Dual-specificity protein phosphatase Yvh1p, which is required for vegetative growth and sporulation, interacts with yeast pescadillo homolog in Saccharomyces cerevisiae

The ribosome assembly factor Nop53 has a structural role in the formation of nuclear pre-60S intermediates, affecting late maturation events

Eukaryotic ribosome biogenesis is an elaborate process during which ribosomal proteins assemble with the pre-rRNA while it is being processed and folded. Hundreds of assembly factors (AF) are required and transiently recruited to assist the sequential remodeling events. One of the most intricate ones is the stepwise removal of the internal transcribed spacer 2 (ITS2), between the 5.8S and 25S rRNAs, that constitutes together with five AFs the pre-60S ‘foot’. In the transition from nucleolus to nucleoplasm, Nop53 replaces Erb1 at the basis of the foot and recruits the RNA exosome for the ITS2 cleavage and foot disassembly. Here we comprehensively analyze the impact of Nop53 recruitment on the pre-60S compositional changes. We show that depletion of Nop53, different from nop53 mutants lacking the exosome-interacting motif, not only causes retention of the unprocessed foot in late pre-60S intermediates but also affects the transition from nucleolar state E particle to subsequent nuclear stages. Additionally, we reveal that Nop53 depletion causes the impairment of late maturation events such as Yvh1 recruitment. In light of recently described pre-60S cryo-EM structures, our results provide biochemical evidence for the structural role of Nop53 rearranging and stabilizing the foot interface to assist the Nog2 particle formation.

Zinc-binding domain mediates pleiotropic functions of Yvh1 in Cryptococcus neoformans

Yvh1 is a dual-specificity phosphatase (DUSP) that is evolutionarily conserved in eukaryotes, including yeasts and humans. Yvh1 is involved in the vegetative growth, differentiation, and virulence of animal and plant fungal pathogens. All Yvh1 orthologs have a conserved DUSP catalytic domain at the N-terminus and a zinc-binding (ZB) domain with two zinc fingers (ZFs) at the C-terminus. Although the DUSP domain is implicated in the regulation of MAPK signaling in humans, only the ZB domain is essential for most cellular functions of Yvh1 in fungi. This study aimed to analyze the functions of the DUSP and ZB domains of Yvh1 in the human fungal pathogen Cryptococcus neoformans, whose Yvh1 (CnYvh1) contains a DUSP domain at the C-terminus and a ZB domain at the N-terminus. Notably, CnYvh1 has an extended internal domain between the two ZF motifs in the ZB domain. To elucidate the function of each domain, we constructed individual domain deletions and swapping strains by complementing the yvh1Δ mutant with wild-type (WT) or mutated YVH1 alleles and examined their Yvh1-dependent phenotypes, including growth under varying stress conditions, mating, and virulence factor production. Here, we found that the complementation of the yvh1Δ mutant with the mutated YVH1 alleles having two ZFs of the ZB domain, but not the DUSP and extended internal domains, restored the WT phenotypic traits in the yvh1Δ mutant. In conclusion, the ZB domain, but not the N-terminal DUSP domain, plays a pivotal role in the pathobiological functions of cryptococcal Yvh1.

Characterization of Single Gene Deletion Mutants Affecting Alternative Oxidase Production in Neurospora crassa: Role of the yvh1 Gene

The Neurospora crassa AOD1 protein is a mitochondrial alternative oxidase that passes electrons directly from ubiquinol to oxygen. The enzyme is encoded by the nuclear aod-1 gene and is produced when the standard electron transport chain is inhibited. We previously identified eleven strains in the N. crassa single gene deletion library that were severely deficient in their ability to produce AOD1 when grown in the presence of chloramphenicol, an inhibitor of mitochondrial translation that is known to induce the enzyme. Three mutants affected previously characterized genes. In this report we examined the remaining mutants and found that the deficiency of AOD1 was due to secondary mutations in all but two of the strains. One of the authentic mutants contained a deletion of the yvh1 gene and was found to have a deficiency of aod-1 transcripts. The YVH1 protein localized to the nucleus and a post mitochondrial pellet from the cytoplasm. A zinc binding domain in the protein was required for rescue of the AOD1 deficiency. In other organisms YVH1 is required for ribosome assembly and mutants have multiple phenotypes. Lack of YVH1 in N. crassa likely also affects ribosome assembly leading to phenotypes that include altered regulation of AOD1 production.

Ribosome-stalk biogenesis is coupled with recruitment of nuclear-export factor to the nascent 60S subunit

Nuclear export of preribosomal subunits is a key step during eukaryotic ribosome formation. To efficiently pass through the FG-repeat meshwork of the nuclear pore complex, the large pre-60S subunit requires several export factors. Here we describe the mechanism of recruitment of the Saccharomyces cerevisiae RNA-export receptor Mex67-Mtr2 to the pre-60S subunit at the proper time. Mex67-Mtr2 binds at the premature ribosomal-stalk region, which later during translation serves as a binding platform for translational GTPases on the mature ribosome. The assembly factor Mrt4, a structural homolog of cytoplasmic-stalk protein P0, masks this site, thus preventing untimely recruitment of Mex67-Mtr2 to nuclear pre-60S particles. Subsequently, Yvh1 triggers Mrt4 release in the nucleus, thereby creating a narrow time window for Mex67-Mtr2 association at this site and facilitating nuclear export of the large subunit. Thus, a spatiotemporal mark on the ribosomal stalk controls the recruitment of an RNA-export receptor to the nascent 60S subunit.

Genome-wide functional analysis of Plasmodium protein phosphatases reveals key regulators of parasite development and differentiation

Reversible protein phosphorylation regulated by kinases and phosphatases controls many cellular processes. Although essential functions for the malaria parasite kinome have been reported, the roles of most protein phosphatases (PPs) during Plasmodium development are unknown. We report a functional analysis of the Plasmodium berghei protein phosphatome, which exhibits high conservation with the P. falciparum phosphatome and comprises 30 predicted PPs with differential and distinct expression patterns during various stages of the life cycle. Gene disruption analysis of P. berghei PPs reveals that half of the genes are likely essential for asexual blood stage development, whereas six are required for sexual development/sporogony in mosquitoes. Phenotypic screening coupled with transcriptome sequencing unveiled morphological changes and altered gene expression in deletion mutants of two N-myristoylated PPs. These findings provide systematic functional analyses of PPs in Plasmodium, identify how phosphatases regulate parasite development and differentiation, and can inform the identification of drug targets for malaria.

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Phylogenetic analysis identifies 30 Plasmodium berghei protein phosphatases (PPs)

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Functional analysis reveals role for six PPs in sexual development/sporogony

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Two N-myristoylated PPs play key roles in sex allocation and parasite transmission

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RNA-Seq highlights significantly altered gene clusters in the N-myristoylated PP mutants

Functional analysis of the Fusarium graminearum phosphatome

Phosphatases are known to play important roles in the regulation of various cellular processes in eukaryotes. However, systematic characterization of the phosphatome has not been reported in phytopathogenic fungi. The wheat scab fungus Fusarium graminearum contains 82 putative phosphatases. The biological functions of each phosphatase were investigated in this study. Although 11 phosphatase genes appeared to be essential, deletion mutants of the other 71 phosphatase genes were obtained and characterized for changes in 15 phenotypes, including vegetative growth, nutrient response and virulence. Overall, the deletion of 63 phosphatase genes resulted in changes in at least one of the phenotypes assayed. Interestingly, the deletion of four genes (Fg06297, Fg03333, Fg03826 and Fg07932) did not dramatically affect hyphal growth, but led to strongly reduced virulence. Western blot analyses showed that three phosphatases (Fg10516, Fg03333 and Fg12867) functioned as negative regulators of the mitogen-activated protein kinase signaling pathways. In addition, we found, for the first time, that FgCdc14 is dispensable for growth, but plays an important role in ribosome biogenesis. Overall, in this first functional characterization of the fungal phosphatome, phosphatases important for various aspects of hyphal growth, development, plant infection and secondary metabolism were identified in the phytopathogenic fungus F. graminearum.

Genetic interactions of ribosome maturation factors Yvh1 and Mrt4 influence mRNA decay, glycogen accumulation, and the expression of early meiotic genes in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Yvh1, a dual-specificity protein phosphatase involved in glycogen accumulation and sporulation, is required for normal vegetative growth. To further elucidate the role of Yvh1, we generated dominant mutants suppressing the slow growth caused by YVH1 disruption. One of the mutant alleles, designated as SVH1-1 (suppressor of Δyvh1 deletion), was identical to MRT4 (mRNA turnover) that contained a single-base substitution causing an amino acid change from Gly(68) to Asp. Mrt4(G68D) restored the deficiencies in growth and rRNA biogenesis that occurs in absence of Yvh1. Here, we report that the interaction between Mrt4 and Yvh1 is also essential for normal glycogen accumulation and mRNA decay as well as the induction of sporulation genes IME2, SPO13 and HOP1. The Mrt4(G68D) could restore the plethora of phenotypes we observed in absence of Yvh1. We found that Yvh1 is not essential for wild-type induction of the transcriptional regulator of these genes, IME1, suggesting that either translation or post-translational modification to activate Ime1 has been compromised. Since a defect in ribosome biogenesis in general can be related to other various defects, the ribosome biogenesis defect caused by absence of Yvh1 might be an indirect cause of observed phenotypes.

Yvh1 is required for a late maturation step in the 60S biogenesis pathway

The step by step assembly process from preribosome in the nucleus to translation-competent 60S ribosome subunit in the cytoplasm is revealed (also see Lo et al. in this issue).

Before entering translation, preribosomal particles undergo sequential late maturation steps. In the case of pre-60S particles, these steps involve the release of shuttling maturation factors and transport receptors. In this study, we report a new maturation step in the 60S biogenesis pathway in budding yeast. We show that efficient release of the nucleolar/nuclear ribosomal-like protein Mrt4 (homologous to the acidic ribosomal P-protein Rpp0) from pre-60S particles requires the highly conserved protein Yvh1, which associates only with late pre-60S particles. Cell biological and biochemical analyses reveal that Mrt4 fails to dissociate from late pre-60S particles in yvh1Δ cells, inducing a delay in nuclear pre–ribosomal RNA processing and a pre-60S export defect in yvh1Δ cells. Moreover, we have isolated gain of function alleles of Mrt4 that specifically bypass the requirement for Yvh1 and rescue all yvh1Δ-associated phenotypes. Together, our data suggest that Yvh1-mediated release of Mrt4 precedes cytoplasmic loading of Rpp0 on pre-60S particles and is an obligatory late step toward construction of translation-competent 60S subunits.

Ribosome stalk assembly requires the dual-specificity phosphatase Yvh1 for the exchange of Mrt4 with P0

The step by step assembly process from preribosome in the nucleus to translation-competent 60S ribosome subunit in the cytoplasm is revealed (also see Kemmler et al. in this issue).

The ribosome stalk is essential for recruitment of translation factors. In yeast, P0 and Rpl12 correspond to bacterial L10 and L11 and form the stalk base of mature ribosomes, whereas Mrt4 is a nuclear paralogue of P0. In this study, we show that the dual-specificity phosphatase Yvh1 is required for the release of Mrt4 from the pre-60S subunits. Deletion of YVH1 leads to the persistence of Mrt4 on pre-60S subunits in the cytoplasm. A mutation in Mrt4 at the protein–RNA interface bypasses the requirement for Yvh1. Pre-60S subunits associated with Yvh1 contain Rpl12 but lack both Mrt4 and P0. These results suggest a linear series of events in which Yvh1 binds to the pre-60S subunit to displace Mrt4. Subsequently, P0 loads onto the subunit to assemble the mature stalk, and Yvh1 is released. The initial assembly of the ribosome with Mrt4 may provide functional compartmentalization of ribosome assembly in addition to the spatial separation afforded by the nuclear envelope.

The dual-specificity phosphatase hYVH1 interacts with Hsp70 and prevents heat-shock-induced cell death

hYVH1 [human orthologue of YVH1 (yeast VH1-related phosphatase)] is an atypical dual-specificity phosphatase that is widely conserved throughout evolution. Deletion studies in yeast have suggested a role for this phosphatase in regulating cell growth. However, the role of the human orthologue is unknown. The present study used MS to identify Hsp70 (heat-shock protein 70) as a novel hYVH1-binding partner. The interaction was confirmed using endogenous co-immunoprecipitation experiments and direct binding of purified proteins. Endogenous Hsp70 and hYVH1 proteins were also found to co-localize specifically to the perinuclear region in response to heat stress. Domain deletion studies revealed that the ATPase effector domain of Hsp70 and the zinc-binding domain of hYVH1 are required for the interaction, indicating that this association is not simply a chaperone-substrate complex. Thermal phosphatase assays revealed hYVH1 activity to be unaffected by heat and only marginally affected by non-reducing conditions, in contrast with the archetypical dual-specificity phosphatase VHR (VH1-related protein). In addition, Hsp70 is capable of increasing the phosphatase activity of hYVH1 towards an exogenous substrate under non-reducing conditions. Furthermore, the expression of hYVH1 repressed cell death induced by heat shock, H2O2 and Fas receptor activation but not cisplatin. Co-expression of hYVH1 with Hsp70 further enhanced cell survival. Meanwhile, expression of a catalytically inactive hYVH1 or a hYVH1 variant that is unable to interact with Hsp70 failed to protect cells from the various stress conditions. The results suggest that hYVH1 is a novel cell survival phosphatase that co-operates with Hsp70 to positively affect cell viability in response to cellular insults.

A mutant plasma membrane protein is stabilized upon loss of Yvh1, a novel ribosome assembly factor

Pma1-10 is a mutant plasma membrane ATPase defective at the restrictive temperature in stability at the cell surface. At 37 degrees, Pma1-10 is ubiquitinated and internalized from the plasma membrane for degradation in the vacuole. YVH1, encoding a tyrosine phosphatase, is a mutant suppressor of pma1-10; in the absence of Yvh1, Pma1-10 remains stable at the plasma membrane, thereby permitting cells to grow. The RING finger domain of Yvh1, but not its phosphatase domain, is required for removal of mutant Pma1-10 from the plasma membrane. Yvh1 is a novel ribosome assembly factor: in yvh1Delta cells, free 60S and 80S ribosomal subunits are decreased, free 40S subunits are increased, and half-mer polysomes are accumulated. Pma1-10 is also stabilized by deletion of 60S ribosomal proteins Rpl19a and Rpl35a. We propose that changes in ribosome biogenesis caused by loss of Yvh1 or specific ribosomal proteins have effects on the plasma membrane, perhaps by producing specific translational changes.

The Candida albicans pescadillo homolog is required for normal hypha-to-yeast morphogenesis and yeast proliferation

A single species, Candida albicans, causes half of all invasive fungal infections in humans. Unlike other fungal pathogens, this organism switches between growth as budding yeast and as pseudohyphal and hyphal filaments in host organs and in vitro. Both cell types play a role in invasive disease: while hyphal and pseudohyphal filaments penetrate host cells and tissues, yeast cells are likely to facilitate dissemination through the bloodstream and establishment of distant foci of infection. Many regulators of the yeast-to-hypha switch have emerged from intensive investigations of this morphogenetic step, but the hypha-to-yeast switch remains poorly understood. Using a forward genetic approach, a novel putative regulator involved in the hypha-to-yeast switch was identified, the C. albicans pescadillo homolog, PES1. In eukaryotes from yeast to human, pescadillo homologs are involved in cell cycle control and ribosome biogenesis, and are essential. We find a pescadillo homolog to act in fungal morphogenesis, specifically in lateral yeast growth on filamentous cells. We also find essentiality of PES1 in C. albicans to be dependent on cell type, because hyphal cells, but not yeast cells, tolerate its loss. PES1 is therefore critical for completion of the C. albicans life cycle, in which the fungus switches between filamentous and yeast growth. Consistent with these in vitro findings, PES1 is required for C. albicans virulence in an in vivo insect model of infection.

Organ-specific requirements for Hdac1 in liver and pancreas formation

Liver, pancreas and lung originate from the presumptive foregut in temporal and spatial proximity. This requires precisely orchestrated transcriptional activation and repression of organ-specific gene expression within the same cell. Here, we show distinct roles for the chromatin remodelling factor and transcriptional repressor Histone deacetylase 1 (Hdac1) in endodermal organogenesis in zebrafish. Loss of Hdac1 causes defects in timely liver specification and in subsequent differentiation. Mosaic analyses reveal a cell-autonomous requirement for hdac1 within the hepatic endoderm. Our studies further reveal specific functions for Hdac1 in pancreas development. Loss of hdac1 causes the formation of ectopic endocrine clusters anteriorly to the main islet, as well as defects in exocrine pancreas specification and differentiation. In addition, we observe defects in extrahepatopancreatic duct formation and morphogenesis. Finally, loss of hdac1 results in an expansion of the foregut endoderm in the domain from which the liver and pancreas originate. Our genetic studies demonstrate that Hdac1 is crucial for regulating distinct steps in endodermal organogenesis. This suggests a model in which Hdac1 may directly or indirectly restrict foregut fates while promoting hepatic and exocrine pancreatic specification and differentiation, as well as pancreatic endocrine islet morphogenesis. These findings establish zebrafish as a tractable system to investigate chromatin remodelling factor functions in controlling gene expression programmes in vertebrate endodermal organogenesis.

Ddi1p and Rad23p play a cooperative role as negative regulators in the PHO pathway in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the PHO pathway regulates expression of phosphate-responsive genes such as PHO5, which encodes repressible acid phosphatase (rAPase). In this pathway, Pho81p functions as an inhibitor of the cyclin-cyclin-dependent kinase (CDK) complex Pho80p-Pho85p. However, the mechanism regulating the inhibitory activity of Pho81p is poorly understood. Through use of the yeast two-hybrid system, we identified the UbL-UbA protein Ddi1p as a Pho81p-binding protein. Further, Pho81p levels were found to be low under high-phosphate condition and high during phosphate starvation, indicating that Pho81p is regulated by phosphate concentration. However, our results revealed that Ddi1p and its associated protein Rad23p are not involved in the decrease in Pho81p level under high-phosphate condition. Significantly, the Deltaddi1Deltarad23 strain exhibited a remarkable increase in rAPase activity at an intermediate-phosphate concentration of 0.4mM, suggesting that Ddi1p and Rad23p play a cooperative role as negative regulators in the PHO pathway.

Light chain 1 of microtubule-associated protein 1B can negatively regulate the action of Pes1

Pes1 was first identified as the locus affected in the zebrafish mutant pescadillo, which exhibits severe defects in gut and liver development. It has since been demonstrated that loss of Pes1 expression in mammals and yeast affects ribosome biogenesis, resulting in a block in cell proliferation. Pes1 contains a BRCA1 C-terminal domain, a structural motif that has been shown to facilitate protein-protein interactions, suggesting that Pes1 has binding partners. We used a yeast two-hybrid screen to identify putative interacting proteins. We found that light chain 1 of the microtubule-associated protein 1B (Mtap1b-LC1) could partner with Pes1, and deletion analyses revealed a specific interaction of Mtap1b-LC1 with the Pes1 BRCA1 C-terminal domain. We confirmed the integrity of the interaction between Pes1 and Mtap1b-LC1 by co-immunoprecipitation experiments. Protein localization studies in NIH3T3 cells revealed that exogenously expressed Pes1 was typically restricted to nuclei and nucleoli. However, exogenous Pes1 was found predominantly in the cytoplasm in cells that were forced to express Mtap1b-LC1. We also observed that the expression of endogenous Pes1 protein was significantly reduced or undetectable in nuclei when Mtap1b-LC1 was overexpressed, implying that a dynamic interaction exists between the two proteins and that Mtap1b-LC1 has the potential to negatively impact Pes1 function. Finally, we demonstrated that, as is the case when Pes1 expression is depleted by shRNA, overexpression of Mtap1b-LC1 resulted in diminished proliferation of NIH3T3 cells, suggesting that Mtap1b-LC1 has the potential to repress cell proliferation by modulating the nucleolar levels of Pes1.

TOR regulates late steps of ribosome maturation in the nucleoplasm via Nog1 in response to nutrients

The protein kinase TOR (target of rapamycin) controls several steps of ribosome biogenesis, including gene expression of rRNA and ribosomal proteins, and processing of the 35S rRNA precursor, in the budding yeast Saccharomyces cerevisiae. Here we show that TOR also regulates late stages of ribosome maturation in the nucleoplasm via the nuclear GTP-binding protein Nog1. Nog1 formed a complex that included 60S ribosomal proteins and pre-ribosomal proteins Nop7 and Rlp24. The Nog1 complex shuttled between the nucleolus and the nucleoplasm for ribosome biogenesis, but it was tethered to the nucleolus by both nutrient depletion and TOR inactivation, causing cessation of the late stages of ribosome biogenesis. Furthermore, after this, Nog1 and Nop7 proteins were lost, leading to complete cessation of ribosome maturation. Thus, the Nog1 complex is a critical regulator of ribosome biogenesis mediated by TOR. This is the first description of a physiological regulation of nucleolus-to-nucleoplasm translocation of pre-ribosome complexes.

The Pyst2-L phosphatase is involved in cell-crowding

The dual-specificity phosphatase Pyst2-L was found to be over expressed in leukocytes derived from AML and ALL patients as well as in certain other solid tumors and lymphoblastoid cell lines. Pyst2-L, binds and dephosphorylates both pERKs and pJNKs proteins, and thus, plays a role in regulating the MAP kinase signaling pathway. In the present study, a comparative genomic application was used and sequence analysis of multi-organisms databases were searched in order to identify genes homologous to Pyst2-L. The Xenopus laevis MAP kinase phosphatase X17c gene and the Yeast nitrogen starvation-induced protein phosphatase Yvh1p gene were revealed to be highly homologous with Pyst2-L. Both X17c and Yvh1p genes play a role in cell cycle regulation. A down regulated expression of the Yvh1p gene occurred in Saccharomyces cerevisiae that were synchronized to the G2-phase of the cell cycle by alpha-factor. In conformity with this result, a reduction in Pyst2-L expression levels was observed in G2-phase-synchronized Human K562 cells. Finally, we were able to show that cells in highly crowded cultures express high levels of the Pyst2-L phosphatase. These observations may indicate that low levels of the Pyst2-L phosphatase are essential for the G2-phase of the cell cycle and that this phosphatase might play a role in signaling cascades induced by cellular crowding.

A putative dual-specific protein phosphatase encoded by YVH1 controls growth, filamentation and virulence in Candida albicans

In response to stimulants, such as serum, the yeast cells of the opportunistic fungal pathogen Candida albicans form germ tubes, which develop into hyphae. Yvh1p, one of the 29 protein phosphatases encoded in the C. albicans genome, has 45% identity with the dual-specific phosphatase Yvh1p of the model yeast Saccharomyces cerevisiae. In this study, Yvh1p expression was not observed during the initial step of germ tube formation, although Yvh1p was expressed constitutively in cell cycle progression of yeast or hyphal cells. In an attempt to analyse the function of Yvh1p phosphatase, the complete ORFs of both alleles were deleted by replacement with hph200-URA3-hph200 and ARG4. Although YVH1 has nine single-nucleotide polymorphisms in its coding sequence, both YVH1 alleles were able to complement the YVH1 gene disruptant. The vegetative growth of Deltayvh1 was significantly slower than the wild-type. The hyphal growth of Deltayvh1 on agar, or in a liquid medium, was also slower than the wild-type because of the delay in nuclear division and septum formation, although germ tube formation was similar between the wild-type and the disruptant. Despite the slow hyphal growth, the expression of several hypha-specific genes in Deltayvh1 was not delayed or repressed compared with that of the wild-type. Infection studies using mouse models revealed that the virulence of Deltayvh1 was less than that of the wild-type. Thus, YVH1 contributes to normal vegetative yeast or hyphal cell cycle progression and pathogenicity, but not to germ tube formation.

Mapping and characterization of the amplicon near APOA2 in 1q23 in human sarcomas by FISH and array CGH

BACKGROUND

Amplification of the q21-q23 region on chromosome 1 is frequently found in sarcomas and a variety of other solid tumours. Previous analyses of sarcomas have indicated the presence of at least two separate amplicons within this region, one located in 1q21 and one located near the apolipoprotein A-II (APOA2) gene in 1q23. In this study we have mapped and characterized the amplicon in 1q23 in more detail.

RESULTS

We have used fluorescence in situ hybridisation (FISH) and microarray-based comparative genomic hybridisation (array CGH) to map and define the borders of the amplicon in 10 sarcomas. A subregion of approximately 800 kb was identified as the core of the amplicon. The amplification patterns of nine possible candidate target genes located to this subregion were determined by Southern blot analysis. The genes activating transcription factor 6 (ATF6) and dual specificity phosphatase 12 (DUSP12) showed the highest level of amplification, and they were also shown to be over-expressed by quantitative real-time reverse transcription PCR (RT-PCR). In general, the level of expression reflected the level of amplification in the different tumours. DUSP12 was expressed significantly higher than ATF6 in a subset of the tumours. In addition, two genes known to be transcriptionally activated by ATF6, glucose-regulated protein 78 kDa and -94 kDa (GRP78 and GRP94), were shown to be over-expressed in the tumours that showed over-expression of ATF6.

CONCLUSION

ATF6 and DUSP12 seem to be the most likely candidate target genes for the 1q23 amplification in sarcomas. Both genes have possible roles in promoting cell growth, which makes them interesting candidate targets.

A zinc-binding dual-specificity YVH1 phosphatase in the malaria parasite, Plasmodium falciparum, and its interaction with the nuclear protein, pescadillo

Biochemical evidence revealed protein tyrosine kinase and phosphatase activities in the human malarial parasite Plasmodium falciparum, a member of the Apicomplexa. A novel cDNA sequence of a dual-specificity phosphatase was identified in both sexual and asexual stages of P. falciparum, and named PfYVH1, since the predicted primary structure of the 278-amino acid polypeptide showed significant similarity to the human and yeast YVH1 phosphatases. The N-terminal half of PfYVH1 contained a conserved tyrosine phosphatase catalytic domain within a dual-specificity phosphatase domain. The C-terminal region, consisting of one histidine and eight cysteines, represented a zinc-binding domain with a potentially unconventional architecture. Recombinant PfYVH1 contained 2mol of zinc per mol protein and dephosphorylated both phosphoserine and phosphotyrosine residues. Mutation of specific Cys residues in the putative zinc finger region abolished zinc binding and drastically reduced phosphatase activity, suggesting an allosteric role of zinc in catalysis. PfYVH1 was expressed in essentially all erythrocytic stages of the parasite, and shuttled between the nucleus and the cytoplasm in a stage-specific manner. A Plasmodium ortholog of the nuclear pescadillo protein (PfPES) was also characterized and shown to interact with PfYVH1, thus implicating PfYVH1 in the regulation of parasitic development. PfYVH1 represents the first dual-specificity zinc-finger phosphatase characterized in the protozoan kingdom.

Yph1p, an ORC-interacting protein: potential links between cell proliferation control, DNA replication, and ribosome biogenesis

Immunoprecipitation of the origin recognition complex (ORC) from yeast extracts identified Yph1p, an essential protein containing a BRCT domain. Two Yph1p complexes were characterized. Besides ORC, MCM proteins, cell-cycle regulatory proteins, checkpoint proteins, 60S ribosomal proteins, and preribosome particle proteins were found to be associated with Yph1p. Yph1p is predominantly nucleolar and is required for 60S ribosomal subunit biogenesis and possibly for translation on polysomes. Proliferating cells depleted of Yph1p arrest in G(1) or G(2), with no cells in S phase, or significantly delay S phase progression after release from a hydroxyurea arrest. Yph1p levels decline as cells commit to exit the cell cycle, and levels vary depending on energy source. Yph1p may link cell proliferation control to DNA replication, ribosome biogenesis, and translation on polysomes.