Although calnexin is essential in S. pombe, its highly conserved central domain is dispensable for viability

In mammalian cells, the calnexin/calreticulin chaperones play a key role in glycoprotein folding and its control within the endoplasmic reticulum (ER), by interacting with folding intermediates via their monoglucosylated glycans. This lectin activity has been mapped in mammalian calnexin/calreticulin chaperones to the central region, which is a highly conserved feature of calnexin/calreticulin molecules across species. The central domain has also been implicated in Ca(2+) binding, and it has been proposed to be involved in the regulation of calcium homeostasis in the ER. Herein, we show that although the Schizosaccharomyces pombe calnexin is essential for viability, cells lacking its 317-amino-acid highly conserved central region are viable under normal growth conditions. However, the central region appears to be necessary for optimal growth under high ER-stress, suggesting that this region is important under extreme folding situations (such as DTT and temperature). The minimal length of calnexin required for viability spans the C-terminal 123 residues. Furthermore, cells with the central domain of the protein deleted were affected in their morphology at 37 degrees C, probably due to a defect in cell wall synthesis, although these mutant cells exhibited the same calcium tolerance as wild-type cells at 30 degrees C.

A molecular target for viral killer toxin: TOK1 potassium channels

Killer strains of S. cerevisiae harbor double-stranded RNA viruses and secrete protein toxins that kill virus-free cells. The K1 killer toxin acts on sensitive yeast cells to perturb potassium homeostasis and cause cell death. Here, the toxin is shown to activate the plasma membrane potassium channel of S. cerevisiae, TOK1. Genetic deletion of TOK1 confers toxin resistance; overexpression increases susceptibility. Cells expressing TOK1 exhibit toxin-induced potassium flux; those without the gene do not. K1 toxin acts in the absence of other viral or yeast products: toxin synthesized from a cDNA increases open probability of single TOK1 channels (via reversible destabilization of closed states) whether channels are studied in yeast cells or X. laevis oocytes.

Mutation of K234 and K236 in the voltage-dependent anion channel 1 impairs its insertion into the mitochondrial outer membrane

Previous in vitro studies indicated that mutation of both K234 and K236 to arginine, glutamine, or glutamic acid impaired the ability of the voltage-dependent anion channel (VDAC1) to insert into the outer membrane of the mitochondria (Smith et al. 1995). These same mutants were expressed in a strain of Saccharomyces cerevisiae with a disruption in the VDAC1 gene. The mutant VDAC1 forms were found in the mitochondria suggesting that they were correctly sorted to the outer membrane. However, only very small amounts of the mutants were inserted into the mitochondrial membranes. Mitochondria isolated from the strains expressing the mutants were capable of catalyzing the translocation of both wild-type VDAC1 and pre-alcohol dehydrogenase III indicating that the translocation apparatus was functional. These results confirm the previously drawn conclusion that K234 and K236 are part of a membrane insertion motif. The failure of the mutant VDAC1 forms to insert did not cause VDAC1 precursors to accumulate in the soluble cell cytoplasm or in the microsomal fraction. The apparent lack of a "precursor pool" suggested that a post-transcriptional control mechanism might limit the amounts of VDAC1 precursors in the cell. Such a control mechanism is consistent with the observation that the amount of VDAC1 was very similar after epichromosomal (gene in a 2u plasmid controlled by a Gal1 promoter) and chromosomal expression (endogenous gene controlled by the endogenous promoter).

Surface charge density and light-scattering of the Plectonema boryanum spheroplasts

The electrokinetic properties of spheroplasts from the cyanobacterium Plectonema boryanum were examined by particle microelectrophoresis technique. The electrophoretic mobility (EPM) of the particles was determined after incubation with CaCl2 in dependence of iron content in culture media as follows: an iron sufficient medium ('control' variant), an iron-deficient medium ('Fe-starved' variant) and an excess of iron supply medium ('20 x Fe' variant). Strong increase in EPM was observed with micromolar concentrations of divalent cations at '20 x Fe' spheroplasts. This pattern of calcium efficiency was not accompanied with the cation influences on the aggregate ability of particles. The EPM of 'control' spheroplasts strongly decreased with addition of calcium cations. The 'Fe-starved' spheroplasts were characterized with a slight reduction in EPM and a mild change in light-scattering properties of the particles. The data is the direct demonstration of the interaction between calcium cations and spheroplast surface, which could be proposed to play a role in the environmental cycling of iron.

Specific cell wall proteins confer resistance to nisin upon yeast cells

The cell wall of a yeast cell forms a barrier for various proteinaceous and nonproteinaceous molecules. Nisin, a small polypeptide and a well-known preservative active against gram-positive bacteria, was tested with wild-type Saccharomyces cerevisiae. This peptide had no effect on intact cells. However, removal of the cell wall facilitated access of nisin to the membrane and led to cell rupture. The roles of individual components of the cell wall in protection against nisin were studied by using synchronized cultures. Variation in nisin sensitivity was observed during the cell cycle. In the S phase, which is the phase in the cell cycle in which the permeability of the yeast wall to fluorescein isothiocyanate dextrans is highest, the cells were most sensitive to nisin. In contrast, the cells were most resistant to nisin after a peak in expression of the mRNA of cell wall protein 2 (Cwp2p), which coincided with the G2 phase of the cell cycle. A mutant lacking Cwp2p has been shown to be more sensitive to cell wall-interfering compounds and Zymolyase (J. M. Van der Vaart, L. H. Caro, J. W. Chapman, F. M. Klis, and C. T. Verrips, J. Bacteriol. 177:3104-3110, 1995). Here we show that of the single cell wall protein knockouts, a Cwp2p-deficient mutant is most sensitive to nisin. A mutant with a double knockout of Cwp1p and Cwp2p is hypersensitive to the peptide. Finally, in yeast mutants with impaired cell wall structure, expression of both CWP1 and CWP2 was modified. We concluded that Cwp2p plays a prominent role in protection of cells against antimicrobial peptides, such as nisin, and that Cwp1p and Cwp2p play a key role in the formation of a normal cell wall.

Functional and structural conservation in the mechanosensitive channel MscL implicates elements crucial for mechanosensation

mscL encodes a channel in Escherichia coli that is opened by membrane stretch force, probably serving as an osmotic gauge. Sequences more or less similar to mscL are found in other bacteria, but the degree of conserved function has been unclear. We subcloned and expressed these putative homologues in E. coli and examined their products under patch clamp. Here, we show that each indeed encodes a conserved mechanosensitive channel activity, consistent with the interpretation that this is an important and primary function of the protein in a wide range of bacteria. Although similar, channels of different bacteria differ in kinetics and their degree of mechanosensitivity. Comparison of the primary sequence of these proteins reveals two highly conserved regions, corresponding to domains previously shown to be important for the function of the wild-type E. coli channel, and a C-terminal region that is not conserved in all species. This structural conservation is providing insight into regions of this molecule that are vital to its role as a mechanosensitive channel and may have broader implications for the understanding of other mechanosensitive systems.

Implication of cell wall constituents in the sensitivity of Kluyveromyces lactis strains to amphotericin B

In Kluyveromyces lactis, the cell wall compositions of Kl (ATCC 96897), a wild sensitive strain, and Klm (ATCC 96896), a strain resistant to amphotericin B (AmB), were shown to be very different, since the walls in the latter were significantly enriched in hexosamine, but had a reduced content in phosphate and amino acid. In both strains, the cell walls limited their sensitivity to this antifungal agent. The absence of cell wall increased the sensitivity of the cells to this polyene by 5 to 10-fold. When the cells were treated with enzymes such as pronase and chitinase in order to change the cell wall structure just before inoculation, the yeasts appeared more resistant to the antibiotic. However, treatments with chymopapain and phospholipase C did not significantly change the sensitivity of the two strains to this agent. Cells treated with acid phosphatase displayed a longer lag phase than the control cells. In addition, when cultured in the presence of AmB, the cells were less sensitive to this agent. The present results reveal that both a change in the ionic charges of the cell wall and an alteration in the cell wall structure modified the sensitivity of these yeast strains to AmB.

Heat shock disassembles the nucleolus and inhibits nuclear protein import and poly(A)+ RNA export

Heat shock causes major positive and negative changes in gene expression, drastically alters the appearance of the nucleolus and inhibits rRNA synthesis. We here show that it causes many yeast nucleolar proteins, including the fibrillarin homolog Nop1p, to relocate to the cytoplasm. Relocation depends on several proteins implicated in mRNA transport (Mtrps) and is reversible. Two observations indicate, surprisingly, that disassembly results from a reduction in Ssa protein (Hsp70) levels: (i) selective depletion of Ssa1p leads to disassembly of the nucleolus; (ii) preincubation at 37 degrees C protects the nucleolus against disassembly by heat shock, unless expression of Ssa proteins is specifically inhibited. We observed that heat shock or reduction of Ssa1p levels inhibits protein import into the nucleus and therefore we propose that inhibition of import leads to disassembly of the nucleolus. These observations provide a simple explanation of the effects of heat shock on the anatomy of the nucleolus and rRNA transcription. They also extend understanding of the path of nuclear export. Since a number of nucleoplasmic proteins also relocate upon heat shock, these observations can provide a general mechanism for regulation of gene expression. Relocation of the hnRNP-like protein Mtr13p (= Npl3p, Nop3p), explains the heat shock sensitivity of export of average poly(A)+ RNA. Strikingly, Hsp mRNA export appears not to be affected.

Chromatin remodeling during Saccharomyces cerevisiae ADH2 gene activation

We have analyzed at both low and high resolution the distribution of nucleosomes over the Saccharomyces cerevisiae ADH2 promoter region in its chromosomal location, both under repressing (high-glucose) conditions and during derepression. Enzymatic treatments (micrococcal nuclease and restriction endonucleases) were used to probe the in vivo chromatin structure during ADH2 gene activation. Under glucose-repressed conditions, the ADH2 promoter was bound by a precise array of nucleosomes, the principal ones positioned at the RNA initiation sites (nucleosome +1), at the TATA box (nucleosome -1), and upstream of the ADR1-binding site (UAS1) (nucleosome -2). The UAS1 sequence and the adjacent UAS2 sequence constituted a nucleosome-free region. Nucleosomes -1 and +1 were destabilized soon after depletion of glucose and had become so before the appearance of ADH2 mRNA. When the transcription rate was high, nucleosomes -2 and +2 also underwent rearrangement. When spheroplasts were prepared from cells grown in minimal medium, detection of this chromatin remodeling required the addition of a small amount of glucose. Cells lacking the ADR1 protein did not display any of these chromatin modifications upon glucose depletion. Since the UAS1 sequence to which Adr1p binds is located immediately upstream of nucleosome -1, Adr1p is presumably required for destabilization of this nucleosome and for aiding the TATA-box accessibility to the transcription machinery.

Morphology of a human-derived YAC in yeast meiosis

In meiosis of human males DNA is packaged along pachytene chromosomes about 20 times more compactly than in meiosis of yeast. Nevertheless, a human-derived yeast artificial chromosome (YAC) shows the same degree of compaction of DNA as endogenous chromosomes in meiotic prophase nuclei of yeast. This suggests that in yeast meiosis, human and yeast DNA adopt a similar organization of chromatin along the pachytene chromosome cores. Therefore meiotic chromatin organization does not seem to be an inherent chromosomal property but is governed by the host-specific cellular environment. We suggest that there is a correlation between the less dense DNA packaging and the increased rate of recombination that has been reported for human-derived YACs as compared with human DNA in its natural environment.

Transfer of yeast artificial chromosomes into mammalian cells and comparative study of their integrity

Yeast artificial chromosomes (YACs) from the CEPH MegaYAC library (Paris, France) ranging in size from 350 to 1600 kb and mapping to the q22.1 and q22.2 regions of human chromosome 21 were transferred into mammalian cells by spheroplast fusion. The integrity of the YACs from two adjacent parts of the region was compared after retrofitting and stable transfer into mammalian cells. We found that large YACs could easily be manipulated to allow transfer of the YAC material into mammalian cells and that the size of the YAC did not appear to be limiting for fusion. However, we show that there was great variability in the integrity of the YACs from the two regions, which was not related to the size of the YACs. Four YACs in region I from sequence-tagged site (STS) G51E05 up to STS LL103 showed, in general, no loss of material and correct gene transfer into mammalian cells. In contrast, the three YACs in the more centromeric region II (from STS G51B09 up to G51E05) frequently showed a loss of human material during handling, retrofitting and transfer. As a YAC from another library covering region II was also found to be unstable, we propose that the integrity of the YACs is highly dependent on the incorporated human chromosomal DNA.

Disruption of the gene for hsp30, an alpha-crystallin-related heat shock protein of Neurospora crassa, causes defects in thermotolerance

The alpha-crystallin-related heat shock proteins are produced by all eukaryotes, but the role of these proteins in thermoprotection remains unclear. To investigate the function of one of these proteins, we disrupted expression of the single-copy hsp30 gene of Neurospora crassa, using repeat-induced point mutagenesis, and we generated and characterized mutant strains that were deficient in hsp30 synthesis. These strains could grow at high temperature and they acquired thermotolerance from a heat shock. However, the hsp30-defective strains proved to be extremely sensitive to the combined stresses of high temperature and carbohydrate limitation, enforced by the addition of a nonmetabolizable glucose analogue. Under these conditions, their survival was reduced by 90% compared with wild-type cells. This sensitive phenotype was reversed by reintroduction of a functional hsp30 gene into the mutant strains. The mutant cells contained mitochondria from which a 22-kDa protein was readily extracted with detergents, in contrast to its retention by the mitochondria of wild-type cells. Antibodies against hsp30 coimmunoprecipitated a protein also of approximately 22 kDa from wild-type cells. Results of this study suggest that hsp30 may be important for efficient carbohydrate utilization during high temperature stress and that it may interact with other mitochondrial membrane proteins and function as a protein chaperone.

Localization of DnaK (chaperone 70) from Escherichia coli in an osmotic-shock-sensitive compartment of the cytoplasm

The chaperone DnaK can be released (up to 40%) by osmotic shock, a procedure which is known to release the periplasmic proteins and a select group of cytoplasmic proteins (including thioredoxin and elongation factor Tu) possibly associated with the inner face of the inner membrane. As distinct from periplasmic proteins, DnaK is retained within spheroplasts prepared with lysozyme and EDTA. The ability to isolate DnaK with a membrane fraction prepared under gentle lysis conditions supports a peripheral association between DnaK and the cytoplasmic membrane. Furthermore, heat shock transiently increases the localization of DnaK in the osmotic-shock-sensitive compartment of the cytoplasm. We conclude that DnaK belongs to the select group of cytoplasmic proteins released by osmotic shock, which are possibly located at Bayer adhesion sites, where the inner and outer membranes are contiguous.

Genetic organization of Acetobacter for acetic acid fermentation

Plasmid vectors for the acetic acid-producing strains of Acetobacter and Gluconobacter were constructed from their cryptic plasmids and the efficient transformation conditions were established. The systems allowed to reveal the genetic background of the strains used in the acetic acid fermentation. Genes encoding indispensable components in the acetic acid fermentation, such as alcohol dehydrogenase, aldehyde dehydrogenase and terminal oxidase, were cloned and characterized. Spontaneous mutations at high frequencies in the acetic acid bacteria to cause the deficiency in ethanol oxidation were analyzed. A new insertion sequence element, IS1380, was identified as a major factor of the genetic instability, which causes insertional inactivation of the gene encoding cytochrome c, an essential component of the functional alcohol dehydrogenase complex. Several genes including the citrate synthase gene of A. aceti were identified to confer acetic acid resistance, and the histidinolphosphate aminotransferase gene was cloned as a multicopy suppressor of an ethanol sensitive mutant. Improvement of the acetic acid productivity of an A. aceti strain was achieved through amplification of the aldehyde dehydrogenase gene with a multicopy vector. In addition, spheroplast fusion of the Acetobacter strains was developed and applied to improve their properties.

Spheroplast formation and plasmid isolation from Rhodococcus spp

The genus Rhodococcus comprises aerobic gram-positive actinomycetes that show considerable morphological and metabolic diversity and are known to be involved in the development of plant diseases and degradation of environmental pollutants. We describe a method for cell lysis and large plasmid DNA isolation from Rhodococcus by creating lysozyme susceptible cells by predigestion with the enzyme mutanolysin. Mutanolysin action resulted in the liberation of reducing sugars and free amino acids from the peptidoglycan layers of the cell wall. A 1-h predigestion with mutanolysin followed by a 0.5-h incubation with lysozyme resulted in spheroplast formation. Complete lysis of cells and efficient isolation of intact large plasmid DNA (108 kb) from wild-type Rhodococcus strains was confirmed.

Two types of mechanosensitive channels in the Escherichia coli cell envelope: solubilization and functional reconstitution

Mechanosensitive ion channels (MSCs) which could provide for fast osmoregulatory responses in bacteria, remain unidentified as molecular entities. MSCs from Escherichia coli (strain AW740) were examined using the patch-clamp technique, either (a) in giant spheroplasts, (b) after reconstitution by fusing native membrane vesicles with asolectin liposomes, or (c) by reassembly of octylglucoside-solubilized membrane extract into asolectin liposomes. MSC activities were similar in all three preparations, consisting of a large nonselective MSC of 3-nS conductance (in 200 mM KCl) that was activated by high negative pressures, and a small weakly anion-selective MSC of 1 nS activated by lower negative pressures. Both channels appeared more sensitive to suction in liposomes than in spheroplasts. After gel filtration of the solubilized membrane extract and reconstituting the fractions, both large MSC and small MSC activities were retrieved in liposomes. The positions of the peaks of channel activity in the column eluate, assayed by patch sampling of individual fractions reconstituted in liposomes, showed an apparent molecular mass under nondenaturing conditions of about 60-80 kDa for the large and 200-400 kDa for the small MSC. We conclude that (a) the large MSC and the small MSC are distinct molecular entities, (b) the fact that both MSCs were functional in liposomes following chromatography strongly suggests that these channels are gated by tension transduced via lipid bilayer, and (c) chromatographic fractionation of detergent-solubilized membrane proteins with subsequent patch sampling of reconstituted fractions can be used to identify and isolate these MS channel proteins.

Site-directed mutagenesis in single cells: transitions produced by DNA carrying a single O6-alkylguanine residue

Using a single burst assay based on a Poisson Distribution, I have determined the mutant virus frequency in single spheroplasts transfected with phi X174 form I' DNA carrying an O6-methyl-, ethyl-, n-propyl- or n-butylguanine residue at position 2401 of the minus strand. One set of experiments was performed with spheroplasts derived from Escherichia coli AB1157, which has normal DNA-repair systems. Of the cells examined after transfection with DNA carrying a methylguanine moiety, 30% produced mutant virus and 12% contained only mutants; with ethylguanine, 55% of the cells had mutants and 41% produced only mutants; with butylguanine, 6% of the cells had mutants and 3% contained only mutants; with propylguanine no mutants were detected in the 33 cells examined. In similar experiments carried out with spheroplasts defective in excision repair (E. coli AB1157 uvrA6) the percentage of cells producing mutant phage after transfection with DNA carrying an O6-butylguanine residue increased from 6 to 21%, and the percentage of cells producing only mutants increased from 3 to 8%; with DNA carrying an O6-methylguanine moiety, the percentage of cells producing mutants decreased from 30 to 6% and the percentage of cells producing only mutants fell from 12% to 0. In order for an individual uvrA cell to produce exclusively mutant phage from a single O6-alkylguanine residue some form of selection must occur during replication because one strand of the transfecting DNA is wild-type, and excision repair, which could lead to a homoduplex with the transition in both strands, is defective in these cells. This selection must also occur in cells with normal DNA repair. The first event in the selection process is critical; if replication of the alkyl-DNA occurs first and if a mutation is produced, then there is a significant probability that the cell will produce only mutant virus regardless of whether or not repair occurs in subsequent events, but the frequency one observes is influenced strongly by the status of the repair systems in the cell.

Germ-line transmission and expression of a human-derived yeast artificial chromosome

Introduction of DNA fragments, hundreds of kilobases in size, into mouse embryonic stem (ES) cells would greatly advance the ability to manipulate the mouse genome. Mice generated from such modified cells would permit investigation of the function and expression of very large or crudely mapped genes. Large DNA molecules cloned into yeast artificial chromosomes (YACs) are stable and genetically manipulable within yeast, suggesting yeast-cell fusion as an ideal method for transferring large DNA segments into mammalian cells. Introduction of YACs into different cell types by this technique has been reported; however, the incorporation of yeast DNA along with the YAC has raised doubts as to whether ES cells, modified in this way, would be able to recolonize the mouse germ line. Here we provide, to our knowledge, the first demonstration of germ-line transmission and expression of a large human DNA fragment, introduced into ES cells by fusion with yeast spheroplasts. Proper development was not impaired by the cointegration of a large portion of the yeast genome with the YAC.

Mitochondrial dysfunction in yeast expressing the cytoplasmic male sterility T-urf13 gene from maize: analysis at the population and individual cell level

The urf13TW gene, which is derived from the mitochondrial T-urf13 gene responsible for Texas cytoplasmic male sterility in maize, was expressed in Saccharomyces cerevisiae by targeting its translation product into mitochondria. Analysis by oxygraphy at the population level revealed that in the presence of methomyl the oxygen uptake of intact yeast cells carrying the targeted protein is strongly stimulated only with ethanol as respiratory substrate and not with glycerol, lactate, pyruvate, or acetate. When malate is the substrate oxidized by isolated mitochondria, interaction between the targeted protein and methomyl results in significant inhibition of oxygen uptake. This inhibition is eliminated and oxygen uptake is stimulated by subsequent addition of NAD+. Using 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] as probe, interactive laser scanning and flow cytometry, which permit analysis at the individual cell level, demonstrated that specific staining of the mitochondrial compartment is obtained and that DiOC6(3) fluorescence serves as a measure of the membrane potential. Finally, it was shown that, as in T cytoplasm maize mitochondria, HmT toxin and methomyl dissipate the membrane potential of yeast mitochondria that carry the foreign protein. Furthermore, the results suggest that the HmT toxin and methomyl response is related to the plasmid copy number per cell and that the deleterious effect induced by HmT toxin is stronger than that of methomyl.

Bioelectrorheological model of the cell. 3. Viscoelastic shear deformation of the membrane

An analytical electromechanical model of a spherical cell exposed to an alternating electric field was used to calculate shear stress generated in the cellular membrane. Shape deformation of Neurospora crassa (slime) spheroplasts was measured. Statistical analysis permitted empirical evaluation of creep of the cellular membrane within the range of infinitesimal stress. Final results were discussed in terms of various rheological models.

The yeast heat shock response is induced by conversion of cells to spheroplasts and by potent transcriptional inhibitors

We report here that procedures commonly used to measure transcription and mRNA decay rates in Saccharomyces cerevisiae induce the heat shock response. First, conversion of cells to spheroplasts with lyticase, a prerequisite for nuclear runoff transcription, induces the expression of HSP70 and HSP90 heat shock genes. The transcript levels of the non-heat-shock gene ACT1 are slightly depressed, consistent with the general yeast stress response. Second, the DNA intercalator, 1,10-phenanthroline, widely employed as a general transcriptional inhibitor in S. cerevisiae, enhances the mRNA abundance of certain heat shock genes (HSP82, SSA1-SSA2) although not of others (HSC82, SSA4, HSP26). Third, the antibiotic thiolutin, previously demonstrated to inhibit all three yeast RNA polymerases both in vivo and in vitro, increases the RNA levels of HSP82 5- to 10-fold, those of SSA4 greater than 25-fold, and those of HSP26 greater than 50-fold under conditions in which transcription of non-heat-shock genes is blocked. By using an episomal HSP82-lacZ fusion gene, we present evidence that lyticase and thiolutin induce heat shock gene expression at the level of transcription, whereas phenanthroline acts at a subsequent step, likely through message stabilization. We conclude that, because of the exquisite sensitivity of the yeast heat shock response, procedures designed to measure the rate of gene transcription or mRNA turnover can themselves impact upon each process.