mechanism involves some rad+ and hus+ genes. However, it is not known how the checkpoint system monitors these events. Recently a multicopy suppressor of a temperature-sensitive DNA polymerase-alpha mutant was isolated. This gene, named cds1+ (checking DNA synthesis), encodes a typical protein kinase. Here we report that this protein kinase is a key component of the DNA replication-monitoring S/G2 checkpoint system. Our data suggest that its primary role is to monitor DNA synthesis by interacting with DNA polymerase alpha and send a signal to block the onset of mitosis while DNA synthesis is in progress.

Expression and functional analysis of a baculovirus gene encoding a truncated protein kinase homolog

Autographa californica nuclear polyhedrosis virus (AcMNPV) potentially encodes a 215-amino acid polypeptide containing 6 out of 11 motifs conserved among eukaryotic protein kinases (Morris et al., Virology 200, 360-369, 1994). We examined the expression of this gene, named pk2, at the transcriptional and translational levels and the possible role of the gene during baculovirus replication in cell culture and insect larvae. Northern (RNA) blot analysis revealed that pk2 was transcribed primarily as an early 1.2-kb RNA. Western blot analysis showed that pk2 was expressed as a 25-kDa protein, PK2, which was present both early and late during virus infection. To examine the function(s) of pk2, we constructed a mutant baculovirus, vKINdel, in which one-third of the PK2-coding region was deleted and then compared the characteristics of vKINdel with wild-type AcMNPV and a marker-rescued revertant. The pk2 deletion mutation had no discernable effect on the number, size, or appearance of plaques, the kinetics of protein synthesis or protein phosphorylation profiles during virus infection of cultured SF-21 cells. Deletion of pk2 also had no significant influence on the infectivity or virulence of the baculovirus in larval bioassays and the level of occluded virus production was normal. Thus, pk2 does not appear to have a significant influence on virus replication in the host systems examined.

Cell cycle-specific induction of an 89 kDa serine/threonine protein kinase activity in Trypanosoma brucei

The cell cycle compartmentalization of specific activities of the protozoan parasite Trypanosoma brucei has remained unexplored due to the lack of a cell synchronization protocol. We report here that stationary phase cells stimulated to enter the cell cycle showed significant synchrony through the first cycle. The pattern of tyrosine phosphorylated proteins, known to undergo alterations during trypanosome development, showed only moderate changes as quiescent cells entered the cycle, particularly an increase in a 77 kDa species. However, the activity of an 89 kDa protein kinase (SPK89), previously demonstrated to be restricted to the proliferative stages of the parasite's life cycle, markedly increased as the population entered S phase. Cell sorting experiments demonstrated that SPK89 activity was highest in S phase cells and moderate in G2/M cells. The entry into S phase and increased SPK89 activity did not depend on serum factors but required protein synthesis for a discrete period after stimulation. Various modulators of protein phosphorylation were tested to determine their effects on progression to S and SPK89 activity. Only staurosporine and genistein were effective. However, both of these compounds inhibited virtually all protein phosphorylation and protein synthesis in the parasites. Thus these drugs cannot be used as specific protein kinase inhibitors in trypanosomes.

RPK1, an essential yeast protein kinase involved in the regulation of the onset of mitosis, shows homology to mammalian dual-specificity kinases

We report here the sequence of RPK1 (for Regulatory cell Proliferation Kinase), a new Saccharomyces cerevisiae gene coding for a protein with sequence similarities to serine/threonine protein kinases. The protein sequence of 764 amino acids includes an amino-terminal domain (residues 1-410), which may be involved in regulation of the kinase domain (residues 411-764). The catalytic domain of Rpk1 is not closely related to other known yeast protein kinases but exhibits strong homology to a newly discovered group of mammalian kinases (PYT, TTK, esk) with serine/threonine/tyrosine kinase activity. Null alleles of RPK1 are lethal and thus this gene belongs to the small group of yeast protein kinase genes that are essential for cell growth. In addition, eliminating the expression of RPK1 gives rise to the accumulation of non-viable cells with less than a 1 N DNA content suggesting that cells proceed into mitosis without completion of DNA synthesis. Therefore, the Rpk1 kinase may function in a checkpoint control which couples DNA replication to mitosis. The level of the RPK1 transcript is extremely low and constant throughout the mitotic cycle. However it is regulated during cellular differentiation, being decreased in alpha-factor-treated a cells and increased late in meiosis in a/alpha diploids. Taken together, our results suggest that Rpk1 is involved in a pathway that coordinates cell proliferation and differentiation.

Cyclic AMP-regulated AChR assembly is independent of AChR subunit phosphorylation by PKA

Forskolin treatment of cells expressing Torpedo acetylcholine receptors leads to enhanced assembly efficiency of subunits, which correlates with increased phosphorylation of the gamma subunit. To determine the role of the two potential protein kinase A sites of the gamma subunit in receptor assembly, cell lines expressing different mutant receptors were established. Mouse fibroblast cell lines stably expressing wild-type Torpedo acetylcholine receptor alpha, beta, delta subunits plus one of three gamma subunit mutations (S353A, S354A, or S353,354A) were established to identify the protein kinase A phosphorylation sites of gamma in vivo, and to determine if increased phosphorylation of the gamma subunit leads to enhanced expression of receptors. We found that both serines (353, 354) in gamma are phosphorylated in vivo by protein kinase A, however, phosphorylation of either or both of these sites does not lead to increased assembly efficiency. We established a cell line expressing alpha, beta, and gamma(S353,354A) subunits only (no delta), and found that the presence of delta (or its phosphorylation) is also not necessary for the observed stimulation by forskolin. alpha beta gamma, alpha gamma, and beta gamma associations were stimulated by forskolin but alpha beta and alpha delta interactions were not. These data imply that the presence of gamma is necessary for forskolin action. We postulate that forskolin may stimulate acetylcholine receptor expression through a cellular protein that is involved in the folding and/or assembly of protein complexes, and that forskolin may regulate the action of such a protein through phosphorylation.

Characterization of a pollen-expressed receptor-like kinase gene of Petunia inflata and the activity of its encoded kinase

From a pollen tube cDNA library of Petunia inflata, we isolated clones encoding a protein with structural features and biochemical properties characteristic of receptor-like kinases. It was designated PRK1 for pollen receptor-like kinase 1. The cytoplasmic domain of PRK1 is highly similar to the kinase domains of other plant receptor-like kinases and contains nearly all of the conserved amino acids for serine/threonine kinases. The extracellular domain of PRK1 contains leucine-rich repeats as found in some other plant receptor-like kinases, but overall its sequence in this region does not share significant similarity. Characterization of a gene encoding PRK1 revealed the presence of two introns. During pollen development, PRK1 mRNA was first detected in anthers containing mostly binucleate microspores; it reached the highest level of mature pollen and remained at a high level in in vitro-germinated pollen tubes. The recombinant cytoplasmic domain of PRK1 autophosphorylated on serine and tyrosine, suggesting that PRK1 may be a dual-specificity kinase. Monospecific immune serum to the recombinant extracellular domain of PRK1 detected a 69-kD protein in microsomal membranes of pollen and pollen tubes. The characteristics of PRK1 suggest that it may play a role in signal transduction events during pollen development and/or pollination.

Dual specificity kinases--a new family of signal transducers

Phosphorylation/dephosphorylation reactions are one of the dynamic mechanisms through which cells modulate protein activity in response to environmental stimuli. The eukaryotic molecules which are responsible for the phosphorylation of serine, threonine and tyrosine residues appear to have co-ordinately evolved from simple prokaryotic enzymes which primarily respond to nutritional cues. In multicellular eukaryotes the complexity of data transfer greatly exceeds that of simple bacteria. The eukaryotic cell needs to exchange information with neighbouring and distant sister cells. Positional, nutritional and hormonal data are transmitted from the extracellular milieu across the plasma membrane and into the cytoplasm. In certain cases the signal must pass into the nucleus or other subcellular organelles where it is decoded and the proper cellular response initiated. All of these events have been shown to have a protein kinase component and it seems likely that in mammalian cells over 1,000 different kinase molecules have evolved to form the requisite signal transducing networks. In this review we describe a previously unappreciated family of protein kinases, the dual specificity or DSK kinases, which play important roles in the regulation of normal cellular growth and differentiation.

Characterization of the KIN2 gene product in Saccharomyces cerevisiae and comparison between the kinase activities of p145KIN1 and p145KIN2

We have isolated two yeast genes, KIN1 and KIN2, by their homology to the protein kinase family of viral oncogenes. Previous studies have identified the yeast KIN1 gene product (pp145KIN1) as a 145 kilodalton (kDa) phosphoprotein with serine/threonine-specific protein kinase activity. To identify and biochemically characterize the KIN2 gene product, antibodies were raised against a bacterial beta-galactosidase/KIN2 fusion polypeptide. In vivo, the KIN2 gene product is a 145 kDa phosphoprotein, pp145KIN2. In immune complexes, pp145KIN2 demonstrates serine/threonine protein kinase activity, transferring phosphate from [gamma-32P]ATP to either itself or the exogenously added substrates alpha-casein, acid-denatured enolase, or phosvitin. In vitro, kinase activity is dependent on either Mn2+ or Mg2+ ions. Both enzymes, pp145KIN1 and pp145KIN2, prefer ATP over GTP as their phosphoryl donor. Since a new class of yeast protein kinases has been identified which are serine/tyrosine-specific, we analysed a wide range of substrates to see if any could be phosphorylated by pp145KIN1 or pp145KIN2 on tyrosine residues. Both enzymes phosphorylate alpha-casein, acid-denatured enolase, and phosvitin on serine and threonine residues. Neither enzyme could phosphorylate tyrosine residues even though good substrates for tyrosine-specific kinases such as enolase, angiotensin II, and the synthetic polymer GLU80TYR20 were used. The biochemical analysis of KIN2 kinase activity shows remarkable similarity to that of its most closely related yeast kinase, KIN1. It remains to be seen if these two yeast protein kinases share any functional relationships or substrates in vivo.