Yeast Actin Gene Research Articles

Formins filter modified actin subunits during processive elongation

Fission yeast cells reject actin subunits tagged with a fluorescent protein from the cytokinetic contractile ring, so cytokinesis fails and the cells die when the native actin gene is replaced by GFP-actin. The lack of a fluorescent actin probe has prevented a detailed study of actin filament dynamics in contractile rings, and left open questions regarding the mechanism of cytokinesis. To incorporate fluorescent actin into the contractile ring to study its dynamics, we introduced the coding sequence for a tetracysteine motif (FLNCCPGCCMEP) at 10 locations in the fission yeast actin gene and expressed the mutant proteins from the native actin locus in diploid cells with wild-type actin on the other chromosome. We labeled these tagged actins inside live cells with the FlAsH reagent. Cells incorporated some of these labeled actins into actin patches at sites of endocytosis, where Arp2/3 complex nucleates all of the actin filaments. However, the cells did not incorporate any of the FlAsH-actins into the contractile ring. Therefore, formin Cdc12p rejects actin subunits with a tag of ~2 kDa, illustrating the stringent structural requirements for this formin to promote the elongation of actin filament barbed ends as it moves processively along the end of a growing filament.

Correction/Completion of the Yeast Actin, Alanine Scan Alleles

IN 1992, the first alanine scanning mutagenesis was performed on the yeast actin gene ACT1 ([Wertman et al. 1992][1]). This collection of mutants has proven to be a very useful set of reagents for probing the structure/function relationships in the actin cytoskeleton. Recently, we realized that

Aroma improving in microvinification processes by the use of a recombinant wine yeast strain expressing the AspergillusnidulansxlnA gene

A recombinant wine yeast strain has been constructed expressing the gene coding for β-(1,4)-endoxylanase from Aspergillus nidulans under the control of the yeast actin gene promoter. The resulting recombinant strain is able to secrete active xylanase enzyme into the culture medium. Wines obtained by microvinification with the control and the recombinant wine yeast strain did not differ in their physicochemical characteristics although an increase in fruity aroma was organoleptically detected in the wine produced by the recombinant yeast. Also, an increase in the concentration of some esters, higher alcohols and terpenes was observed in the case of the recombinant strain.

Expression and secretion of Bacillus polymyxa neopullulanase in Saccharomyces cerevisiae

We have isolated the gene encoding the neopullulanase enzyme from Bacillus polymyxa CECT 155. It consists of an open reading frame of 1545 bp that could code for a protein of 515 amino acids. This open reading frame was expressed in Bacillus subtilis and the corresponding transformants produced extracellular neopullulanase. The neopullulanase gene was also expressed in Saccharomyces cerevisiae placing it under the control of the yeast actin gene ( ACT1) promoter. Clones containing the intact neopullulanase gene, including its own bacterial signal sequence, gave rise to the synthesis of active, but intracellular, enzyme by S. cerevisiae transformants. When sequences specifying the signal sequence and leader region of the yeast mating pheromone α-factor ( MFα 1) were fused upstream of the gene encoding the neopullulanase enzyme, the enzyme was secreted by S. cerevisiae. The secreted protein presented the same biochemical properties and the same apparent molecular mass as the Bacillus polymyxa original enzyme. The predicted amino acid sequence of the neopullulanase protein contained sequence motifs conserved among amylolytic enzymes. Northern blot analysis indicated that the transcription of the neopullulanase gene in B. polymyxa was induced by the presence of the substrate, pullulan, in the culture, and was repressed by glucose.

Expression and secretion of Bacillus polymyxa neopullulanase in Saccharomyces cerevisiae.

We have isolated the gene encoding the neopullulanase enzyme from Bacillus polymyxa CECT 155. It consists of an open reading frame of 1545 bp that could code for a protein of 515 amino acids. This open reading frame was expressed in Bacillus subtilis and the corresponding transformants produced extracellular neopullulanase. The neopullulanase gene was also expressed in Saccharomyces cerevisiae placing it under the control of the yeast actin gene (ACT1) promoter. Clones containing the intact neopullulanase gene, including its own bacterial signal sequence, gave rise to the synthesis of active, but intracellular, enzyme by S. cerevisiae transformants. When sequences specifying the signal sequence and leader region of the yeast mating pheromone alpha-factor (MF alpha 1) were fused upstream of the gene encoding the neopullulanase enzyme, the enzyme was secreted by S. cerevisiae. The secreted protein presented the same biochemical properties and the same apparent molecular mass as the Bacillus polymyxa original enzyme. The predicted amino acid sequence of the neopullulanase protein contained sequence motifs conserved among amylolytic enzymes. Northern blot analysis indicated that the transcription of the neopullulanase gene in B. polymyxa was induced by the presence of the substrate, pullulan, in the culture, and was repressed by glucose.

Multiple functions for actin during filamentous growth of Saccharomyces cerevisiae.

Saccharomyces cerevisiae is dimorphic and switches from a yeast form to a pseudohyphal (PH) form when starved for nitrogen. PH cells are elongated, bud in a unipolar manner, and invade the agar substrate. We assessed the requirements for actin in mediating the dramatic morphogenetic events that accompany the transition to PH growth. Twelve "alanine scan" alleles of the single yeast actin gene (ACT1) were tested for effects on filamentation, unipolar budding, agar invasion, and cell elongation. Some act1 mutations affect all phenotypes, whereas others affect only one or two aspects of PH growth. Tests of intragenic complementation among specific act1 mutations support the phenotypic evidence for multiple actin functions in filamentous growth. We present evidence that interaction between actin and the actin-binding protein fimbrin is important for PH growth and suggest that association of different actin-binding proteins with actin mediates the multiple functions of actin in filamentous growth. Furthermore, characterization of cytoskeletal structure in wild type and act1/act1 mutants indicates that PH cell morphogenesis requires the maintenance of a highly polarized actin cytoskeleton. Collectively, this work demonstrates that actin plays a central role in fungal dimorphism.

Functional analysis of a 38 kilobase region on chromosome XVI in Saccharomyces cerevisiae.

In this report we assess the functional importance of 16 open reading frames (ORFs) contained within a 38 780 base-pair region immediately adjacent to the centromere on the right arm of chromosome XVI in Saccharomyces cerevisiae. This analysis involved replacing one copy of each ORF in a diploid strain with a cassette encoding the green fluorescent protein from the jellyfish Aequorea victoria and HIS3. Each replacement cassette was generated by PCR using oligonucleotide pairs with 45-base extensions complementary to sequences immediately upstream and downstream of the target gene's coding region. After replacement of the targeted genes, each gene-replacement strain was subjected to a series of genetic and phenotypic tests to assess the functional importance of the deleted gene. This analysis showed that two ORFs were essential, one for spores to germinate and another for vegetative growth. A third gene encoded a copper-fist-like transcription factor that was required for proper bud-site selection. One of the 16 ORFs was duplicated, a situation not observed in the strain used to sequence the yeast genome (S288C). RNA analysis showed 11 of the 16 ORFs in this region expressed steady-state poly(A+) RNA levels that were greater than or equal to 2% of the level expressed from the yeast actin gene, ACT1.

Expression in a wine yeast strain of the Aspergillus niger abfB gene

A recombinant wine yeast strain has been constructed expressing the gene coding for a-L-arabinofuranosidase B from Aspergillus niger under the control of the yeast actin gene promoter. The protein is efficiently secreted by the recombinant yeast, allowing its purification and characterisation. The heterologous α- l-arabinofuranosidase B shows similar physico-chemical properties to the native enzyme. The wine produced in microvinification experiments using the recombinant yeast presents the same oenological characteristics as obtained with the untransformed strain. The a-L-arabinofuranosidase B protein is detected throughout the fermentation.

Expression in a wine yeast strain of the Aspergillus niger abfB gene.

A recombinant wine yeast strain has been constructed expressing the gene coding for alpha-L-arabinofuranosidase B from Aspergillus niger under the control of the yeast actin gene promoter. The protein is efficiently secreted by the recombinant yeast, allowing its purification and characterisation. The heterologous alpha-L-arabinofuranosidase B shows similar physico-chemical properties to the native enzyme. The wine produced in microvinification experiments using the recombinant yeast presents the same oenological characteristics as obtained with the untransformed strain. The alpha-L-arabinofuranosidase B protein is detected throughout the fermentation.

Mapping actin surfaces required for functional interactions in vivo.

An in vivo strategy to identify amino acids of actin required for functional interactions with actin-binding proteins was developed. This approach is based on the assumption that an actin mutation that specifically impairs the interaction with an actin-binding protein will cause a phenotype similar to a null mutation in the gene that encodes the actin-binding protein. 21 actin mutations were analyzed in budding yeast, and specific regions of actin subdomain 1 were implicated in the interaction with fimbrin, an actin filament-bundling protein. Mutations in this actin subdomain were shown to be, like a null allele of the yeast fimbrin gene (SAC6), lethal in combination with null mutations in the ABP1 and SLA2 genes, and viable in combination with a null mutation in the SLA1 gene. Biochemical experiments with act1-120 actin (E99A, E100A) verified a defect in the fimbrin-actin interaction. Genetic interactions between mutant alleles of the yeast actin gene and null alleles of the SAC6, ABP1, SLA1, and SLA2 genes also demonstrated that the effects of the 21 actin mutations are diverse and allowed four out of seven pseudo-wild-type actin alleles to be distinguished from the wild-type gene for the first time, providing evidence for functional redundancy between different surfaces of actin.

Construction of a recombinant wine yeast strain expressing beta-(1,4)-endoglucanase and its use in microvinification processes

A genetic transformation system for an industrial wine yeast strain is presented here. The system is based on the acquisition of cycloheximide resistance and is a direct adaptation of a previously published procedure for brewing yeasts (L. Del Pozo, D. Abarca, M. G. Claros, and A. Jiménez, Curr. Genet. 19:353-358, 1991). Transformants arose at an optimal frequency of 0.5 transformant per microgram of DNA, are stable in the absence of selective pressure, and produce wine in the same way as the untransformed industrial strain. By using this transformation protocol, a filamentous fungal beta-(1,4)-endoglucanase gene has been expressed in an industrial wine yeast under the control of the yeast actin gene promoter. Endoglucanolytic wine yeast secretes the fungal enzyme to the must, producing a wine with an increased fruity aroma.

Site-directed mutagenesis of the yeast actin gene: a test for actin function in vivo.

The yeast Saccharomyces cerevisiae has a single actin gene, ACT1, whose protein product is essential for cell viability. To study the structure-function relationship of this evolutionarily highly conserved protein, we have introduced into the gene several mutations leading to substitutions of amino acids that, by chemical crosslinking experiments, have previously been identified as potential sites for the interaction of actin with several actin-binding proteins and of actin monomers in filaments. The in vitro mutated actin genes were used to replace one chromosomal ACT1 allele in diploid cells. From diploid transformants, haploids that solely depended on mutant actins were isolated and their phenotypic alterations studied. The replacement of the N-terminal acidic residues (Asp2 and Glu4) with valine was functionally neutral. Substitutions of Asp11 led to dominant lethality. Substitutions of Lys191, Lys336, Trp356, Lys373 and Cys374 were without observable effect on cell growth, proliferation and morphology. Deletion of the C-terminal end, Lys-Cys-Phe-COOH, was lethal, whereas successive removal of the C-terminal Phe375 or Cys374 and Phe375 resulted in temperature sensitivity. At the nonpermissive temperature, the mutant cells were characterized by an increase in size, a tendency to lyse and significant alterations of the actin cytoskeleton.

Use of interplasmid recombination to generate stable selectable markers for yeast transformation: application to studies of actin gene control

A plasmid recombination system has been developed that relies upon interplasmid exchanges for yeast cell viability. Two types of plasmids, one carrying the LEU2 allele inserted within yeast actin gene sequences and the other carrying 2-microns plasmid DNA and an intact actin gene, were constructed. Neither plasmid alone yielded transformants in the haploid Leu- strain AH22, but when cotransformed, a number of colonies were obtained. Southern blot analysis revealed that transformants arose because of recombination events within the homologous actin sequences that transferred the LEU2 gene to the actin gene on the 2-microns plasmid. The recombinant plasmids could be recovered, and sequence analysis of one recombination site revealed that the exchange event was faithful at the nucleotide level. The resulting recombinant plasmids carried a defective actin gene and presumably arose because of a double-crossover event. Deletion mutations that prevented actin gene expression on one donor plasmid enabled the recovery at a high frequency of transformants resulting primarily from single-crossover events between the two plasmids. This was presumably because such events no longer generated an intact actin gene on a multicopy plasmid. Infrequently a transformant from a plasmid with an intact gene was recovered, but in these cases the plasmid was not present in multiple copies. These cells exhibited a slower growth rate, and Northern blot analysis revealed an elevated level of actin mRNA.

The intron of the yeast actin gene contains the promoter for an antisense RNA

Using Northern blot analysis we have detected an approximately 840 nucleotide-long RNA which is complementary to the 5' leader sequence and the first ten nucleotides of the coding sequence of the yeast actin (ACT1) messenger RNA. We have determined two transcription start sites for this actin antisense RNA (ASR1), both within the ACT1 intron, at about 80 and 90 nucleotides downstream from the 5' splice site. Analysis of a cDNA clone showed that this RNA species overlaps the entire trailer sequence and approximately 20 nucleotides of the coding sequence of the nearby yeast YPT1 gene.

Disruption of the actin cytoskeleton in yeast capping protein mutants

Capping protein controls the addition of actin subunits to the barbed end of actin filaments and nucleates actin polymerization in vitro. Capping protein has been identified in all eukaryotic cells examined so far; it is a heterodimer with subunits of relative molecular masses 32,000-36,000 (alpha-subunit) and 28,000-32,000 (beta-subunit). In skeletal muscle, capping protein (CapZ) probably binds the barbed ends of actin filaments at the Z line. The in vivo role of this protein in non-muscle cells is not known. We report here the characterization of CAP2, the single gene encoding the beta-subunit of capping protein in Saccharomyces cerevisiae. Yeast cells in which the CAP2 gene was disrupted by an insertion or a deletion had an abnormal actin distribution, including the loss of actin cables. The mutant cells were round and large, with a heterogeneous size distribution, and, although viable, grew more slowly than congenic wild-type cells. Chitin, a cell wall component restricted to the mother-bud junction in wild-type budding yeast, was found on the entire mother cell surface in the mutants. The phenotype of CAP2 disruption resembled that of temperature-sensitive mutations in the yeast actin gene ACT1, indicating that capping protein regulates actin-filament distribution in vivo.

DNA sequences required for yeast actin gene transcription do not include conserved CCAAT motifs

Sequences required for Saccharomyces cerevisiae actin gene transcription were mapped and compared to the regulatory region of the actin gene from a thermophilic fungus, Thermomyces lanuginosus. Two CCAAT motifs conserved in position in these two species could be mutated without affecting promoter activity, regardless of whether the yeast were grown in fermentable or non-fermentable carbon sources. Two TATA-like sequences and an upstream activation sequence (UAS) composed of multiple elements were identified. The contribution of sequence motifs within these elements to UAS activity varied depending on the carbon source. The Thermomyces gene contains sequences highly homologous to this UAS, but in the opposite orientation.

Analysis of expression of hybrid yeast genes containing ARS elements

In an attempt to devise a new assay for ARS-binding proteins we have inserted the HO ARS between the upstream activation site and the TATA region of the yeast CYC1 promoter. A marked reduction in promoter activity is observed. Inactivation of the HO ARS element by point mutation does not restore promoter activity to its original level, although a modest activation is seen. We have also inserted the HO ARS into the intron of the yeast actin gene; although there is no apparent deleterious effect on transcription, the activity of the ARS is abolished in this new environment.

A yeast actin-binding protein is encoded by SAC6, a gene found by suppression of an actin mutation.

The protein encoded by SAC6, a gene that can mutate to suppress a temperature-sensitive defect in the yeast actin gene, has been identified as a 67-kilodalton actin-binding protein (ABP 67) that associates with all identifiable actin structures. This finding demonstrates the in vivo functional importance of the actin-ABP 67 interaction previously established in vitro and illustrates the use of suppressor analysis to identify physically interacting proteins.

Impairment of yeast pre-mRNA splicing by potential secondary structure-forming sequences near the conserved branchpoint sequence.

The absolutely conserved TACTAAC box within introns of RNA polymerase II-transcribed genes of the yeast Saccharomyces cerevisiae serves an indispensable role in lariat formation. We show in this report that rather short palindromic sequences inserted into the yeast actin gene intron immediately 3' to the TACTAAC box block the second but not the first splicing step. In contrast, a palindromic sequence inserted some 23 bp 3' of the TACTAAC box did not affect correct and efficient splicing. The data suggest that hairpin structures that might form adjacent to the branchsite sequence interfere with some necessary alteration of the spliceosome required for 3' intron cleavage and exon ligation.

Yeast pre-mRNA splicing requires a minimum distance between the 5' splice site and the internal branch acceptor site.

We have generated several deletions within the intron of a yeast actin gene construct which have lead to different splicing efficiencies as measured by Northern blot (RNA blot) and primer extension analyses. Our data especially demonstrate that a minimum distance from the 5' splice site to the internal branch acceptor site is required for accurate and efficient splicing. In a construct in which splicing was completely abolished, splicing could be restored by expanding the distance from the 5' splice site to the internal branch acceptor site with heterologous sequences. Alternative splicing, i.e., exon skipping and the use of a cryptic 5' splice site, was observed when the mRNA precursor was derived from a tandem repeat of a truncated intron with flanking exon sequences.