Membrane-displayed peptide ligand activates the pheromone response pathway in Saccharomyces cerevisiae

Novel strategy for anchorage position control of GPI-attached proteins in the yeast cell wall using different GPI-anchoring domains

The yeast cell surface provides space to display functional proteins. Heterologous proteins can be covalently anchored to the yeast cell wall by fusing them with the anchoring domain of glycosylphosphatidylinositol (GPI)-anchored cell wall proteins (GPI-CWPs). In the yeast cell-surface display system, the anchorage position of the target protein in the cell wall is an important factor that maximizes the capabilities of engineered yeast cells because the yeast cell wall consists of a 100- to 200-nm-thick microfibrillar array of glucan chains. However, knowledge is limited regarding the anchorage position of GPI-attached proteins in the yeast cell wall. Here, we report a comparative study on the effect of GPI-anchoring domain-heterologous protein fusions on yeast cell wall localization. GPI-anchoring domains derived from well-characterized GPI-CWPs, namely Sed1p and Sag1p, were used for the cell-surface display of heterologous proteins in the yeast Saccharomyces cerevisiae. Immunoelectron-microscopic analysis of enhanced green fluorescent protein (eGFP)-displaying cells revealed that the anchorage position of the GPI-attached protein in the cell wall could be controlled by changing the fused anchoring domain. eGFP fused with the Sed1-anchoring domain predominantly localized to the external surface of the cell wall, whereas the anchorage position of eGFP fused with the Sag1-anchoring domain was mainly inside the cell wall. We also demonstrate the application of the anchorage position control technique to improve the cellulolytic ability of cellulase-displaying yeast. The ethanol titer during the simultaneous saccharification and fermentation of hydrothermally-processed rice straw was improved by 30% after repositioning the exo- and endo-cellulases using Sed1- and Sag1-anchor domains. This novel anchorage position control strategy will enable the efficient utilization of the cell wall space in various fields of yeast cell-surface display technology.

REFINING CELLULAR PATHWAY MODELS USING AN ENSEMBLE OF HETEROGENEOUS DATA SOURCES

Improving current models and hypotheses of cellular pathways is one of the major challenges of systems biology and functional genomics. There is a need for methods to build on established expert knowledge and reconcile it with results of new high-throughput studies. Moreover, the available sources of data are heterogeneous, and the data need to be integrated in different ways depending on which part of the pathway they are most informative for. In this paper, we introduce a compartment specific strategy to integrate edge, node and path data for refining a given network hypothesis. To carry out inference, we use a local-move Gibbs sampler for updating the pathway hypothesis from a compendium of heterogeneous data sources, and a new network regression idea for integrating protein attributes. We demonstrate the utility of this approach in a case study of the pheromone response MAPK pathway in the yeast S. cerevisiae.

Recovery of platinum(0) through the reduction of platinum ions by hydrogenase-displaying yeast

Biological technologies for recycling rare metals, which are essential for high-tech products, have attracted much attention because they could prove to be more environmentally friendly and energy-saving than other methods. We have developed biological recycling technologies by cell surface engineering for the selective recovery of toxic heavy metal ions and rare metal ions from aqueous wastes. In this study, we aimed to construct a unique biological technique to recover rare metals ‘in solid’ form by reducing rare metal ions, leading to a practical next-generation recovery system. Sulfate-reducing bacteria (SRB) can reduce Pt(II) to Pt(0), and hydrogenases of SRB contribute to the reduction. Therefore, we constructed yeasts displaying their hydrogenases on the ‘cell membrane’, and reduction experiments were performed under anaerobic conditions without any electron donors. As a result, hydrogenase-displaying yeasts produced black precipitates in PtCl₄²⁻ solution. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) observations, the constructed yeasts were found to successfully produce the precipitates of Pt(0) through the reduction of Pt(II). Interestingly, the precipitates of Pt(0) were formed as nanoparticles, suitable for industrial usage.

Establishment of cell surface engineering and its development

Cell surface display of proteins/peptides has been established based on mechanisms of localizing proteins to the cell surface. In contrast to conventional intracellular and extracellular (secretion) expression systems, this method, generally called an arming technology, is particularly effective when using yeasts as a host, because the control of protein folding that is often required for the preparation of proteins can be natural. This technology can be employed for basic and applied research purposes. In this review, I describe various strategies for the construction of engineered yeasts and provide an outline of the diverse applications of this technology to industrial processes such as the production of biofuels and chemicals, as well as bioremediation and health-related processes. Furthermore, this technology is suitable for novel protein engineering and directed evolution through high-throughput screening, because proteins/peptides displayed on the cell surface can be directly analyzed using intact cells without concentration and purification. Functional proteins/peptides with improved or novel functions can be created using this beneficial, powerful, and promising technique.

A design for the control of apoptosis in genetically modified Saccharomyces cerevisiae

We have engineered a system that holds potential for use as a safety switch in genetically modified yeasts. Human apoptotic factor BAX (no homolog in yeast), under the control of the FBP1 (gluconeogenesis enzyme) promoter, was conditionally expressed to induce yeast cell apoptosis after glucose depletion. Such systems might prove useful for the safe use of genetically modified organisms.

Bright fluorescence monitoring system utilizing Zoanthus sp. green fluorescent protein (ZsGreen) for human G-protein-coupled receptor signaling in microbial yeast cells

G-protein-coupled receptors (GPCRs) are currently the most important pharmaceutical targets for drug discovery because they regulate a wide variety of physiological processes. Consequently, simple and convenient detection systems for ligands that regulate the function of GPCR have attracted attention as powerful tools for new drug development. We previously developed a yeast-based fluorescence reporter ligand detection system using flow cytometry. However, using this conventional detection system, fluorescence from a cell expressing GFP and responding to a ligand is weak, making detection of these cells by fluorescence microscopy difficult. We here report improvements to the conventional yeast fluorescence reporter assay system resulting in the development of a new highly-sensitive fluorescence reporter assay system with extremely bright fluorescence and high signal-to-noise (S/N) ratio. This new system allowed the easy detection of GPCR signaling in yeast using fluorescence microscopy. Somatostatin receptor and neurotensin receptor (implicated in Alzheimer's disease and Parkinson's disease, respectively) were chosen as human GPCR(s). The facile detection of binding to these receptors by cognate peptide ligands was demonstrated. In addition, we established a highly sensitive ligand detection system using yeast cell surface display technology that is applicable to peptide screening, and demonstrate that the display of various peptide analogs of neurotensin can activate signaling through the neurotensin receptor in yeast cells. Our system could be useful for identifying lead peptides with agonistic activity towards targeted human GPCR(s).

Arming Technology in Yeast-Novel Strategy for Whole-cell Biocatalyst and Protein Engineering

Cell surface display of proteins/peptides, in contrast to the conventional intracellular expression, has many attractive features. This arming technology is especially effective when yeasts are used as a host, because eukaryotic modifications that are often required for functional use can be added to the surface-displayed proteins/peptides. A part of various cell wall or plasma membrane proteins can be genetically fused to the proteins/peptides of interest to be displayed. This technology, leading to the generation of so-called "arming technology", can be employed for basic and applied research purposes. In this article, we describe various strategies for the construction of arming yeasts, and outline the diverse applications of this technology to industrial processes such as biofuel and chemical productions, pollutant removal, and health-related processes, including oral vaccines. In addition, arming technology is suitable for protein engineering and directed evolution through high-throughput screening that is made possible by the feature that proteins/peptides displayed on cell surface can be directly analyzed using intact cells without concentration and purification. Actually, novel proteins/peptides with improved or developed functions have been created, and development of diagnostic/therapeutic antibodies are likely to benefit from this powerful approach.

Membrane-tethered ligands: tools for cell-autonomous pharmacological manipulation of biological circuits

Detection of secreted signaling molecules by cognate cell surface receptors is a major intercellular communication pathway in cellular circuits that control biological processes. Understanding the biological significance of these connections would allow us to understand how cellular circuits operate as a whole. Membrane-tethered ligands are recombinant transgenes with structural modules that allow them to act on cell-surface receptors and ion channel subtypes with pharmacological specificity in a cell-autonomous manner. Membrane-tethered ligands have been successful in the specific manipulation of ion channels as well as G-protein-coupled receptors, and, in combination with cell-specific promoters, such manipulations have been restricted to genetically defined subpopulations within cellular circuits in vivo to induce specific phenotypes controlled by those circuits. These studies establish the membrane-tethering approach as a generally applicable method for dissecting neural and physiological circuits.

Membrane-displayed somatostatin activates somatostatin receptor subtype-2 heterologously produced in Saccharomyces cerevisiae

The G-protein-coupled receptor (GPCR) superfamily, which includes somatostatin receptors (SSTRs), is one of the most important drug targets in the pharmaceutical industry. The yeast Saccharomyces cerevisiae is an attractive host for the ligand screening of human GPCRs. Here, we demonstrate the utility of the technology that was developed for displaying peptide ligands on yeast plasma membrane, termed "PepDisplay", which triggers signal transduction upon GPCR activation. A yeast strain that heterologously produced human somatostatin receptor subtype-2 (SSTR2) and chimeric Gα protein was constructed along with membrane-displayed somatostatin; somatostatin was displayed on the yeast plasma membrane by linking it to the anchoring domain of the glycosylphosphatidylinositol anchored plasma membrane protein Yps1p. We demonstrate that the somatostatin displayed on the plasma membrane successfully activated human SSTR2 in S. cerevisiae. The methodology presented here provides a new platform for identifying novel peptide ligands for both liganded and orphan mammalian GPCRs.