Effects of polymyxin B sulfate and polymyxin B nonapeptide on growth and permeability of the yeast Saccharomyces cerevisiae

A two-hybrid approach to identify inhibitors of the RAS-RAF interaction

MCP compounds were developed with the idea to inhibit RAS/RAF interaction. They were identified by carrying out high-throughput screens of chemical compounds for their ability to inhibit RAS/RAF interaction in the yeast two-hybrid assay. A number of compounds including MCP1, MCP53, and MCP110 were identified as active compounds. Their inhibition of the RAS signaling was demonstrated by examining RAF and MEK activities, phosphorylation of ERK as well as characterizing their effects on events downstream of RAF. Direct evidence for the inhibition of RAS/RAF interaction was obtained by carrying out co-IP experiments. MCP compounds inhibit proliferation of a wide range of human cancer cell lines. Combination studies with other drugs showed that MCP compounds synergize with MAPK pathway inhibitors as well as with microtubule-targeting chemotherapeutics. In particular, a strong synergy with paclitaxel was observed. Efficacy to inhibit tumor formation was demonstrated using mouse xenograft models. Combination of MCP110 and paclitaxel was particularly effective in inhibiting tumor growth in a mouse xenograft model of colorectal carcinoma.

Amplification of vacuole-targeting fungicidal activity of antibacterial antibiotic polymyxin B by allicin, an allyl sulfur compound from garlic

A cationic antibacterial peptide, polymyxin B (PMB), was evaluated as an antifungal antibiotic against various yeasts and filamentous fungi when used in combination with allicin, an allyl sulfur compound from garlic. Allicin was not lethal but could markedly amplify the fungicidal activity of PMB, which was weakly detected with the increase in the plasma membrane permeability in Saccharomyces cerevisiae. Their combined actions caused a dynamic structural damage to the yeast vacuole as judged by the disappearance of its swollen spherical architecture. The vacuole-targeting activity of PMB was similarly amplified in medium with t-butyl hydroperoxide as a substitute for the action of allicin. These findings suggest that the allicin-mediated lipoperoxide production in fungal plasma membrane is the cause of the enhancement in the cellular uptake of PMB as well as its action against the vacuole.

Development of a yeast two-hybrid screen for selection of human Ras-Raf protein interaction inhibitors

A yeast two-hybrid screening system was developed to screen for small molecules that inhibit the interaction of the Ras and the Raf proteins. Hyperpermeable yeast strains useful for high-throughput screening (HTS) for the two-hybrid system were created. Differential inhibition of the Ras-Raf vs the hsRPB4-hsRPB7 interaction allowed the identification of selective inhibitors.

The yeast translation release factors Mrf1p and Sup45p (eRF1) are methylated, respectively, by the methyltransferases Mtq1p and Mtq2p

The translation release factors (RFs) RF1 and RF2 of Escherichia coli are methylated at the N5-glutamine of the GGQ motif by PrmC methyltransferase. This motif is conserved in organisms from bacteria to higher eukaryotes. The Saccharomyces cerevisiae RFs, mitochondrial Mrf1p and cytoplasmic Sup45p (eRF1), have sequence similarities to the bacterial RFs, including the potential site of glutamine methylation in the GGQ motif. A computational analysis revealed two yeast proteins, Mtq1p and Mtq2p, that have strong sequence similarity to PrmC. Mass spectrometric analysis demonstrated that Mtq1p and Mtq2p methylate Mrf1p and Sup45p, respectively, in vivo. A tryptic peptide of Mrf1p, GGQHVNTTDSAVR, containing the GGQ motif was found to be approximately 50% methylated at the glutamine residue in the normal strain but completely unmodified in the peptide from mtq1-Delta. Moreover, Mtq1p methyltransferase activity was observed in an in vitro assay. In similar experiments, it was determined that Mtq2p methylates Sup45p. The Sup45p methylation by Mtq2p was recently confirmed independently (Heurgue-Hamard, V., Champ, S., Mora, L., Merkulova-Rainon, T., Kisselev, L. L., and Buckingham, R. H. (2005) J. Biol. Chem. 280, 2439-2445). Analysis of the deletion mutants showed that although mtq1-Delta had only moderate growth defects on nonfermentable carbon sources, the mtq2-Delta had multiple phenotypes, including cold sensitivity and sensitivity to translation fidelity antibiotics paromomycin and geneticin, to high salt and calcium concentrations, to polymyxin B, and to caffeine. Also, the mitochondrial mit(-) mutation, cox2-V25, containing a premature stop mutation, was suppressed by mtq1-Delta. Most interestingly, the mtq2-Delta was significantly more resistant to the anti-microtubule drugs thiabendazole and benomyl, suggesting that Mtq2p may also methylate certain microtubule-related proteins.

Yeast as a model system for anticancer drug discovery

Saccharomyces cerevisiae has been used extensively as a model for higher eukaryotes in the study of basic cellular processes. The high degree of conservation in terms of sequence similarity and function has made this organism useful in elucidating biological pathways, both yeast and human. Among these are pathways responsible for DNA damage repair and cell cycle control. This review presents an overview of opportunities for using yeast as a model system for anticancer drug discovery. It covers screens directed against specific cancer-related targets as well as contexts created by cancer-related alterations. The methodologies covered include pharmacological and genetic screens, as well as genome-wide approaches to drug target identification.

Understanding gene and allele function with two-hybrid methods

Two-hybrid schemes for detecting protein-protein interactions have deepened our understanding of biology by allowing scientists to identify individual important proteins. Recent developments will allow biologists to chart regulatory networks and to rapidly generate hypotheses for the function of genes, allelic variants, and the connections between proteins that make up these networks. Future developments will allow biologists to test inferences about the function of network elements, and allow global approaches to questions of biological function.

The yeast two-hybrid system and its pharmaceutical significance

The detected phenotypes in many diseases are caused from dysfunction in protein-protein, protein-DNA and receptor-ligand interactions. Therefore, determination of these molecular interactions followed by designing or screening the compounds to target these interactions provides a significant challenge in drug development. This review aims to highlight the yeast two-hybrid system in determination of protein-protein interactions and its possible outcomes in pharmaceutical research. The variations of the basic methodology as one- and three-hybrid systems are also discussed in relation to their potential pharmaceutical applications.

A novel colorimetric yeast bioassay for detecting trichothecene mycotoxins

A novel colorimetric microbial bioassay for toxicity has been developed; it shows particular sensitivity to trichothecene mycotoxins. The assay uses inhibition of expression of beta-galactosidase activity within the yeast Kluyveromyces marxianus as a sensitive toxicity indicator, cultures remaining yellow, rather than turning deep green-blue, in the presence of X-gal, a chromogenic substrate. The assay is conducted in standard microtitre plates, permitting small volumes (160 microl) and many replicates, and can be scored either automatically by a plate-reader, or by eye. Factors likely to affect the efficacy of the bioassay, including carbon source, solvents, inoculum cell density, and the use of membrane-modulating agents (MMAs), were assessed. Polymyxin B nonapeptide was the most effective toxicity-enhancing MMA tested, enabling the trichothecene mycotoxin, verrucarin A, to be detected at a concentration of about 1 ng/ml. The assay's reproducibility was examined using polymyxin B sulfate, a cheaper MMA, and another trichothecene mycotoxin, T2 toxin: reproducibility and sensitivity were better for the beta-galactosidase X-gal endpoint than for an alternative chromogenic toxicity indicator, the respiratory substrate 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT).

The impact of two-hybrid and related methods on biotechnology

Two-hybrid technology has contributed significantly to the unraveling of molecular regulatory networks by facilitating the discovery of protein interactions. Outgrowths of these methods are developing rapidly, including interaction mating to identify false positives and map protein networks, two-bait systems, systems not based on transcription, and systems permitting the selection of peptide aptamers to manipulate gene and allele function. These advances promise to have a significant impact on industrial biotechnology and drug development.

Fus2 localizes near the site of cell fusion and is required for both cell fusion and nuclear alignment during zygote formation

Zygote formation occurs through tightly coordinated cell and nuclear fusion events. Genetic evidence suggests that the FUS2 gene product promotes cell fusion during zygote formation in Saccharomyces cerevisiae, functioning with the Fus1 plasma membrane protein at or before cell wall and plasma membrane fusion. Here we report the sequence of the FUS2 gene, localization of Fus2 protein, and show that fus1 and fus2 mutants have distinct defects in cell fusion. FUS2 encodes a unique open reading frame of 617 residues that only is expressed in haploid cells in response to mating pheromone. Consistent with a role in cell fusion, Fus2 protein localizes with discrete structures that could be of cytoskeletal or vesicular origin that accumulate at the tip of pheromone-induced shmoos and at the junction of paired cells in zygotes. Fus2 is predicted to be a coiled-coil protein and fractionates with a 100,000 g pellet, suggesting that it is associated with cytoskeleton, membranes, or other macromolecular structures. Fus2 may interact with structures involved in the alignment of the nuclei during cell fusion, because fus2 mutants have strong defects in karyogamy and fail to orient microtubules between parental nuclei in zygotes. In contrast, fus1 mutants show no karyogamy defects. These, and other results suggest that Fus2 defines a novel cell fusion function and subcellular structure that is also required for the alignment of parental nuclei before nuclear fusion.

Applications of interaction traps/two-hybrid systems to biotechnology research

Two-hybrid methods provide a simple and sensitive means to detect the interaction between two proteins in living cells. Their use has resulted in the isolation of new proteins and has facilitated characterization of particular protein-protein interactions. These techniques have already resulted in the identification of important targets for pharmaceutical intervention, and it is likely that their extension in coming years will facilitate the development of new drugs.

Identification of cryptic lox sites in the yeast genome by selection for Cre-mediated chromosome translocations that confer multiple drug resistance

The Cre recombinase efficiently causes site-specific DNA recombination at loxP sites placed into the eukaryotic genome. Since the loxP site of phage P1 is 34 base-pairs in size, the natural occurrence of this exact sequence is unlikely in any eukaryotic genome. However, related sequences may exist in eukaryotic genomes that could recombine at low efficiency with an authentic loxP site. This work identifies such cryptic lox sites in the yeast genome using a positive selection procedure that allows the detection of events occurring at a frequency of less than 1 x 10(-7). The selection is based on the disruption/reconstruction of the yeast gene YGL022. Disruption of YGL022 confers multiple drug sensitivity. Recombination events at a loxP site 5' to the structural gene restore expression of YGL022 and result in a multiple drug resistant phenotype. These drug resistant mutants all display chromosomal rearrangements resulting from low-frequency Cre-mediated recombination with an endogenous cryptic lox site. Ten such sites have been found and they have been mapped physically to a number of different yeast chromosomes. Although the efficiency of Cre-mediated recombination between loxP and such endogenous sites is quite low, it may be possible to redesign recombination substrates to improve recombination efficiency. Because of the greater complexity of the human and mouse genomes compared with yeast, an analogous situation is likely to exist in these organisms. The availability of such sites would be quite useful in the development of alternative strategies for gene therapy and in the generation of transgenic animals.

Effect of polymyxin B nonapeptide and polymyxin B sulphate on trichothecene mycotoxin sensitivity of yeasts using a conductimetric instrument

The addition of polymyxin B sulphate (PBS), or an inactive by-product, polymyxin B nonapeptide (PBN) to a yeast bioassay system, increased its sensitivity to various toxic agents. The nil effect level (NEL) of T-2 toxin was reduced from 0.1 to 0.01 microgram/ml for Kluyveromyces fragilis GK 1005 in the presence of these agents when using a Malthus AT 192 conductimetric instrument. Other synergistic agents (DMSO, ethanol, cetyl trimethyl ammonium bromide and Triton X-100) gave poor results in the conductimetric system. PBN also increased sensitivity of K. fragilis GK 1005 towards cycloheximide in the Malthus system, and PBS reduced the NEL of T-2 toxin for K. fragilis GK 1005 in a disc diffusion assay from 0.2 to 0.04 microgram per disc. No yeasts were found sensitive to the trichothecene deoxynivalenol (DON) even at a DON concentration of 10 micrograms/ml, except in the presence of PBN and PBS. The minimal inhibitory concentration (mic) of DON in the presence of PBS was 2 micrograms/ml for K. fragilis GK 1005.

Complete nucleotide sequence of a gene conferring polymyxin B resistance on yeast: similarity of the predicted polypeptide to protein kinases

Polymyxin B is an antibiotic that kills sensitive cells by disrupting their membranes. We have cloned a wild-type yeast gene that, when present on a high-copy-number plasmid, renders the cells resistant to the drug. The nucleotide sequence of this gene is presented. A single open reading frame within the sequence has the potential to encode a polypeptide (molecular mass of 77.5 kDa) that shows strong homologies to polypeptides of the protein kinase family. The gene, PBS2, located on chromosome X, is not allelic to the previously described PBS1 gene (where PBS signifies polymyxin B sensitivity). Although pbs1 mutations confer resistance to high levels of polymyxin B, double mutants, pbs1 pbs2, are not resistant to the drug, indicating that PBS2 is essential for pbs1 activity. Models based on the proposed protein kinase activity of the PBS2 gene product are presented to explain the interaction between PBS1 and PBS2 gene products involved in conferring polymyxin B resistance on yeast cells.

Polymyxin B nonapeptide inhibits mating in Saccharomyces cerevisiae

Polymyxin B nonapeptide enhanced susceptibility of yeast cells to various hydrophobic antibiotics and to mating pheromones. At much lower concentrations, the nonapeptide severely inhibited mating. The inhibition was caused by interference with sexual agglutination.