Characterization of the Prk1 protein kinase from Schizosaccharomyces pombe

Identification of Flo11-like Adhesin in Schizosaccharomyces pombe and the Mechanism of Small-Molecule Compounds Mediating Biofilm Formation in Yeasts

Fungal infection is initiated by the adhesion of pathogens to biotic and abiotic surfaces, with various manifestations including biofilm formation and invasive growth, etc. A previous report, though devoid of functional data, speculated that the Schizosaccharomyces pombe glycoprotein SPBPJ4664.02 could be the homology of Saccharomyces cerevisiae Flo11. Here, our studies with S. pombe substantiated the previously proposed speculation by (1) the deletion of SPBPJ4664.02 attenuated biofilm formation and invasive growth in S. pombe; (2) the S. pombe's lack of SPBPJ4664.02 could be complemented by expressing S. cerevisiae flo11. Furthermore, indole-3-acetic acid (IAA) and dodecanol were examined in S. pombe for their respective effects on biofilm formation. IAA and dodecanol at high concentrations could inhibit biofilm formation, whereas opposing effects were observed with low concentrations of these molecules. Mechanism studies with the SPBPJ4664.02Δ and SPBPJ4664.02Δ/flo11OE versus the wild type have demonstrated that IAA or dodecanol might exert regulatory effects downstream of SPBPJ4664.02 in the signaling pathway for biofilm formation. Moreover, our research extrapolated to Candida albicans has pinpointed that IAA inhibited biofilm formation at high concentrations, consistent with the transcriptional downregulation of the biofilm-related genes. Dodecanol suppressed C. albicans biofilm formation at all the concentrations tested, in accord with the downregulation of biofilm-related transcripts.

The Dual-Specificity LAMMER Kinase Affects Stress-Response and Morphological Plasticity in Fungi

The morphological plasticity of fungal pathogens has long been implicated in their virulence and is often influenced by extracellular factors. Complex signal transduction cascades are critical for sensing stresses imposed by external cues such as antifungal drugs, and for mediating appropriate cellular responses. Many of these signal transduction cascades are well-conserved and involve in the distinct morphogenetic processes during the life cycle of the pathogenic fungi. The dual-specificity LAMMER kinases are evolutionarily conserved across species ranging from yeasts to mammals and have multiple functions in various physiological processes; however, their functions in fungi are relatively unknown. In this review, we first describe the involvement of LAMMER kinases in cell surface changes, which often accompany alterations in growth pattern and differentiation. Then, we focus on the LAMMER kinase-dependent molecular machinery responsible for the stress responses and cell cycle regulation. Last, we discuss the possible cross-talk between LAMMER kinases and other signaling cascades, which integrates exogenous and host signals together with genetic factors to affect the morphological plasticity and virulence in fungi.

LAMMER Kinase Lkh1 Is an Upstream Regulator of Prk1-Mediated Non-Sexual Flocculation in Fission Yeast

The cation-dependent galactose-specific flocculation activity of the Schizosaccharomyces pombe null mutant of lkh1 ⁺, the gene encoding LAMMER kinase homolog, has previously been reported by our group. Here, we show that disruption of prk1 ⁺, another flocculation associated regulatory kinase encoding gene, also resulted in cation-dependent galactose-specific flocculation. Deletion of prk1 increased the flocculation phenotype of the lkh1 ⁺ null mutant and its overexpression reversed the flocculation of cells caused by lkh1 deletion. Transcript levels of prk1 ⁺ were also decreased by lkh1 ⁺ deletion. Cumulatively, these results indicate that Lkh1 is one of the negative regulators acting upstream of Prk1, regulating non-sexual flocculation in fission yeast.

Glycan Alteration Imparts Cellular Resistance to a Membrane-Lytic Anticancer Peptide

Although resistance toward small-molecule chemotherapeutics has been well studied, the potential of tumor cells to avoid destruction by membrane-lytic compounds remains unexplored. Anticancer peptides (ACPs) are a class of such agents that disrupt tumor cell membranes through rapid and non-stereospecific mechanisms, encouraging the perception that cellular resistance toward ACPs is unlikely to occur. We demonstrate that eukaryotic cells can, indeed, develop resistance to the model oncolytic peptide SVS-1, which preferentially disrupts the membranes of cancer cells. Utilizing fission yeast as a model organism, we show that ACP resistance is largely controlled through the loss of cell-surface anionic saccharides. A similar mechanism was discovered in mammalian cancer cells where removal of negatively charged sialic acid residues directly transformed SVS-1-sensitive cell lines into resistant phenotypes. These results demonstrate that changes in cell-surface glycosylation play a major role in tumor cell resistance toward oncolytic peptides.

Two conserved modules of Schizosaccharomyces pombe Mediator regulate distinct cellular pathways

Mediator is an evolutionary conserved coregulator complex required for transcription of almost all RNA polymerase II-dependent genes. The Schizosaccharomyces pombe Mediator consists of two dissociable components—a core complex organized into a head and middle domain as well as the Cdk8 regulatory subcomplex. In this work we describe a functional characterization of the S. pombe Mediator. We report the identification of the S. pombe Med20 head subunit and the isolation of ts alleles of the core head subunit encoding med17⁺. Biochemical analysis of med8^ts, med17^ts, Δmed18, Δmed20 and Δmed27 alleles revealed a stepwise head domain molecular architecture. Phenotypical analysis of Cdk8 and head module alleles including expression profiling classified the Mediator mutant alleles into one of two groups. Cdk8 module mutants flocculate due to overexpression of adhesive cell-surface proteins. Head domain-associated mutants display a hyphal growth phenotype due to defective expression of factors required for cell separation regulated by transcription factor Ace2. Comparison with Saccharomyces cerevisiae Mediator expression data reveals that these functionally distinct modules are conserved between S. pombe and S. cerevisiae.

Molecular phylogenetics of ascomycotal adhesins--a novel family of putative cell-surface adhesive proteins in fission yeasts

In this work, we identify a family of putative adhesins in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. The members of this family share a conserved tandem repeat related to those found in the Candida albicans Als family of adhesins. Unlike previously characterised adhesins that possess conserved ligand-binding domains at the N-terminus, this group of proteins carry ligand-binding domains at their C-termini. We demonstrate that one such domain--the uncharacterised GLEYA domain, is related to the lectin-like ligand-binding domain found in the Saccharomyces cerevisiae Flo proteins. Unlike the Flo and Als proteins, the fission yeast adhesins do not contain detectable glycosyl phosphatidyl inositol (GPI) membrane anchor signals to mediate their attachment to the cell wall, which may suggest a novel cell wall attachment mechanism. Further sequence analysis identified several putative adhesins in the sub-phylum of Pezizomycotina, where only a few adhesins have been described to date.

Cdk8 is essential for preimplantation mouse development

The Cdk8 kinase and associated proteins form a nonessential transcriptional repressor module of the Mediator in the budding yeast Saccharomyces cerevisiae. Genetic analyses of this module have demonstrated functions ranging from environmental responses in budding yeast to organogenesis and development in worms, flies, and zebrafish. Here we have investigated the function of mammalian Cdk8 using mice harboring a gene trap insertion at the Cdk8 locus inactivating this kinase. No phenotypes were noted in heterozygote Cdk8+/- mice, but intercrossing these did not produce homozygous Cdk8-/- offspring. Developmental analysis demonstrated a requirement for Cdk8 prior to implantation at embryonic days 2.5 to 3.0. Cdk8-/- preimplantation embryos had fragmented blastomeres and did not proceed to compaction. As Cdk8 deficiency in cultured metazoan cells did not affect cell viability, the results suggest that transcriptional repression of genes critical for early-cell-fate determination underlies the requirement of Cdk8 in embryogenesis.

The cyclin-dependent kinase 8 module sterically blocks Mediator interactions with RNA polymerase II

CDK8 (cyclin-dependent kinase 8), along with CycC, Med12, and Med13, form a repressive module (the Cdk8 module) that prevents RNA polymerase II (pol II) interactions with Mediator. Here, we report that the ability of the Cdk8 module to prevent pol II interactions is independent of the Cdk8-dependent kinase activity. We use electron microscopy and single-particle reconstruction to demonstrate that the Cdk8 module forms a distinct structural entity that binds to the head and middle region of Mediator, thereby sterically blocking interactions with pol II.

TRAP230/ARC240 and TRAP240/ARC250 Mediator subunits are functionally conserved through evolution

In Saccharomyces cerevisiae Mediator, a subgroup of proteins (Srb8, Srb9, Srb10, and Srb11) form a module, which is involved in negative regulation of transcription. Homologues of Srb10 and Srb11 are found in some mammalian Mediator preparations, whereas no clear homologues have been reported for Srb8 and Srb9. Here, we identify a TRAP240/ARC250 homologue in Schizosaccharomyces pombe and demonstrate that this protein, spTrap240, is stably associated with a larger form of Mediator, which also contains conserved homologues of Srb8, Srb10, and Srb11. We find that spTrap240 and Sch. pombe Srb8 (spSrb8) regulate the same distinct subset of genes and have indistinguishable phenotypic characteristics. Importantly, Mediator containing the spSrb8/spTrap240/spSrb10/spSrb11 subunits is isolated only in free form, devoid of RNA polymerase II. In contrast, Mediator lacking this module associates with the polymerase. Our findings provide experimental evidence for recent suggestions that TRAP230/ARC240 and TRAP240/ARC250 may indeed be the Srb8 and Srb9 homologues of mammalian Mediator. Apparently Srb8/TRAP230/ARC240, Srb9/TRAP240/ARC250, Srb10, and Srb11 constitute a conserved Mediator submodule, which is involved in negative regulation of transcription in all eukaryotes.

Cyclin-dependent kinase 8 is expressed both maternally and zygotically during zebrafish embryo development

Cyclin-dependent kinase 8 (cdk8) regulates transcription by phosphorylating RNA polymerase II and TFIIH. The mechanism of zygotic transcription activation during vertebrate embryonic development is poorly understood. Here we describe the cloning and developmental expression pattern of zebrafish cdk8 mRNA. It is highly conserved, sharing 79% DNA and 95% amino acid sequence identity with human cdk8, thereby indicating an important role for the protein. Northern blotting and whole mount in situ hybridisation revealed expression of zebrafish cdk8 maternally, following the onset of zygotic transcription at the mid-blastula transition (MBT) and throughout embryonic development.

Negative regulation of filamentous growth and flocculation by Lkh1, a fission yeast LAMMER kinase homolog

We have isolated a full-length cDNA clone that encodes for a Schizosaccharomyces pombe homolog of the dual-specificity protein kinase of the LAMMER family, lkh1 (lammer kinase homolog). The proposed Lkh1 protein contains 575 amino acids. The lkh1(+) null mutant is viable, but exhibits flocculation upon reaching stationary phase in liquid media and filamentous adhesion growth on solid media. Analysis of the flocculation activity of the lkh1(+) null mutant indicates that asexual aggregation of S. pombe cells into floccules is divalent cation-dependent and galactose-specific. We also demonstrate that the Saccharomyces cerevisiae LAMMER kinase homolog, Kns1, can substitute for the Lkh1 function in S. pombe.

Functional analysis of the C-terminal cytoplasmic region of the M-factor receptor in fission yeast

BACKGROUND

Yeast mating-pheromone receptors facilitate the study of G protein-coupled signal transduction. To date, molecular dissection of the budding yeast alpha-factor receptor has been done extensively, but little analysis has been performed with pheromone receptors of fission yeast, another genetically tractable yeast species.

RESULTS

We analysed the fission yeast M-factor receptor Map3p. Truncation of the C-terminal 54 amino acids made Map3p dominant-negative over the wild-type. This form, called Map3-dn9p, was competent in the induction of pheromone-dependent gene expression, although it could not direct proper conjugation. Map3-dn9p failed both to provoke the orientated projection of conjugation tubes and to induce adaptation to the pheromone signal associated with endocytosis of the receptor. Deletion and substitution analyses suggested that the integrity of the C-terminal region, rather than a specific subgroup of amino acid residues therein, was vital for the respective Map3p activities. Ubiquitination of the C-terminus was not absolutely essential for Map3p function.

CONCLUSIONS

The C-terminal region of Map3p is dispensable for the pheromone signalling per se, but is pivotal for adaptation and pheromone-induced conjugation tube formation, as is true with the budding yeast alpha-factor receptor. However, the mechanisms which induce adaptation appear to differ between fission and budding yeast concerning the necessity of ubiquitination.

Gene disruption in Schizosaccharomyces pombe using a temperature-sensitive Ura4p

We have generated a temperature-sensitive form of the Ura4p protein from the fission yeast Schizosaccharomyces pombe. A single T-to-C mutation at nucleotide 782 (relative to the initiator ATG codon of ura4) changes the leucine residue at position 261 in Ura4p to a proline. The mutant Ura4p(ts) supports growth at 30 degrees C but is unable to allow growth at 37 degrees C in the absence of uracil when a single copy of the gene is integrated into the host chromosome. Using the ura4(ts) cassette for gene replacements simplifies the identification of transformants in which the disruption construct has undergone homologous integration into the host chromosome, as these individuals contain a single copy of the ura4(ts) gene and fail to grow when replicated to 37 degrees C in the absence of uracil.

Cell surface galactosylation is essential for nonsexual flocculation in Schizosaccharomyces pombe

We have isolated fission yeast mutants that constitutively flocculate upon growth in liquid media. One of these mutants, the gsf1 mutant, was found to cause dominant, nonsexual, and calcium-dependent aggregation of cells into flocs. Its flocculation was inhibited by the addition of galactose but was not affected by the addition of mannose or glucose, unlike Saccharomyces cerevisiae FLO mutants. The gsf1 mutant coflocculated with Schizosaccharomyces pombe wild-type cells, while no coflocculation was found with galactose-deficient (gms1Delta) cells. Moreover, flocculation of the gsf1 mutant was also inhibited by addition of cell wall galactomannan from wild-type cells but not from gms1Delta cells. These results suggested that galactose residues in the cell wall glycoproteins may be receptors of gsf1-mediated flocculation, and therefore cell surface galactosylation is required for nonsexual flocculation in S. pombe.