Identification and characterization of the Komagataella phaffii mating pheromone genes

Advances in Metabolic Engineering of Pichia pastoris Strains as Powerful Cell Factories

Pichia pastoris is the most widely used microorganism for the production of secreted industrial proteins and therapeutic proteins. Recently, this yeast has been repurposed as a cell factory for the production of chemicals and natural products. In this review, the general physiological properties of P. pastoris are summarized and the readily available genetic tools and elements are described, including strains, expression vectors, promoters, gene editing technology mediated by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, and adaptive laboratory evolution. Moreover, the recent achievements in P. pastoris-based biosynthesis of proteins, natural products, and other compounds are highlighted. The existing issues and possible solutions are also discussed for the construction of efficient P. pastoris cell factories.

Extending the reach of homology by using successive computational filters to find yeast pheromone genes

The mating of fungi depends on pheromones that mediate communication between two mating types. Most species use short peptides as pheromones, which are either unmodified (e.g., α-factor in Saccharomyces cerevisiae) or C-terminally farnesylated (e.g., a-factor in S. cerevisiae). Peptide pheromones have been found by genetics or biochemistry in a small number of fungi, but their short sequences and modest conservation make it impossible to detect homologous sequences in most species. To overcome this problem, we used a four-step computational pipeline to identify candidate a-factor genes in sequenced genomes of the Saccharomycotina, the fungal clade that contains most of the yeasts: we require that candidate genes have a C-terminal prenylation motif, are shorter than 100 amino acids long, and contain a proteolytic-processing motif upstream of the potential mature pheromone sequence and that closely related species contain highly conserved homologs of the potential mature pheromone sequence. Additional manual curation exploits the observation that many species carry more than one a-factor gene, encoding identical or nearly identical pheromones. From 332 Saccharomycotina genomes, we identified strong candidate pheromone genes in 241 genomes, covering 13 clades that are each separated from each other by at least 100 million years, the time required for evolution to remove detectable sequence homology among small pheromone genes. For one small clade, the Yarrowia, we demonstrated that our algorithm found the a-factor genes: deleting all four related genes in the a-mating type of Yarrowia lipolytica prevents mating.

Structural and functional investigation of ABC transporter STE6-2p from Pichia pastoris reveals unexpected interaction with sterol molecules

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are multidomain transmembrane proteins, which facilitate the transport of various substances across cell membranes using energy derived from ATP hydrolysis. They are important drug targets since they mediate decreased drug susceptibility during pharmacological treatments. For the methylotrophic yeast Pichia pastoris, a model organism that is a widely used host for protein expression, the role and function of its ABC transporters is unexplored. In this work, we investigated the Pichia ABC-B transporter STE6-2p. Functional investigations revealed that STE6-2p is capable of transporting rhodamines in vivo and is active in the presence of verapamil and triazoles in vitro. A phylogenetic analysis displays homology among multidrug resistance (MDR) transporters from pathogenic fungi to human ABC-B transporters. Further, we present high-resolution single-particle electron cryomicroscopy structures of an ABC transporter from P. pastoris in the apo conformation (3.1 Å) and in complex with verapamil and adenylyl imidodiphosphate (AMP-PNP) (3.2 Å). An unknown density between transmembrane helices 4, 5, and 6 in both structures suggests the presence of a sterol-binding site of unknown function.

Fermenting Futures: an artistic view on yeast biotechnology

BioArt is a new discipline where artists employ materials and techniques of modern life sciences and create novel meanings of biology, often involving living organisms such as tissue culture, bacteria and yeasts, which may also be genetically engineered. The authors have engaged in a collaboration to develop 'Fermenting Futures', a project designed to explore the significance of yeast for early human history by enabling baking and brewing, all the way to industrial biotechnology and synthetic biology with their potential contributions to fight the climate change. Research in two of the authors' lab provides the materials and thematic lines for the artists to develop their installations. The two main pieces reflect on fermentation as a metabolic trait of baker's yeast and its enormous transformational power for human society, and on the application of synthetic biology to enable yeast to grow and produce materials from carbon dioxide. The role of BioArt to support public engagement and science dissemination is discussed, highlighting the importance of collaborations of scientists and artists on equal terms, as showcased here.

Hungry for Sex: Differential Roles for Ustilago maydisb Locus Components in Haploid Cells vis à vis Nutritional Availability

Mating-types allow single-celled eukaryotic organisms to distinguish self from non-self in preparation for sexual reproduction. The components of mating-type loci provide initial self/non-self-recognition through pheromone and receptor interactions that control early cell fusion events. However, they may also provide a second level of scrutiny that requires differences in alleles leading to production of a transcription factor required for successful downstream developmental pathways after initial cell fusion. Interestingly, the protein subunits of these transcription factors have not been thoroughly examined for their roles, if any, in the haploid cells themselves. In Ustilago maydis, the causative agent of galls in maize plants, the b locus, encoding bEast (bE) and bWest (bW), components of the eventual requisite transcription factor, has been extensively studied for its role in formation of the stable dikaryon after mating and subsequent pathogenic program. Little is known, however, about any roles for bE or bW in haploid cells. Since mating in fungi is often induced under conditions of nitrogen starvation, we have explored connections between the b locus and the nitrogen-sensing and response pathways in U. maydis. We previously identified a connection in haploid cells between the b locus and Ump2, the high-affinity transceptor, a protein that both transports ammonium and triggers filamentous growth as a response to nitrogen starvation. Deletion of the entire b locus abrogates the filamentous response to low ammonium, a phenotype that is rescued by overexpression of Ump2. Here we further investigated the individual roles of bE and bW in haploid cells. We show that bE and bW are expressed differentially in haploid cells starved for ammonium. Their respective deletion elicits different effects on transcription of mating and pathogenic-related genes and, importantly, on the degree of pathogenic development in host plants. This is the first demonstration of a role for these mating locus components on haploid development and the first to demonstrate a connection to the ammonium transceptors.

Komagataella phaffii as Emerging Model Organism in Fundamental Research

Komagataella phaffii (Pichia pastoris) is one of the most extensively applied yeast species in pharmaceutical and biotechnological industries, and, therefore, also called the biotech yeast. However, thanks to more advanced strain engineering techniques, it recently started to gain attention as model organism in fundamental research. So far, the most studied model yeast is its distant cousin, Saccharomyces cerevisiae. While these data are of great importance, they limit our knowledge to one organism only. Since the divergence of the two species 250 million years ago, K. phaffii appears to have evolved less rapidly than S. cerevisiae, which is why it remains more characteristic of the common ancient yeast ancestors and shares more features with metazoan cells. This makes K. phaffii a valuable model organism for research on eukaryotic molecular cell biology, a potential we are only beginning to fully exploit. As methylotrophic yeast, K. phaffii has the intriguing property of being able to efficiently assimilate methanol as a sole source of carbon and energy. Therefore, major efforts have been made using K. phaffii as model organism to study methanol assimilation, peroxisome biogenesis and pexophagy. Other research topics covered in this review range from yeast genetics including mating and sporulation behavior to other cellular processes such as protein secretion, lipid biosynthesis and cell wall biogenesis. In this review article, we compare data obtained from K. phaffii with S. cerevisiae and other yeasts whenever relevant, elucidate major differences, and, most importantly, highlight the big potential of using K. phaffii in fundamental research.

Komagataella phaffii YPS1-5 encodes the alpha-factor degrading protease Bar1

Yeast mating pheromones are small secreted peptides required for efficient mating between cells of opposite mating type. Pheromone gradients allow the cells to detect potential mating partners. Secreted pheromone degrading proteases steepen local gradients and allow fast recovery from the pheromone signal. The methylotrophic yeast Komagataella phaffii is a preferentially haploid species. Only under nitrogen starvation, mating genes are activated and the cells are able to undergo a full sexual cycle of mating and sporulation. It has been shown that, similar to other yeasts, K. phaffii requires the mating pheromone and pheromone surface receptor genes for efficient mating. The analysis of so far uncharacterized mating-type-specific genes allowed us to identify the K. phaffii α-factor protease gene YPS1–5. It encodes an aspartic protease of the yapsin family and is upregulated only in a-type cells under mating conditions. The phenotype of K. phaffiia-type strains with a deletion in the protease gene was found to be highly similar to the phenotype of Saccharomyces cerevisiae α-factor protease BAR1 deletion strains. They are highly sensitive to α-factor pheromone in pheromone sensitivity assays and were found to mate with reduced efficiency. Based on our results, we propose to rename the gene into K. phaffii BAR1.

Selecting for Altered Substrate Specificity Reveals the Evolutionary Flexibility of ATP-Binding Cassette Transporters

ATP-binding cassette (ABC) transporters are the largest family of ATP-hydrolyzing transporters, which import or export substrates across membranes, and have members in every sequenced genome. Structural studies and biochemistry highlight the contrast between the global structural similarity of homologous transporters and the enormous diversity of their substrates. How do ABC transporters evolve to carry such diverse molecules and what variations in their amino acid sequence alter their substrate selectivity? We mutagenized the transmembrane domains of a conserved fungal ABC transporter that exports a mating pheromone and selected for mutants that export a non-cognate pheromone. Mutations that alter export selectivity cover a region that is larger than expected for a localized substrate-binding site. Individual selected clones have multiple mutations, which have broadly additive contributions to specific transport activity. Our results suggest that multiple positions influence substrate selectivity, leading to alternative evolutionary paths toward selectivity for particular substrates and explaining the number and diversity of ABC transporters.

Srikant et al. find that mutations at many different positions in an ABC transporter of fungal mating pheromone have roughly additive effects on substrate recognition. This helps explain the evolvability of ABC transporters to transport a remarkable variety of substrates and their presence as the largest protein family across all domains of life.

Genomic diversity and meiotic recombination among isolates of the biotech yeast Komagataella phaffii (Pichia pastoris)

BACKGROUND

Komagataella phaffii is a yeast widely used in the pharmaceutical and biotechnology industries, and is one of the two species that were previously called Pichia pastoris. However, almost all laboratory work on K. phaffii has utilized strains derived from a single natural isolate, CBS7435. There is little information about the sequence diversity of K. phaffii or the genetic properties of this species.

RESULTS

We sequenced the genomes of all the known isolates of K. phaffii. We made a genetic cross between derivatives of two isolates that differ at 44,000 single nucleotide polymorphism sites, and used this cross to analyze the rate and landscape of meiotic recombination. We conducted tetrad analysis by making use of the property that K. phaffii haploids do not mate in rich media, which enabled us to isolate and sequence the four types of haploid cell that are present in the colony that forms when a tetra-type ascus germinates.

CONCLUSIONS

We found that only four distinct natural isolates of K. phaffii exist in public yeast culture collections. The meiotic recombination rate in K. phaffii is approximately 3.5 times lower than in Saccharomyces cerevisiae, with an average of 25 crossovers per meiosis. Recombination is suppressed, and genetic diversity among natural isolates is low, in a region around centromeres that is much larger than the centromeres themselves. Our work lays a foundation for future quantitative trait locus analysis in K. phaffii.

Pichia pastoris protease-deficient and auxotrophic strains generated by a novel, user-friendly vector toolbox for gene deletion

Targeted gene knockouts play an important role in the study of gene function. For the generation of knockouts in the industrially important yeast Pichia pastoris, several protocols have been published to date. Nevertheless, creating a targeted knockout in P. pastoris still is a time-consuming process, as the existing protocols are labour intensive and/or prone to accumulate nucleotide mutations. In this study, we introduce a novel, user-friendly vector-based system for the generation of targeted knockouts in P. pastoris. Upon confirming the successful knockout, respective selection markers can easily be recycled. Excision of the marker is mediated by Flippase (Flp) recombinase and occurs at high frequency (≥95%). We validated our knockout system by deleting 20 (confirmed and putative) protease genes and five genes involved in biosynthetic pathways. For the first time, we describe gene deletions of PRO3 and PHA2 in P. pastoris, genes involved in proline, and phenylalanine biosynthesis, respectively. Unexpectedly, knockout strains of PHA2 did not display the anticipated auxotrophy for phenylalanine but rather showed a bradytroph phenotype on minimal medium hinting at an alternative but less efficient pathway for production of phenylalanine exists in P. pastoris. Overall, all knockout vectors can easily be adapted to the gene of interest and strain background by efficient exchange of target homology regions and selection markers in single cloning steps. Average knockout efficiencies for all 25 genes were shown to be 40%, which is comparably high.