The Dictyostelium dual-specificity kinase splA is essential for spore differentiation

Physiological and proteomic analysis reveals the impact of boron deficiency and surplus on alfalfa (Medicago sativa L.) reproductive organs

Boron (B), an essential element for increasing seed yield and germinability in alfalfa (Medicago sativa L.), plays a vital role in its reproductive processes. However, effects of B stress on physiological and proteomic changes in reproductive organs related to alfalfa seed yield and germinability are poorly understood. In order to gain a better insight into B response or tolerance mechanisms, field trials were designed for B deficiency (0 mg B L^-1), B sufficiency (800 mg B L^-1), and B surplus (1600 mg B L^-1) application during alfalfa flowering to analyze the proteomics and physiological responses of alfalfa 'Aohan' reproductive organs. Results showed that B deficiency weakened the stress-responsive ability in these organs, while B surplus reduced the sugar utilization of 'Aohan' flowers and caused lipid membrane peroxidation in 'Aohan' seeds. In addition, four upregulated stress responsive proteins (ADF-like protein, IMFP, NAD(P)-binding Rossmann-fold protein and NAD-dependent ALDHs) might play pivotal roles in the response of 'Aohan' reproductive organs to conditions of B deficiency and B surplus. All of the above results would be helpful to understand the tolerance mechanisms of alfalfa reproductive organs to both B deficiency and B surplus conditions, and also to give insight into the regulatory role of B in improving seed yield and germinability in alfalfa seed production. In summary, B likely plays a structural and regulatory role in relation to lipid metabolism, carbohydrate metabolism, amino acid metabolism, and signal transduction, thus regulates alfalfa reproductive processes eventually affecting the seed yield and germinability of alfalfa seeds.

Analysis of DrkA kinase's role in STATa activation

Dictyostelium STATa is a homologue of metazoan signal transducers and activators of transcription (STATs) and is important for morphogenesis. STATa is activated by phosphorylation on Tyr702 when cells are exposed to extracellular cAMP. Although two tyrosine kinase-like (TKL) proteins, Pyk2 and Pyk3, have been definitively identified as STATc kinases, no kinase is known for STATa activation. Based on homology to the previously identified tyrosine-selective TKLs, we identified DrkA, a member of the TKL family and the Dictyostelium receptor-like kinase (DRK) subfamily, as a candidate STATa kinase. The drkA gene is almost exclusively expressed in prestalk A (pstA) cells, where STATa is activated. Transient over-expression of DrkA increased STATa phosphorylation, although over-expression of the protein causes a severe growth defect and cell death. Furthermore, recombinant DrkA protein is auto-phosphorylated on tyrosine and threonine residues, and an in vitro kinase assay shows that DrkA can phosphorylate STATa on Tyr702 in a STATa-SH2 (phosphotyrosine binding) domain-dependent manner. These observations strongly suggest that DrkA is one of the key regulators of STATa tyrosine phosphorylation and is consistent with it being the kinase that directly activates STATa.

Proteomic Identification of Differentially Expressed Proteins during Alfalfa (Medicago sativa L.) Flower Development

Flower development, pollination, and fertilization are important stages in the sexual reproduction process of plants; they are also critical steps in the control of seed formation and development. During alfalfa (Medicago sativa L.) seed production, some distinct phenomena such as a low seed setting ratio, serious flower falling, and seed abortion commonly occur. However, the causes of these phenomena are complicated and largely unknown. An understanding of the mechanisms that regulate alfalfa flowering is important in order to increase seed yield. Hence, proteomic technology was used to analyze changes in protein expression during the stages of alfalfa flower development. Flower samples were collected at pre-pollination (S1), pollination (S2), and the post-pollination senescence period (S3). Twenty-four differentially expressed proteins were successfully identified, including 17 down-regulated in pollinated flowers, one up-regulated in pollinated and senesced flowers, and six up-regulated in senesced flowers. The largest proportions of the identified proteins were involved in metabolism, signal transduction, defense response, oxidation reduction, cell death, and programmed cell death (PCD). Their expression profiles demonstrated that energy metabolism, carbohydrate metabolism, and amino acid metabolism provided the nutrient foundation for pollination in alfalfa. Furthermore, there were three proteins involved in multiple metabolic pathways: dual specificity kinase splA-like protein (kinase splALs), carbonic anhydrase, and NADPH: quinone oxidoreductase-like protein. Expression patterns of these proteins indicated that MAPK cascades regulated multiple processes, such as signal transduction, stress response, and cell death. PCD also played an important role in the alfalfa flower developmental process, and regulated both pollination and flower senescence. The current study sheds some light on protein expression profiles during alfalfa flower development and contributes to the understanding of the basic molecular mechanisms during the alfalfa flowering process. These results may offer insight into potential strategies for improving seed yield, quality, and stress tolerance in alfalfa.

Gene discovery by chemical mutagenesis and whole-genome sequencing in Dictyostelium

Whole-genome sequencing is a useful approach for identification of chemical-induced lesions, but previous applications involved tedious genetic mapping to pinpoint the causative mutations. We propose that saturation mutagenesis under low mutagenic loads, followed by whole-genome sequencing, should allow direct implication of genes by identifying multiple independent alleles of each relevant gene. We tested the hypothesis by performing three genetic screens with chemical mutagenesis in the social soil amoeba Dictyostelium discoideum Through genome sequencing, we successfully identified mutant genes with multiple alleles in near-saturation screens, including resistance to intense illumination and strong suppressors of defects in an allorecognition pathway. We tested the causality of the mutations by comparison to published data and by direct complementation tests, finding both dominant and recessive causative mutations. Therefore, our strategy provides a cost- and time-efficient approach to gene discovery by integrating chemical mutagenesis and whole-genome sequencing. The method should be applicable to many microbial systems, and it is expected to revolutionize the field of functional genomics in Dictyostelium by greatly expanding the mutation spectrum relative to other common mutagenesis methods.

Phloem sap proteome studied by iTRAQ provides integrated insight into salinity response mechanisms in cucumber plants

Cucumber is an economically important crop as well as a model system for plant vascular biology. Salinity is one of the major environmental factors limiting plant growth. Here, we used an iTRAQ-based quantitative proteomics approach for comparative analysis of protein abundances in cucumber phloem sap in response to salt. A total of 745 distinct proteins were identified and 111 proteins were differentially expressed upon salinity in sensitive and tolerant cultivars, of which 69 and 65 proteins changed significantly in sensitive and tolerant cultivars, respectively. A bioinformatics analysis indicated that cucumber phloem employed a combination of induced metabolism, protein turnover, common stress response, energy and transport, signal transduction and regulation of transcription, and development proteins as protection mechanisms against salinity. The proteins that were mapped to the carbon fixation pathway decreased in abundance in sensitive cultivars and had no change in tolerant cultivars under salt stress, suggesting that this pathway may promote salt tolerance by stabilizing carbon fixation and maintaining the essential energy and carbohydrates in tolerant cultivars. This study leads to a better understanding of the salinity mechanism in cucumber phloem and provides a list of potential gene targets for the further engineering of salt tolerance in plants.

Two Dictyostelium tyrosine kinase-like kinases function in parallel, stress-induced STAT activation pathways

When Dictyostelium cells are hyperosmotically stressed, STATc is activated by tyrosine phosphorylation. Unusually, activation is regulated by serine phosphorylation and consequent inhibition of a tyrosine phosphatase: PTP3. The identity of the cognate tyrosine kinase is unknown, and we show that two tyrosine kinase-like (TKL) enzymes, Pyk2 and Pyk3, share this function; thus, for stress-induced STATc activation, single null mutants are only marginally impaired, but the double mutant is nonactivatable. When cells are stressed, Pyk2 and Pyk3 undergo increased autocatalytic tyrosine phosphorylation. The site(s) that are generated bind the SH2 domain of STATc, and then STATc becomes the target of further kinase action. The signaling pathways that activate Pyk2 and Pyk3 are only partially overlapping, and there may be a structural basis for this difference because Pyk3 contains both a TKL domain and a pseudokinase domain. The latter functions, like the JH2 domain of metazoan JAKs, as a negative regulator of the kinase domain. The fact that two differently regulated kinases catalyze the same phosphorylation event may facilitate specific targeting because under stress, Pyk3 and Pyk2 accumulate in different parts of the cell; Pyk3 moves from the cytosol to the cortex, whereas Pyk2 accumulates in cytosolic granules that colocalize with PTP3.

Knights in action: lectin receptor-like kinases in plant development and stress responses

The Receptor-Like Kinase (RLK) is a vast protein family with over 600 genes in Arabidopsis and 1100 in rice. The Lectin RLK (LecRLK) family is believed to play crucial roles in saccharide signaling as well as stress perception. All the LecRLKs possess three domains: an N-terminal lectin domain, an intermediate transmembrane domain, and a C-terminal kinase domain. On the basis of lectin domain variability, LecRLKs have been subgrouped into three subclasses: L-, G-, and C-type LecRLKs. While the previous studies on LecRLKs were dedicated to classification, comparative structural analysis and expression analysis by promoter-based studies, most of the recent studies on LecRLKs have laid special emphasis on the potential of this gene family in regulating biotic/abiotic stress and developmental pathways in plants, thus making the prospects of studying the LecRLK-mediated regulatory mechanism exceptionally promising. In this review, we have described in detail the LecRLK gene family with respect to a historical, evolutionary, and structural point of view. Furthermore, we have laid emphasis on the LecRLKs roles in development, stress conditions, and hormonal response. We have also discussed the exciting research prospects offered by the current knowledge on the LecRLK gene family. The multitude of the LecRLK gene family members and their functional diversity mark these genes as both interesting and worthy candidates for further analysis, especially in the field of crop improvement.

Identification of the kinase that activates a nonmetazoan STAT gives insights into the evolution of phosphotyrosine-SH2 domain signaling

SH2 domains are integral to many animal signaling pathways. By interacting with specific phosphotyrosine residues, they provide regulatable protein-protein interaction domains. Dictyostelium is the only nonmetazoan with functionally characterized SH2 domains, but the cognate tyrosine kinases are unknown. There are no orthologs of the animal tyrosine kinases, but there are very many tyrosine kinase-like kinases (TKLs), a group of kinases which, despite their family name, are classified mainly as serine-threonine kinases. STATs are transcription factors that dimerize via phosphotyrosine-SH2 domain interactions. STATc is activated by phosphorylation on Tyr922 when cells are exposed to the prestalk inducer differentiation inducing factor (DIF-1), a chlorinated hexaphenone. We show that in a null mutant for Pyk2, a tyrosine-specific TKL, exposure to DIF-1 does not activate STATc. Conversely, overexpression of Pyk2 causes constitutive STATc activation. Pyk2 phosphorylates STATc on Tyr922 in vitro and complexes with STATc both in vitro and in vivo. This demonstration that a TKL directly activates a STAT has significant implications for understanding the evolutionary origins of SH2 domain-phosphotyrosine signaling. It also has mechanistic implications. Our previous work suggested that a predicted constitutive STATc tyrosine kinase activity is counterbalanced in vivo by the DIF-1-regulated activity of PTP3, a Tyr922 phosphatase. Here we show that the STATc-Pyk2 complex is formed constitutively by an interaction between the STATc SH2 domain and phosphotyrosine residues on Pyk2 that are generated by autophosphorylation. Also, as predicted, Pyk2 is constitutively active as a STATc kinase. This observation provides further evidence for this highly atypical, possibly ancestral, STAT regulation mechanism.

Serum-dependent selective expression of EhTMKB1-9, a member of Entamoeba histolytica B1 family of transmembrane kinases

Entamoeba histolytica transmembrane kinases (EhTMKs) can be grouped into six distinct families on the basis of motifs and sequences. Analysis of the E. histolytica genome revealed the presence of 35 EhTMKB1 members on the basis of sequence identity (≥95%). Only six homologs were full length containing an extracellular domain, a transmembrane segment and an intracellular kinase domain. Reverse transcription followed by polymerase chain reaction (RT-PCR) of the kinase domain was used to generate a library of expressed sequences. Sequencing of randomly picked clones from this library revealed that about 95% of the clones were identical with a single member, EhTMKB1-9, in proliferating cells. On serum starvation, the relative number of EhTMKB1-9 derived sequences decreased with concomitant increase in the sequences derived from another member, EhTMKB1-18. The change in their relative expression was quantified by real time PCR. Northern analysis and RNase protection assay were used to study the temporal nature of EhTMKB1-9 expression after serum replenishment of starved cells. The results showed that the expression of EhTMKB1-9 was sinusoidal. Specific transcriptional induction of EhTMKB1-9 upon serum replenishment was further confirmed by reporter gene (luciferase) expression and the upstream sequence responsible for serum responsiveness was identified. EhTMKB1-9 is one of the first examples of an inducible gene in Entamoeba. The protein encoded by this member was functionally characterized. The recombinant kinase domain of EhTMKB1-9 displayed protein kinase activity. It is likely to have dual specificity as judged from its sensitivity to different kinase inhibitors. Immuno-localization showed EhTMKB1-9 to be a surface protein which decreased on serum starvation and got relocalized on serum replenishment. Cell lines expressing either EhTMKB1-9 without kinase domain, or EhTMKB1-9 antisense RNA, showed decreased cellular proliferation and target cell killing. Our results suggest that E. histolytica TMKs of B1 family are functional kinases likely to be involved in serum response and cellular proliferation.

The presence of a vast array of putative transmembrane kinase genes suggests an extensive network of signaling systems in E. histolytica, particularly the ability to perceive signals from the extracellular environment and transduce these intracellularly. However, it has been very difficult to work with these molecules due to the presence of a large number of homologs. It is also not clear if these molecules are indeed protein kinases, as no kinase activity has yet been shown associated with these molecules. In this report, we show that EhTMKB1-9 is a protein kinase and it is one of the early serum-induced genes. It is a predominant EhTMKB1 molecule that is expressed in proliferating cells and its expression is modulated by serum. Cells containing a reduced level of EhTMKB1-9 or high level of a mutant protein result in decreased proliferation, target cell killing and adherence. The results presented in this report suggest that EhTMKB1-9 is an important signaling molecule likely to be involved in E. histolytica proliferation and virulence. We have also identified a serum starvation induced response where expression of EhTMKB1-18 was found to be induced.

A new protein carrying an NmrA-like domain is required for cell differentiation and development in Dictyostelium discoideum

We have isolated a Dictyostelium mutant unable to induce expression of the prestalk-specific marker ecmB in monolayer assays. The disrupted gene, padA, leads to a range of phenotypic defects in growth and development. We show that padA is essential for growth, and we have generated a thermosensitive mutant allele, padA(-). At the permissive temperature, mutant cells grow poorly; they remain longer at the slug stage during development and are defective in terminal differentiation. At the restrictive temperature, growth is completely blocked, while development is permanently arrested prior to culmination. padA(-) slugs are deficient in prestalk A cell differentiation and present an abnormal ecmB expression pattern. Sequence comparisons and predicted three-dimensional structure analyses show that PadA carries an NmrA-like domain. NmrA is a negative transcriptional regulator involved in nitrogen metabolite repression in Aspergillus nidulans. PadA predicted structure shows a NAD(P)(+)-binding domain, which we demonstrate that is essential for function. We show that padA(-) development is more sensitive to ammonia than wild-type cells and two ammonium transporters, amtA and amtC, appear derepressed during padA(-) development. Our data suggest that PadA belongs to a new family of NAD(P)(+)-binding proteins that link metabolic changes to gene expression and is required for growth and normal development.

The P450 oxidoreductase, RedA, controls development beyond the mound stage in Dictyostelium discoideum

BACKGROUND

NADPH-cytochrome-P450 oxidoreductase (CPR) is a ubiquitous enzyme that belongs to a family of diflavin oxidoreductases and is required for activity of the microsomal cytochrome-P450 monooxygenase system. CPR gene-disruption experiments have demonstrated that absence of this enzyme causes developmental defects both in mouse and insect.

RESULTS

Annotation of the sequenced genome of D. discoideum revealed the presence of three genes (redA, redB and redC) that encode putative members of the diflavin oxidoreductase protein family. redA transcripts are present during growth and early development but then decline, reaching undetectable levels after the mound stage. redB transcripts are present in the same levels during growth and development while redC expression was detected only in vegetative growing cells. We isolated a mutant strain of Dictyostelium discoideum following restriction enzyme-mediated integration (REMI) mutagenesis in which redA was disrupted. This mutant develops only to the mound stage and accumulates a bright yellow pigment. The mound-arrest phenotype is cell-autonomous suggesting that the defect occurs within the cells rather than in intercellular signaling.

CONCLUSION

The developmental arrest due to disruption of redA implicates CPR in the metabolism of compounds that control cell differentiation.

Regulation of ammonia homeostasis by the ammonium transporter AmtA in Dictyostelium discoideum

Ammonia has been shown to function as a morphogen at multiple steps during the development of the cellular slime mold Dictyostelium discoideum; however, it is largely unknown how intracellular ammonia levels are controlled. In the Dictyostelium genome, there are five genes that encode putative ammonium transporters: amtA, amtB, amtC, rhgA, and rhgB. Here, we show that AmtA regulates ammonia homeostasis during growth and development. We found that cells lacking amtA had increased levels of ammonia/ammonium, whereas their extracellular ammonia/ammonium levels were highly decreased. These results suggest that AmtA mediates the excretion of ammonium. In support of a role for AmtA in ammonia homeostasis, AmtA mRNA is expressed throughout the life cycle, and its expression level increases during development. Importantly, AmtA-mediated ammonia homeostasis is critical for many developmental processes. amtA(-) cells are more sensitive to NH(4)Cl than wild-type cells in inhibition of chemotaxis toward cyclic AMP and of formation of multicellular aggregates. Furthermore, even in the absence of exogenously added ammonia, we found that amtA(-) cells produced many small fruiting bodies and that the viability and germination of amtA(-) spores were dramatically compromised. Taken together, our data clearly demonstrate that AmtA regulates ammonia homeostasis and plays important roles in multiple developmental processes in Dictyostelium.

Staurosporine induces tyrosine phosphorylation in Dictyostelium discoideum proteins

The treatment of cells with staurosporine results in inhibition and less frequently activation of protein kinases, in a cell-type specific manner. In the social amoeba Dictyostelium discoideum, staurosporine induces marked changes in cell morphology affecting growth and development. Here we describe that incubation of D. discoideum growing or starved cells with staurosporine results in a rapid and unexpected tyrosine phosphorylation on two polypeptides of approximately 64 and approximately 62 kDa. These proteins emerge as novel substrates for tyrosine phosphorylation opening up new perspectives for the study of cell signalling in D. discoideum.

The dictyostelium kinome--analysis of the protein kinases from a simple model organism

Dictyostelium discoideum is a widely studied model organism with both unicellular and multicellular forms in its developmental cycle. The Dictyostelium genome encodes 285 predicted protein kinases, similar to the count of the much more advanced Drosophila. It contains members of most kinase classes shared by fungi and metazoans, as well as many previously thought to be metazoan specific, indicating that they have been secondarily lost from the fungal lineage. This includes the entire tyrosine kinase–like (TKL) group, which is expanded in Dictyostelium and includes several novel receptor kinases. Dictyostelium lacks tyrosine kinase group kinases, and most tyrosine phosphorylation appears to be mediated by TKL kinases. About half of Dictyostelium kinases occur in subfamilies not present in yeast or metazoa, suggesting that protein kinases have played key roles in the adaptation of Dictyostelium to its habitat. This study offers insights into kinase evolution and provides a focus for signaling analysis in this system.

Protein kinases are eukaryotic enzymes involved in cell communication pathways, and transmit information from outside the cell or between subcellular components within the cell. About 2.5% of genes code for protein kinases, and mutations in many of these cause human disease. The authors characterize the complete set of protein kinases (kinome) from Dictyostelium discoideum, a social amoeba that responds to starvation by forming aggregates of cells, which then differentiate into multicellular fruiting bodies. Dictyostelium branched from the vertebrate lineage after plants but before fungi, and thus illuminates an interesting period in evolutionary history. By comparing the Dictyostelium kinome to those of other organisms, the authors find 46 types of kinases that appear to be conserved in all organisms, and are likely to be involved in fundamental cellular processes. Dictyostelium is an established model organism for studying many aspects of cell biology that are conserved in humans, and this exposition of conserved kinases will help to guide future studies. The Dictyostelium kinome also contains an impressive degree of creativity—almost half of the kinases are unique to Dictyostelium. Many of these Dictyostelium-specific kinases may be related to this organism's distinctive mechanism for coping with starvation.

Genome-wide analysis and experimentation of plant serine/ threonine/tyrosine-specific protein kinases

Protein tyrosine phosphorylation plays an important role in cell growth, development and oncogenesis. No classical protein tyrosine kinase has hitherto been cloned from plants. Does protein tyrosine kinase exist in plants? To address this, we have performed a genomic survey of protein tyrosine kinase motifs in plants using the delineated tyrosine phosphorylation motifs from the animal system. The Arabidopsis thaliana genome encodes 57 different protein kinases that have tyrosine kinase motifs. Animal non-receptor tyrosine kinases, SRC, ABL, LYN, FES, SEK, KIN and RAS have structural relationship with putative plant tyrosine kinases. In an extended analysis, animal receptor and non-receptor kinases, Raf and Ras kinases, mixed lineage kinases and plant serine/threonine/tyrosine (STY) protein kinases, form a well-supported group sharing a common origin within the superfamily of STY kinases. We report that plants lack bona fide tyrosine kinases, which raise an intriguing possibility that tyrosine phosphorylation is carried out by dual-specificity STY protein kinases in plants. The distribution pattern of STY protein kinase families on Arabidopsis chromosomes indicates that this gene family is partly a consequence of duplication and reshuffling of the Arabidopsis genome and of the generation of tandem repeats. Genome-wide analysis is supported by the functional expression and characterization of At2g24360 and phosphoproteomics of Arabidopsis. Evidence for tyrosine phosphorylated proteins is provided by alkaline hydrolysis, anti-phosphotyrosine immunoblotting, phosphoamino acid analysis and peptide mass fingerprinting. These results report the first comprehensive survey of genome-wide and tyrosine phosphoproteome analysis of plant STY protein kinases.

Identification and gene expression analysis of a large family of transmembrane kinases related to the Gal/GalNAc lectin in Entamoeba histolytica

We identified in the Entamoeba histolytica genome a family of over 80 putative transmembrane kinases (TMKs). The TMK extracellular domains had significant similarity to the intermediate subunit (Igl) of the parasite Gal/GalNAc lectin. The closest homolog to the E. histolytica TMK kinase domain was a cytoplasmic dual-specificity kinase, SplA, from Dictyostelium discoideum. Sequence analysis of the TMK family demonstrated similarities to both serine/threonine and tyrosine kinases. TMK genes from each of six phylogenetic groups were expressed as mRNA in trophozoites, as assessed by spotted oligoarray and real-time PCR assays, suggesting nonredundant functions of the TMK groups for sensing and responding to extracellular stimuli. Additionally, we observed changes in the expression profile of the TMKs in continuous culture. Antisera produced against the conserved kinase domain identified proteins of the expected molecular masses of the expressed TMKs. Confocal microscopy with anti-TMK kinase antibodies revealed a focal distribution of the TMKs on the cytoplasmic face of the trophozoite plasma membrane. We conclude that E. histolytica expresses members of each subgroup of TMKs. The presence of multiple receptor kinases in the plasma membrane offers for the first time a potential explanation of the ability of the parasite to respond to the changing environment of the host.

Comparative analysis of spore coat formation, structure, and function in Dictyostelium

Dictyostelium produces spores at the end of its developmental cycle to propagate the lineage. The spore coat is an essential feature of spore biology contributing a semipermeable chemical and physical barrier to protect the enclosed amoeba. The coat is assembled from secreted proteins and a polysaccharide, and from cellulose produced at the cell surface. They are organized into a polarized molecular sandwich with proteins forming layers surrounding the microfibrillar cellulose core. Genetic and biochemical studies are beginning to provide insight into how the deliveries of protein and cellulose to the cell surface are coordinated and how cysteine-rich domains of the proteins interact to form the layers. A multidomain inner layer protein, SP85/PsB, seems to have a central role in regulating coat assembly and contributing to a core structural module that bridges proteins to cellulose. Coat formation and structure have many parallels in walls from plant, algal, yeast, protist, and animal cells.

Outside-in signaling of cellulose synthesis by a spore coat protein in Dictyostelium

The spore coat of Dictyostelium is formed de novo from proteins secreted from vesicles and cellulose synthesized across the plasma membrane as differentiating spores rise up the stalk. The mechanism by which these events are coordinated is not understood. In the course of experiments designed to test the function of the inner layer coat protein SP85 (PsB), expression of a specific partial length fragment was found to interrupt coat assembly after protein secretion and prior to cellulose synthesis in 85% of the cells. This fragment consisted of SP85's N-terminal domain, containing prespore vesicle targeting information, and its Cys-rich C1 domain. The effect of the NC1 fusion was not cell autonomous in interstrain chimeras, suggesting that it acted at the cell surface. SP85-null spores presented an opposite phenotype in which spores differentiated prematurely before reaching the top of the stalk, and cellulose was slightly overproduced in a disorganized fashion. A similar though less severe phenotype occurred when a fusion of the N and C2 domains was expressed. In a double mutant, absence of SP85 was epistatic to NC1 expression, suggesting that NC1 inhibited SP85 function. Together, these results suggest the existence of an outside-in signaling pathway that constitutes a checkpoint to ensure that cellulose synthesis does not occur until coat proteins are properly organized at the cell surface and stalk formation is complete. Checkpoint execution is proposed to be regulated by SP85, which is in turn under the influence of other coat proteins that interact with SP85 via its C1 and C2 domains.

TaCRK3 encodes a novel Theileria annulata protein kinase with motifs characteristic of the family of eukaryotic cyclin dependent kinases: a comparative analysis of its expression with TaCRK2 during the parasite life cycle

The TaCRK3 gene from the bovine apicomplexan parasite Theileria annulata, encodes a 46 kDa polypeptide with strong homology to the eukaryotic family of cyclin-dependent kinases. TaCRK3 does not show significant alignment with any particular CDK group, other than the Pfmrk kinases from the related apicomplexans Plasmodium falciparum and Plasmodium yoelii. It has a putative bipartite nuclear localization signal and is located to parasite nuclei by IFAT. Protein levels are constitutive throughout differentiation of the intra-lymphocytic macroschizont. This contrasts with the expression pattern of TaCRK2 (Kinnaird et al., 1996, Mol. Microbiol., 22, 293-302) which is closely related to the eukaryotic CDK1 /2 families involved in regulation of cell cycle progression. TaCRK2 is also located to the parasite nuclei but has no nuclear localization signal and exhibits transient up-regulation in protein levels during mid-merogony. However compared to TaCRK3, it shows down-regulation near the end of merogony. We predict that TaCRK3 may have a role in regulation of gene transcription while TaCRK2 is more likely to be involved in control of parasite nuclear division.

Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases

Plant receptor-like kinases (RLKs) are proteins with a predicted signal sequence, single transmembrane region, and cytoplasmic kinase domain. Receptor-like kinases belong to a large gene family with at least 610 members that represent nearly 2.5% of Arabidopsis protein coding genes. We have categorized members of this family into subfamilies based on both the identity of the extracellular domains and the phylogenetic relationships between the kinase domains of subfamily members. Surprisingly, this structurally defined group of genes is monophyletic with respect to kinase domains when compared with the other eukaryotic kinase families. In an extended analysis, animal receptor kinases, Raf kinases, plant RLKs, and animal receptor tyrosine kinases form a well supported group sharing a common origin within the superfamily of serine/threonine/tyrosine kinases. Among animal kinase sequences, Drosophila Pelle and related cytoplasmic kinases fall within the plant RLK clade, which we now define as the RLK/Pelle family. A survey of expressed sequence tag records for land plants reveals that mosses, ferns, conifers, and flowering plants have similar percentages of expressed sequence tags representing RLK/Pelle homologs, suggesting that the size of this gene family may have been close to the present-day level before the diversification of land plant lineages. The distribution pattern of four RLK subfamilies on Arabidopsis chromosomes indicates that the expansion of this gene family is partly a consequence of duplication and reshuffling of the Arabidopsis genome and of the generation of tandem repeats.

Human ERMAP: an erythroid adhesion/receptor transmembrane protein

A human cDNA and gene encoding for human ERMAP, a putative erythroid transmembrane adhesion/receptor protein, is reported. The predicted protein is made up of 475 amino acids and shares high homology with the murine ERMAP (73% identity and 14% conservative changes). Human Ermap is highly expressed in erythroid tissues and the protein localizes to the plasma membrane, particularly in sites of cell contact, and "cytoplasmic bodies." The extracellular segment contains one IgV fold that shares high homology with the butyrophilin family of milk proteins, autoantigens, and avian blood group antigens. In the intracellular region, there is a conserved B30.2 domain that is encoded by a single exon and is highly homologous with a similar domain in a diverse group of proteins, including butyrophilin, pyrin, and MID 1. The human Ermap gene is composed of 11 exons spanning 19 kb on chromosome 1p34.

cAMP and DIF-1 repress the expression of the Dictyostelium MADS-box gene srfA at early stages of development

The MADS-box-containing gene srfA from Dictyostelium discoideum codes for a putative transcription factor that plays multiple roles in the development of this social amoeba. We have investigated the regulation of srfA gene expression after disaggregation of the cells from developing structures. The steady-state level of srfA mRNA was strongly and transiently induced shortly after disaggregation. srfA is maximally expressed 20 min after cell disaggregation and decreases thereafter. Induction was not dependent on protein synthesis, PKA, the kinase SplA and SrfA itself. This phenomena does not occur when cells are disaggregated in a small volume of buffer, suggesting the presence of extracellular molecules that repress srfA gene expression. To test this hypothesis, several well-known extracellular signaling molecules were studied. We found that srfA mRNA induction can be efficiently repressed by addition of exogenous cAMP and DIF-1 to the buffer in which the cells were disaggregated. Addition of other extracellular compounds such as ammonia, adenosine, SDF-1, and SDF-2 had no effect. srfA promoter P2, specifically induced during slug migration, was responsible for this regulation by extracellular compounds.

Protein tyrosine phosphatase PTP1 negatively regulates Dictyostelium STATa and is required for proper cell-type proportioning

The protein tyrosine phosphatase PTP1, which mediates reversible phosphorylation on tyrosine, has been shown to play an important regulatory role during Dictyostelium development. Mutants lacking PTP1 develop more rapidly than normal, while strains that overexpress PTP1 display aberrant morphology. However, the signalling pathways involved have not been characterised. In reexamining these strains, we have found that there is an inverse correlation between levels of PTP1 activity, the extent of tyrosine phosphorylation on Dictyostelium STATa after treatment with cAMP, and the proportion of the slug population exhibiting STATa nuclear enrichment in vivo. This suggests that PTP1 acts to attenuate the tyrosine phosphorylation of STATa and downstream STATa-mediated pathways. Consistent with this, we show that when PTP1 is overexpressed, there is increased expression of a prestalk cell marker at the slug posterior, a phenocopy of STATa null slugs. In ptp1 null strains, STATa tyrosine phosphorylation and nuclear enrichment in the slug anterior is increased. There is also a change in the prestalk to prespore cell ratio. Synergy experiments suggest that this is due to a cell-autonomous defect in forming the subset of prespore cells that are located in the anterior prespore region.

An SH2-domain-containing kinase negatively regulates the phosphatidylinositol-3 kinase pathway

SHK1 is a novel dual-specificity kinase that contains an SH2 domain in its C-terminal region. We demonstrate that SHK1 is required for proper chemotaxis and phagocytosis. Mutant shk1 null cells lack polarity, move very slowly, and exhibit an elevated and temporally extended chemoattractant-mediated activation of the kinase Akt/PKB. GFP fusions of the PH domain of Akt/PKB or the PH-domain-containing protein CRAC, which become transiently associated with the plasma membrane after a global stimulation with a chemoattractant, remain associated with the plasma membrane for an extended period of time in shk1 null cells. These results suggest that SHK1 is a negative regulator of the PI3K (phosphatidylinositol-3 kinase) pathway. Furthermore, when a chemoattractant gradient is applied to a wild-type cell, these PH-domain-containing proteins and the F-actin-binding protein coronin localize to its leading edge, but in an shk1 null cell they become randomly associated with the plasma membrane and cortex, irrespective of the direction of the chemoattractant gradient, suggesting that SHK1 is required for the proper spatiotemporal control of F-actin levels in chemotaxing cells. Consistent with such functions, SHK1 is localized at the plasma membrane/cortex, and we show that its SH2 domain is required for this localization and the proper function of SHK1.

Loss of a member of the aquaporin gene family, aqpA affects spore dormancy in Dictyostelium

We isolated and characterized a gene from Dictyostelium discoideum, which encodes a protein of 279 amino acids (30.6kDa) containing six transmembrane domains with two highly conserved motifs of asparagine-proline-alanine (NPA) found in the aquaporin family of water-channel proteins, although the second motif of the protein has been modified into NPV (asparagine-proline-valine). The deduced amino acid sequence of the gene, which we have named aqpA, is 39% identical to D. discoideum WacA, 26% identical to human Aqp5, 26% identical to Oryza sativa PIP2a, 25% identical to yeast Aqy1 and 24% identical to E.coli AqpZ. Southern analyses indicated that aqpA is present as a single copy in the genome. Northern blot analysis showed that the developmentally regulated 1kb mRNA transcript first appears at the tight mound stage (12h), and is abundant in fingers (16h) and late culminants (20h). In-situ hybridization of slugs revealed that aqpA mRNA accumulated in cells of the prespore region but not in those of the prestalk region. Disruption of aqpA by homologous recombination did not significantly affect growth or developmental morphogenesis. Although mutant spores were viable, when assayed soon after encapsulation, they became permeable to propidium iodide and lost viability after a week on the top of a fruiting body. Thus, AqpA is essential to maintain spore dormancy perhaps through the regulation of water flow.

Precocious sporulation and developmental lethality in yelA null mutants of Dictyostelium

A novel developmental gene, yelA, has been found that plays as essential role in regulating terminal differentiation of Dictyostelium discoideum. Strains in which yelA is disrupted by plasmid insertion are arrested at the tight mound stage but accumulate the bright yellow pigment characteristic of mature sori. Although these mutant strains do not form fruiting bodies, many of the cells encapsulate within the mounds. Sporulation occurs about 6 hours earlier in yelA- cells than in wild-type cells, accompanied by precocious expression of a prespore gene, spiA. However, the spores are defective and lose viability over a period of several hours. Unencapsulated cells also die unless they are dissociated from the mounds and shaken in suspension. The yelA gene was isolated by plasmid rescue and found to encode a protein of 102 kDa in which the N-terminal sequence shows significant similarity to domains found in the eIF-4G subunits of the translational initiation complex eIF-4F. In wild-type cells yelA mRNA first accumulates at 8 hours of development and is maintained in both prespore and prestalk cells until culmination when it is found only is stalk cells. Mutations in yelA can partially suppress the block to sporulation in mutant strains in which either of the prestalk genes tagB or tagC is disrupted such that an encapsulation signal is not produced. It appears that premature encapsulation is normally inhibited by YelA until a signal is received from prestalk cells during culmination.

Dictyostelium development-socializing through cAMP

Starving Dictyostelium amoebae use cAMP as a chemoattractant to gather into aggregates, as a hormone-like signal to induce cell differentiation, and as an intracellular messenger to control stalk- and spore cell maturation and germination of spores. In this chapter we describe the respective roles of the three adenylyl cyclases ACA, ACB and ACG in controlling cAMP signaling during development and we discuss how cAMP signals are processed by the cells to trigger the large repertoire of gene regulatory events that is under control of this signal molecule.

High levels of actin tyrosine phosphorylation: correlation with the dormant state of Dictyostelium spores

Upon removal of nutrients, the amoebae of the cellular slime mold Dictyostelium discoideum differentiate into dormant spores which survive starvation stress. In this study, we demonstrate that half of the actin molecules in the spores are tyrosine-phosphorylated. The phosphorylated actin is distributed around immobile crenate mitochondria and vesicles, as well as in the cytoplasm of the spores. The actin isolated from spore lysates contains phosphorylated and unphosphorylated forms at the same molar ratio as that of the original whole spore lysate. Under actin polymerizing conditions they form actin filaments and then they are completely depolymerized under actin depolymerizing conditions, indicating that tyrosine phosphorylation of actin may not prohibit actin polymerization nor stimulate depolymerization. The phosphorylation levels increase at the end of the culmination stage when spores have matured morphologically and physiologically, and reach maximum levels after an additional 12 hours of development. The levels are stable for 20 days following spore maturation, and decline to undetectable levels within the next 10 days. Spores having high levels of phosphorylation show high viability, and vice versa. Following activation of spores with nutrient medium containing spore germination promoters, the phosphorylation levels quickly decrease with a half-life of about 5 minutes. After 20 minutes spores begin to swell. At this later time, most of the phosphorylated actin already has been dephosphorylated. Also, in heat-activated spores actin dephosphorylation occurs prior to spore swelling. However, addition of phosphatase inhibitors following heat-activation, prevented spore swelling and dephosphorylation of actin. Our data indicate that the high levels of actin tyrosine phosphorylation, specific to the spore stage, may be required for maintaining dormancy to withstand starvation stress. The rapid dephosphorylation of actin leads to a reactivated dynamic actin system which participates in spore swelling, vesicle movement, and mitochondrial shape changes during the spore germination process.

A Serum Response Factor homolog is required for spore differentiation in Dictyostelium

A homolog of the Serum Response Factor (SRF) has been isolated from Dictyostelium discoideum and its function studied by analyzing the consequences of its gene disruption. The MADS-box region of Dictyostelium SRF (DdSRF) is highly conserved with those of the human, Drosophila and yeast homologs. srfA is a developmentally regulated gene expressed in prespore and spore cells. This gene plays an essential role in sporulation as its disruption leads to abnormal spore morphology and loss of viability. The mutant spores were round and cellulose deposition seemed to be partially affected. Initial prestalk and prespore cell differentiation did not seem to be compromised in the mutant since the expression of several cell-type-specific markers were found to be unaffected. However, the mRNA level of the spore marker spiA was greatly reduced. Activation of the cAMP-dependent protein kinase (PKA) by 8-Br-cAMP was not able to fully bypass the morphological defects of srfA- mutant spores, although this treatment induced spiA mRNA expression. Our results suggest that DdSRF is required for full maturation of spores and participates in the regulation of the expression of the spore-coat marker spiA and probably other maturation genes necessary for proper spore cell differentiation.

The Croonian Lecture 1997. The phosphorylation of proteins on tyrosine: its role in cell growth and disease

The reversible phosphorylation of tyrosines in proteins plays a key role in regulating many different processes in eukaryotic organisms, such as growth control, cell cycle control, differentiation cell shape and movement, gene transcription, synaptic transmission, and insulin action. Phosphorylation of proteins is brought about by enzymes called protein-tyrosine kinases that add phosphate to specific tyrosines in target proteins; phosphate is removed from phosphorylated tyrosines by enzymes called protein-tyrosine phosphatases. Phosphorylated tyrosines are recognized by specialized binding domains on other proteins, and such interactions are used to initiate intracellular signaling pathways. Currently, more than 95 protein-tyrosine kinases and more than 55 protein-tyrosine phosphatase genes are known in Homo sapiens. Aberrant tyrosine phosphorylation is a hallmark of many types of cancer and other human diseases. Drugs are being developed that antagonize the responsible protein-tyrosine kinases and phosphatases in order to combat these diseases.

SH2 signaling in a lower eukaryote: a STAT protein that regulates stalk cell differentiation in dictyostelium

The TTGA-binding factor is a transcriptional regulator activated by DIF, the chlorinated hexaphenone that induces prestalk cell differentiation in Dictyostelium. The same activity also functions as a repressor, controlling stalk cell differentiation. We show that the TTGA-binding factor is a STAT protein. Like the metazoan STATs, it functions via the reciprocal interaction of a phosphotyrosine residue on one molecule with an SH2 domain on a dimerizing partner. Furthermore, it will bind specifically to a mammalian interferon-stimulated response element. In Saccharomyces cerevisiae, where the entire genomic sequence is known, SH2 domains have not been identified. It would seem, therefore, that SH2 signaling pathways arose very early in the evolution of multicellular organisms, perhaps to facilitate intercellular comunication.