AG alpha 1 is the structural gene for the Saccharomyces cerevisiae alpha-agglutinin, a cell surface glycoprotein involved in cell-cell interactions during mating

The Flo Adhesin Family

The first step in the infection of fungal pathogens in humans is the adhesion of the pathogen to host tissue cells or abiotic surfaces such as catheters and implants. One of the main players involved in this are the expressed cell wall adhesins. Here, we review the Flo adhesin family and their involvement in the adhesion of these yeasts during human infections. Firstly, we redefined the Flo adhesin family based on the domain architectures that are present in the Flo adhesins and their functions, and set up a new classification of Flo adhesins. Next, the structure, function, and adhesion mechanisms of the Flo adhesins whose structure has been solved are discussed in detail. Finally, we identified from Pfam database datamining yeasts that could express Flo adhesins and are encountered in human infections and their adhesin architectures. These yeasts are discussed in relation to their adhesion characteristics and involvement in infections.

Enzymatic Analysis of Yeast Cell Wall-Resident GAPDH and Its Secretion

In yeast, many proteins are found in both the cytoplasmic and extracellular compartments, and consequently it can be difficult to distinguish nonconventional secretion from cellular leakage. Therefore, we monitored the extracellular glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity of intact cells as a specific marker for nonconventional secretion. Extracellular GAPDH activity was proportional to the number of cells assayed, increased with incubation time, and was dependent on added substrates. Preincubation of intact cells with 100 μM dithiothreitol increased the reaction rate, consistent with increased access of the enzyme after reduction of cell wall disulfide cross-links. Such treatment did not increase cell permeability to propidium iodide, in contrast to effects of higher concentrations of reducing agents. An amine-specific membrane-impermeant biotinylation reagent specifically inactivated extracellular GAPDH. The enzyme was secreted again after a 30- to 60-min lag following the inactivation, and there was no concomitant increase in propidium iodide staining. There were about 4 × 10⁴ active GAPDH molecules per cell at steady state, and secretion studies showed replenishment to that level 1 h after inactivation. These results establish conditions for specific quantitative assays of cell wall proteins in the absence of cytoplasmic leakage and for subsequent quantification of secretion rates in intact cells.IMPORTANCE Eukaryotic cells secrete many proteins, including many proteins that do not follow the classical secretion pathway. Among these, the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is unexpectedly found in the walls of yeasts and other fungi and in extracellular space in mammalian cell cultures. It is difficult to quantify extracellular GAPDH, because leakage of just a little of the very large amount of cytoplasmic enzyme can invalidate the determinations. We used enzymatic assays of intact cells while also maintaining membrane integrity. The results lead to estimates of the amount of extracellular enzyme and its rate of secretion to the wall in intact cells. Therefore, enzyme assays under controlled conditions can be used to investigate nonconventional secretion more generally.

Engineering Escherichia coli for Direct Production of 1,2-Propanediol and 1,3-Propanediol from Starch

Diols are versatile chemicals used for multiple manufacturing products. In some previous studies, Escherichia coli has been engineered to produce 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) from glucose. However, there are no reports on the direct production of these diols from starch instead of glucose as a substrate. In this study, we directly produced 1,2-PDO and 1,3-PDO from starch using E. coli engineered for expressing a heterologous α-amylase, along with the expression of 1,2-PDO and 1,3-PDO synthetic genes. For this, the recombinant plasmids, pVUB3-SBA harboring amyA gene for α-amylase production, pSR5 harboring pct, pduP, and yahK genes for 1,2-PDO production, and pSR8 harboring gpd1-gpp2, dhaB123, gdrAB, and dhaT genes for 1,3-PDO production, were constructed. Subsequently, E. coli BW25113 (ΔpflA) and BW25113 strains were transformed with pVUB3-SBA, pSR5, and/or pSR8. Using these transformants, direct production of 1,2-PDO and 1,3-PDO from starch was demonstrated under microaerobic condition. As a result, the maximum production titers of 1,2-PDO and 1,3-PDO from 1% glucose as a sole carbon source were 13 mg/L and 150 mg/L, respectively. The maximum production titers from 1% starch were similar levels (30 mg/L 1,2-PDO and 120 mg/L 1,3-PDO). These data indicate that starch can be an alternative carbon source for the production of 1,2-PDO and 1,3-PDO in engineered E. coli. This technology could simplify the upstream process of diol bioproduction.

Adhesins of Yeasts: Protein Structure and Interactions

The ability of yeast cells to adhere to other cells or substrates is crucial for many yeasts. The budding yeast Saccharomyces cerevisiae can switch from a unicellular lifestyle to a multicellular one. A crucial step in multicellular lifestyle adaptation is self-recognition, self-interaction, and adhesion to abiotic surfaces. Infectious yeast diseases such as candidiasis are initiated by the adhesion of the yeast cells to host cells. Adhesion is accomplished by adhesin proteins that are attached to the cell wall and stick out to interact with other cells or substrates. Protein structures give detailed insights into the molecular mechanism of adhesin-ligand interaction. Currently, only the structures of a very limited number of N-terminal adhesion domains of adhesins have been solved. Therefore, this review focuses on these adhesin protein families. The protein architectures, protein structures, and ligand interactions of the flocculation protein family of S. cerevisiae; the epithelial adhesion family of C. glabrata; and the agglutinin-like sequence protein family of C. albicans are reviewed and discussed.

Gamete signalling underlies the evolution of mating types and their number

The gametes of unicellular eukaryotes are morphologically identical, but are nonetheless divided into distinct mating types. The number of mating types varies enormously and can reach several thousand, yet most species have only two. Why do morphologically identical gametes need to be differentiated into self-incompatible mating types, and why is two the most common number of mating types? In this work, we explore a neglected hypothesis that there is a need for asymmetric signalling interactions between mating partners. Our review shows that isogamous gametes always interact asymmetrically throughout sex and argue that this asymmetry is favoured because it enhances the efficiency of the mating process. We further develop a simple mathematical model that allows us to study the evolution of the number of mating types based on the strength of signalling interactions between gametes. Novel mating types have an advantage as they are compatible with all others and rarely meet their own type. But if existing mating types coevolve to have strong mutual interactions, this restricts the spread of novel types. Similarly, coevolution is likely to drive out less attractive mating types. These countervailing forces specify the number of mating types that are evolutionarily stable.

This article is part of the themed issue ‘Weird sex: the underappreciated diversity of sexual reproduction’.

Mapping of endoglucanases displayed on yeast cell surface using atomic force microscopy

The surface of yeast cells has been an attractive interface for the effective use of cellulose. Surface enzymes, however, are difficult to visualize and evaluate. In this study, two kinds of unique anchoring regions were used to display the cellulase, endoglucanase (EG), on a yeast cell surface. Differences in the display level and the localization of EG were observed by atomic force microscopy. By surveying the yeast cell surface with a chemically modified cantilever, the interactive force between the cellulose and EG was measured. Force curve mapping revealed differences in the display levels and the localization of EG according to anchoring regions. The proposed methodology enables visualization of displayed enzymes such as EG on the yeast cell surface.

Candida albicans Agglutinin-Like Sequence (Als) Family Vignettes: A Review of Als Protein Structure and Function

Approximately two decades have passed since the description of the first gene in the Candida albicans ALS (agglutinin-like sequence) family. Since that time, much has been learned about the composition of the family and the function of its encoded cell-surface glycoproteins. Solution of the structure of the Als adhesive domain provides the opportunity to evaluate the molecular basis for protein function. This review article is formatted as a series of fundamental questions and explores the diversity of the Als proteins, as well as their role in ligand binding, aggregative effects, and attachment to abiotic surfaces. Interaction of Als proteins with each other, their functional equivalence, and the effects of protein abundance on phenotypic conclusions are also examined. Structural features of Als proteins that may facilitate invasive function are considered. Conclusions that are firmly supported by the literature are presented while highlighting areas that require additional investigation to reveal basic features of the Als proteins, their relatedness to each other, and their roles in C. albicans biology.

Engineering of Immunoglobulin Fc Heterodimers Using Yeast Surface-Displayed Combinatorial Fc Library Screening

Immunoglobulin Fc heterodimers, which are useful scaffolds for the generation of bispecific antibodies, have been mostly generated through structure-based rational design methods that introduce asymmetric mutations into the CH3 homodimeric interface to favor heterodimeric Fc formation. Here, we report an approach to generate heterodimeric Fc variants through directed evolution combined with yeast surface display. We developed a combinatorial heterodimeric Fc library display system by mating two haploid yeast cell lines, one haploid cell line displayed an Fc chain library (displayed FcCH3A) with mutations in one CH3 domain (CH3A) on the yeast cell surface, and the other cell line secreted an Fc chain library (secreted FcCH3B) with mutations in the other CH3 domain (CH3B). In the mated cells, secreted FcCH3B is displayed on the cell surface through heterodimerization with the displayed FcCH3A, the detection of which enabled us to screen the library for heterodimeric Fc variants. We constructed combinatorial heterodimeric Fc libraries with simultaneous mutations in the homodimer-favoring electrostatic interaction pairs K370-E357/S364 or D399-K392/K409 at the CH3 domain interface. High-throughput screening of the libraries using flow cytometry yielded heterodimeric Fc variants with heterodimer-favoring CH3 domain interface mutation pairs, some of them showed high heterodimerization yields (~80-90%) with previously unidentified CH3 domain interface mutation pairs, such as hydrogen bonds and cation-π interactions. Our study provides a new approach for engineering Fc heterodimers that could be used to engineer other heterodimeric protein-protein interactions through directed evolution combined with yeast surface display.

The Candida albicans agglutinin-like sequence family of adhesins: functional insights gained from structural analysis

Candida albicans colonizes many host sites suggesting its interaction with diverse ligands. Candida albicans adhesion is mediated by a number of proteins including those in the Als (agglutinin-like sequence) family, which have been studied intensively. The recent solution of the Als binding domain structure ended years of speculation regarding the molecular mechanism for Als adhesive function. Als adhesins bind flexible C termini from a broad collection of proteins, providing the basis for adhesion to various cell types and perhaps for C. albicans broad tissue tropism. Understanding adhesive functions at the molecular level will reveal the sequence of events in C. albicans pathogenesis, from host recognition to complex interactions such as development of polymicrobial biofilms or disseminated disease.

Display of heterologous proteins on the Saccharomyces cerevisiae surface display system using a single constitutive expression vector

In this study, we constructed a novel and simple yeast surface display system with a single expression vector. The newly established system uses a bidirectional expression vector carrying the AGA1 gene driven by the PGK1 promoter in one direction and the AGA2-expression cassette driven by the TEF1 promoter in the reverse direction, and uses the geneticin, a G418-resistant gene, as the selection marker for transformants. Because all the display elements are put into one expression vector, the new system is much simpler to use, and there is no need for any genetic modification of the host strains; therefore, the new system can be used in wild type as well as laboratory strains of Saccharomyces cerevisiae. The display efficiency of heterologous proteins using the new system has been confirmed by displaying enhanced green fluorescent protein and Eimeria tenella (a chicken protozoan parasite) microneme protein2 (EtMic2) on several S. cerevisiae strains. We also tested the new system with an aga2 mutant strain of S. cerevisiae. The results indicate that the native expressed Aga2 protein has no effect on the display efficiency of heterologous proteins.

Therapeutic antibody engineering by high efficiency cell screening

In recent years, several cell-based screening technologies for the isolation of antibodies with prescribed properties emerged. They rely on the multi-copy display of antibodies or antibody fragments on a cell surface in functional form followed by high through put screening and isolation of cell clones that carry an antibody variant with the desired affinity, specificity, and stability. Particularly yeast surface display in combination with high-throughput fluorescence-activated cell sorting has proven successful in the last fifteen years as a very powerful technology that has some advantages over classical generation of monoclonals using the hybridoma technology or bacteriophage-based antibody display and screening. Cell-based screening harbours the benefit of single-cell online and real-time analysis and characterisation of individual library candidates. Moreover, when using eukaryotic expression hosts, intrinsic quality control machineries for proper protein folding and stability exist that allow for co-selection of high-level expression and stability simultaneously to the binding functionality. Recently, promising technologies emerged that directly rely on antibody display on higher eukaryotic cell lines using lentiviral transfection or direct screening on B-cells. The combination of immunisation, B-cell screening and next generation sequencing may open new avenues for the isolation of therapeutic antibodies with prescribed physicochemical and functional characteristics.

Expression of Pneumocystis jirovecii major surface glycoprotein in Saccharomyces cerevisiae

The major surface glycoprotein (Msg), which is the most abundant protein expressed on the cell surface of Pneumocystis organisms, plays an important role in the attachment of this organism to epithelial cells and macrophages. In the present study, we expressed Pneumocystis jirovecii Msg in Saccharomyces cerevisiae, a phylogenetically related organism. Full-length P. jirovecii Msg was expressed with a DNA construct that used codons optimized for expression in yeast. Unlike in Pneumocystis organisms, recombinant Msg localized to the plasma membrane of yeast rather than to the cell wall. Msg expression was targeted to the yeast cell wall by replacing its signal peptide, serine-threonine-rich region, and glycophosphatidylinositol anchor signal region with the signal peptide of cell wall protein α-agglutinin of S. cerevisiae, the serine-threonine-rich region of epithelial adhesin (Epa1) of Candida glabrata, and the carboxyl region of the cell wall protein (Cwp2) of S. cerevisiae, respectively. Immunofluorescence analysis and treatment with β-1,3 glucanase demonstrated that the expressed Msg fusion protein localized to the yeast cell wall. Surface expression of Msg protein resulted in increased adherence of yeast to A549 alveolar epithelial cells. Heterologous expression of Msg in yeast will facilitate studies of the biologic properties of Pneumocystis Msg.

Accelerated and adaptive evolution of yeast sexual adhesins

There is a recent emergence of interest in the genes involved in gametic recognition as drivers of reproductive isolation. The recent population genomic sequencing of two species of sexually primitive yeasts (Liti G, Carter DM, Moses AM, Warringer J, Parts L, James SA, Davey RP, Roberts IN, Burt A, Koufopanou V et al. [23 co-authors]. 2009. Population genomics of domestic and wild yeasts. Nature 458:337-341.) has provided data for systematic study of the roles these genes play in the early evolution of sex and speciation. Here, we discovered that among genes encoding cell surface proteins, the sexual adhesin genes have evolved significantly more rapidly than others, both within and between Saccharomyces cerevisiae and its closest relative S. paradoxus. This result was supported by analyses using the PAML pairwise model, a modified McDonald-Kreitman test, and the PAML branch model. Moreover, using a combination of a new statistic of neutrality, an information theory-based measure of evolutionary variability, and functional characterization of amino acid changes, we found that a higher proportion of amino acid changes are fixed in the sexual adhesins than in other proteins and a greater proportion of the fixed amino acid changes either between the two species or the two subgroups of S. paradoxus are functionally dissimilar or radically different. These results suggest that the accelerated evolution of sexual adhesin genes may facilitate speciation, or incipient speciation, and promote sexual selection in general.

A screen for deficiencies in GPI-anchorage of wall glycoproteins in yeast

Many of the genes and enzymes critical for assembly and biogenesis of yeast cell walls remain unidentified or poorly characterized. Therefore, we designed a high throughput genomic screen for defects in anchoring of GPI-cell wall proteins (GPI-CWPs), based on quantification of a secreted GFP-Sag1p fusion protein. Saccharomyces cerevisiae diploid deletion strains were transformed with a plasmid expressing the fusion protein under a GPD promoter, then GFP fluorescence was determined in culture supernatants after mid-exponential growth. Variability in the amount of fluorescent marker secreted into the medium was reduced by growth at 18 degrees C in buffered defined medium in the presence of sorbitol. Secondary screens included immunoblotting for GFP, fluorescence emission spectra, cell surface fluorescence, and cell integrity. Of 167 mutants deleted for genes affecting cell wall biogenesis or structure, eight showed consistent hyper-secretion of GFP relative to parental strain BY4743: tdh3 (glyceraldehyde-3-phosphate dehydrogenase), gda1 (guanosine diphosphatase), gpi13 and mcd4 (both ethanolamine phosphate-GPI-transferases), kre5 and kre1 (involved in synthesis of beta1,6 glucan), dcw1(implicated in GPI-CWP cross-linking to cell wall glucan), and cwp1 (a major cell wall protein). In addition, deletion of a number of genes caused decreased secretion of GFP. These results elucidate specific roles for specific genes in cell wall biogenesis, including differentiating among paralogous genes.

Discovering the secrets of the Candida albicans agglutinin-like sequence (ALS) gene family--a sticky pursuit

The agglutinin-like sequence (ALS) family of Candida albicans includes eight genes that encode large cell-surface glycoproteins. The high degree of sequence relatedness between the ALS genes and the tremendous allelic variability often present in the same C. albicans strain complicated definition and characterization of the gene family. The main hypothesis driving ALS family research is that the genes encode adhesins, primarily involved in host-pathogen interactions. Although adhesive function has been demonstrated for several Als proteins, the challenge of studying putative adhesins in a highly adhesive organism like C. albicans has led to varying ideas about how best to pursue such investigations, and results that are sometimes contradictory. Recent analysis of alsdelta/alsdelta strains suggested roles for Als proteins outside of adhesion to host surfaces, and a broader scope of Als protein function than commonly believed. The availability and use of experimental methodologies to study C. albicans at the genomic level, and the ALS family en masse, have advanced knowledge of these genes and emphasized their importance in C. albicans biology and pathogenesis.

Molecular phylogenetics of ascomycotal adhesins--a novel family of putative cell-surface adhesive proteins in fission yeasts

In this work, we identify a family of putative adhesins in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. The members of this family share a conserved tandem repeat related to those found in the Candida albicans Als family of adhesins. Unlike previously characterised adhesins that possess conserved ligand-binding domains at the N-terminus, this group of proteins carry ligand-binding domains at their C-termini. We demonstrate that one such domain--the uncharacterised GLEYA domain, is related to the lectin-like ligand-binding domain found in the Saccharomyces cerevisiae Flo proteins. Unlike the Flo and Als proteins, the fission yeast adhesins do not contain detectable glycosyl phosphatidyl inositol (GPI) membrane anchor signals to mediate their attachment to the cell wall, which may suggest a novel cell wall attachment mechanism. Further sequence analysis identified several putative adhesins in the sub-phylum of Pezizomycotina, where only a few adhesins have been described to date.

Candida albicans Als3p is required for wild-type biofilm formation on silicone elastomer surfaces

Candida albicans ALS3 encodes a large cell-surface glycoprotein that has adhesive properties. Immunostaining of cultured C. albicans germ tubes showed that Als3p is distributed diffusely across the germ tube surface. Two-photon laser scanning microscopy of model catheter biofilms grown using a PALS3-green fluorescent protein (GFP) reporter strain showed GFP production in hyphae throughout the biofilm structure while biofilms grown using a PTPI1-GFP reporter strain showed GFP in both hyphae and yeast-form cells. Model catheter biofilms formed by an als3 Delta/als3 Delta strain were weakened structurally and had approximately half the biomass of a wild-type biofilm. Reintegration of a wild-type ALS3 allele restored biofilm mass and wild-type biofilm structure. Production of an Als3p-Ag alpha 1p fusion protein under control of the ALS3 promoter in the als3 Delta/als3 Delta strain restored some of the wild-type biofilm structural features, but not the wild-type biofilm mass. Despite its inability to restore wild-type biofilm mass, the Als3p-Ag alpha 1p fusion protein mediated adhesion of the als3 Delta/als3 Delta C. albicans strain to human buccal epithelial cells (BECs). The adhesive role of the Als3p N-terminal domain was further demonstrated by blocking adhesion of C. albicans to BECs with immunoglobulin reactive against the Als3p N-terminal sequences. Together, these data suggest that portions of Als3p that are important for biofilm formation may be different from those that are important in BEC adhesion, and that Als3p may have multiple functions in biofilm formation. Overexpression of ALS3 in an efg1 Delta/efg1 Delta strain that was deficient for filamentous growth and biofilm formation resulted in growth of elongated C. albicans cells, even under culture conditions that do not favour filamentation. In the catheter biofilm model, the ALS3 overexpression strain formed biofilm with a mass similar to that of a wild-type control. However, C. albicans cells in the biofilm had yeast-like morphology. This result uncouples the effect of cellular morphology from biofilm formation and underscores the importance of Als3p in biofilm development on silicone elastomer surfaces.

Role of cell cycle-regulated expression in the localized incorporation of cell wall proteins in yeast

The yeast cell wall is an essential organelle that protects the cell from mechanical damage and antimicrobial peptides, participates in cell recognition and adhesion, and is important for the generation and maintenance of normal cell shape. We studied the localization of three covalently bound cell wall proteins in Saccharomyces cerevisiae. Tip1p was found only in mother cells, whereas Cwp2p was incorporated in small-to-medium-sized buds. When the promoter regions of TIP1 and CWP2 (responsible for transcription in early G1 and S/G2 phases, respectively) were exchanged, the localization patterns of Tip1p and Cwp2p were reversed, indicating that the localization of cell wall proteins can be completely determined by the timing of transcription during the cell cycle. The third protein, Cwp1p, was incorporated into the birth scar, where it remained for several generations. However, we could not detect any role of Cwp1p in strengthening the birth scar wall or any functional interaction with the proteins that mark the birth scar pole as a potential future budding site. Promoter-exchange experiments showed that expression in S/G2 phase is necessary but not sufficient for the normal localization of Cwp1p. Studies of mutants in which septum formation is perturbed indicate that the normal asymmetric localization of Cwp1p also depends on the normal timing of septum formation, composition of the septum, or both.

Ethanol fermentation from lignocellulosic hydrolysate by a recombinant xylose- and cellooligosaccharide-assimilating yeast strain

The sulfuric acid hydrolysate of lignocellulosic biomass, such as wood chips, from the forest industry is an important material for fuel bioethanol production. In this study, we constructed a recombinant yeast strain that can ferment xylose and cellooligosaccharides by integrating genes for the intercellular expressions of xylose reductase and xylitol dehydrogenase from Pichia stipitis, and xylulokinase from Saccharomyces cerevisiae and a gene for displaying beta-glucosidase from Aspergillus acleatus on the cell surface. In the fermentation of the sulfuric acid hydrolysate of wood chips, xylose and cellooligosaccharides were completely fermented after 36 h by the recombinant strain, and then about 30 g/l ethanol was produced from 73 g/l total sugar added at the beginning. In this case, the ethanol yield of this recombinant yeast was much higher than that of the control yeast. These results demonstrate that the fermentation of the lignocellulose hydrolysate is performed efficiently by the recombinant Saccharomyces strain with abilities for xylose assimilation and cellooligosaccharide degradation.

ALS3 and ALS8 represent a single locus that encodes a Candida albicans adhesin; functional comparisons between Als3p and Als1p

The ALS (agglutinin-like sequence) gene family of Candida albicans encodes eight cell-surface glycoproteins, some of which are involved in adherence to host surfaces. A mutational analysis of each ALS gene is currently being performed to deduce the functions of the encoded proteins and to better understand the role of these proteins in C. albicans biology and pathogenesis. This paper describes construction of an als3/als3 mutant and comparison of its phenotype to an als1/als1 strain. Efforts to disrupt ALS3 indicated that the gene could be deleted in two transformation steps, suggesting that the gene is encoded by a single locus and that the ALS3-like locus, ALS8, does not exist. Strains lacking ALS3 or ALS1 did not exhibit a defect in germ tube formation when grown in RPMI 1640 medium, but the als1/als1 mutant formed significantly fewer germ tubes in Lee medium. Analysis of ALS3 and ALS1 promoter activity using green fluorescent protein (GFP) reporter strains and flow cytometry showed that when cells are placed into medium that promotes germ tube formation, ALS1 is transcribed prior to ALS3. Comparison of the mutant strains in adhesion assays showed that the als3/als3 strain was defective in adhesion to both human umbilical vein endothelial cells (HUVEC) and buccal epithelial cells (BEC), but not to fibronectin-coated plastic plates. In contrast, the als1/als1 strain showed decreased adherence to HUVEC, but adherence to BEC and fibronectin were the same as wild-type controls. Inoculation of the buccal reconstituted human epithelium (RHE) model of oral candidiasis with the mutant strains showed nearly a total lack of adhesion and epithelial destruction by the als3/als3 mutant while the als1/als1 strain showed only a slightly reduced degree of epithelial destruction compared to the wild-type control. Adhesion data presented here suggest that, in the assays performed, loss of Als3p affects C. albicans adhesion more than loss of Als1p. Collectively, these results demonstrate functional similarities and differences between Als1p and Als3p, and suggest the potential for more complex interrelationships between the ALS genes and their encoded proteins.

Deficiencies in the endoplasmic reticulum (ER)-membrane protein Gab1p perturb transfer of glycosylphosphatidylinositol to proteins and cause perinuclear ER-associated actin bar formation

The essential GAB1 gene, which encodes an endoplasmic reticulum (ER)-membrane protein, was identified in a screen for mutants defective in cellular morphogenesis. A temperature-sensitive gab1 mutant accumulates complete glycosylphosphatidylinositol (GPI) precursors, and its temperature sensitivity is suppressed differentially by overexpression of different subunits of the GPI transamidase, from strong suppression by Gpi8p and Gpi17p, to weak suppression by Gaa1p, and to no suppression by Gpi16p. In addition, both Gab1p and Gpi17p localize to the ER and are in the same protein complex in vivo. These findings suggest that Gab1p is a subunit of the GPI transamidase with distinct relationships to other subunits in the same complex. We also show that depletion of Gab1p or Gpi8p, but not Gpi17p, Gpi16p, or Gaa1p causes accumulation of cofilin-decorated actin bars that are closely associated with the perinuclear ER, which highlights a functional interaction between the ER network and the actin cytoskeleton.

Identification and study of a Candida albicans protein homologous to Saccharomyces cerevisiae Ssr1p, an internal cell-wall protein

After screening of a Candida albicans genome database, the product of an ORF (IPF 3054) that has 62 % homology with Saccharomyces cerevisiae Ssr1p, an internal cell-wall protein, was identified and named CaSsr1p. The deduced amino acid sequence shows that CaSsr1p contains an N-terminal hydrophobic signal peptide, is rich in Ser and Thr amino acids and has a potential glycosylphosphatidylinositol-attachment signal. CaSsr1p is released following degradation of isolated cell walls by zymolyase (mainly a 1,3-beta-glucanase) and therefore seems to be covalently linked to the beta-glucan of the cell walls. Both disruption and overexpression of the CaSSR1 gene caused an increased sensitivity to calcofluor white, Congo red and zymolyase digestion. These results suggest that CaSsr1p has a structural role associated with the cell-wall beta-glucan.

Roles of sexual cell agglutination in yeast mass mating

The agalpha1 mutant MAT alpha cells specifically lack the cell surface alpha-type sexual agglutination substance, which is also called alpha-agglutinin. Because the mutant cells (MATalpha agalpha1) can not form aggregates with MATa cells, MATalpha agalpha1 cells are unable to mate with MATa cells when they are co-inoculated in a liquid medium, and the mating is attenuated on solid medium. The attenuated mating ability shown in the previous studies gave us a vague idea about a physiological function of the sexual agglutinability. In order to solve the question, mating behavior of MATalpha agalpha1 cells was investigated here under conditions where the contact between MATa and MAT alpha cells is assisted by physical methods. A synthetic mutation agalpha1::URA3 was constructed and used as well as agalpha1-1 for this study to ensure the genetic defect. When a mixture of MATa and MAT alpha cells was kept on filter membrane placed on relatively dry agar medium, even agalpha1::URA3 mutant cells mated as efficiently as the wild type (AGalpha1) cells did. On filter membrane placed on moist agar medium, agalpha1 mutants mated 10-fold less efficiently than wild type cells did. The mutant cells mated 10000-time less efficiently than the wild type cells in a pellet formed by brief low speed centrifugation. In contrast, the wild type MATalpha cells mated well under all conditions tested. Under the pellet condition, a mixture of MATa and MATalpha AG alpha1 cells formed an extended and cotton-like pellet while a mixture of MATa and MATalpha agalpha1 cells formed a compact and tight pellet. These results suggest that sexual cell agglutination contributes not only to cell contact between MATa and MAT alpha cells thereby stabilizing a-alpha cell pairs, but also to construction of a uniquely organized ultra structure favorable for zygote formation and subsequent growth of diploid cells. The mating specific extended pellet formation was observed also in 4 pairs of a and alpha strains in ascosporogenous yeast genera Hansenula and Pichia.

Two homologous genes, DCW1 (YKL046c) and DFG5, are essential for cell growth and encode glycosylphosphatidylinositol (GPI)-anchored membrane proteins required for cell wall biogenesis in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae consists of glucan, chitin and various kinds of mannoproteins. Major parts of mannoproteins are synthesized as glycosylphosphatidylinositol (GPI)-anchored proteins and are then transferred to cell wall beta-1,6-glucan. A glycosyltransferase has been hypothesized to catalyse this transfer reaction. A database search revealed that the products of YKL046c and DFG5 are homologous to bacterial mannosidase. These genes are homologous to each other and have primary structures characteristic of GPI-anchored proteins. Although single disruptants of ykl046c and dfg5 were viable, ykl046cDelta was hypersensitive to a cell wall-digesting enzyme (zymolyase), suggesting that this gene is involved in cell wall biosynthesis. We therefore designated this gene as DCW1 (defective cell wall). A double disruptant of dcw1 and dfg5 was synthetically lethal, indicating that the functions of these gene products are redundant, and at least one of them is required for cell growth. Cells deficient in both Dcw1p and Dfg5p were round and large, had cell walls that contained an increased amount of chitin and secreted a major cell wall protein, Cwp1p, into the medium. Biochemical analyses showed that epitope-tagged Dcw1p is an N-glycosylated, GPI-anchored membrane protein and is localized in the membrane fraction including the cell surface. These results suggest that both Dcw1p and Dfg5p are GPI-anchored membrane proteins and are required for normal biosynthesis of the cell wall.

Genetic characterization of genes encoding enzymes catalyzing addition of phospho-ethanolamine to the glycosylphosphatidylinositol anchor in Saccharomyces cerevisiae

MPC1/GPI13/YLL031C, one of the genes involved in the addition of phospho-ethanolamine to the glycosylphosphatidylinositol (GPI) anchor core, is an essential gene. Three available temperature-sensitive mutant alleles, mpc1-3, mpc1-4, and mpc1-5, displayed different phenotypes to each other and, correspondingly, these mutants were found to have different mutations in the MPC1 ORF. Temperature-sensitivity of mpc1-5 mutants was suppressed by 5 mM ZnSO(4) and by 5 mM MnCl(2). Multicopy suppressors were isolated from mpc1-5 mutant. Suppressors commonly effective to mpc1-4 and mpc1-5 mutations are PSD1, encoding phosphatidylserine decarboxylase, and ECM33, which were found to suppress the temperature-sensitive phenotype shown by the fsr2-1 and las21delta mutants, those of which have defects in the GPI anchor synthesis. PSD2, encoding another phosphatidylserine decarboxylase that is localized in Golgi/vacuole, was found to be able to serve as a multicopy suppressor of mpc1 and fsr2-1 mutants but not of the las21 delta mutant. In contrast to psd1delta, psd2delta showed a synthetic growth defect with mpc1 mutants but not with fsr2-1 or las21delta. Furthermore, psd1delta psd2delta mpc1 triple mutants did not form colonies on nutrient medium unless ethanolamine was supplied to the medium, whereas psd1delta psd2 delta fsr2-1 or psd1delta psd2 delta las21delta triple mutants grew on nutrient medium without supplementation of ethanolamine. These observations suggest that Mpc1 preferentially utilizes phosphatidylethanolamine produced by Psd2 that is localized in Golgi/vacuole. fsr2-1 dpl1 Delta psd1delta strains showed slower growth than fsr2-1 dpl1delta psd2 delta, suggesting that Fsr2 enzyme depends more on Dpl1 and Psd1 for production of phosphatidylethanolamine. Las21 did not show preference for the metabolic pathway to produce phosphatidylethanolamine.

Characterization of a cell-wall acid phosphatase (PhoAp) in Aspergillus fumigatus

In the filamentous fungus Aspergillus fumigatus, the vast majority of the cell-wall-associated proteins are secreted proteins that are in transit in the cell wall. These proteins can be solubilized by detergents and reducing agents. Incubation of a SDS/beta-mercaptoethanol-treated cell-wall extract with various recombinant enzymes that hydrolyse cell-wall polysaccharides resulted in the release of a unique protein in minute amounts only after incubation of the cell wall in the presence of 1,3-beta-glucanase. Sequence analysis and biochemical studies showed that this glycoprotein, with an apparent molecular mass of 80 kDa, was an acid phosphatase (PhoAp) that was active on both phosphate monoesters and phosphate diesters. PhoAp is a glycosylphosphatidylinositol-anchored protein that was recovered in the culture filtrate and cell-wall fraction of A. fumigatus after cleavage of its anchor. It is also a phosphate-repressible acid phosphatase. The absence of PhoAp from a phosphate-rich medium was not associated with a reduction in fungal growth, indicating that this cell-wall-associated protein does not play a role in the morphogenesis of A. fumigatus.

Delineation of functional regions within the subunits of the Saccharomyces cerevisiae cell adhesion molecule a-agglutinin

a-Agglutinin from Saccharomyces cerevisiae is a cell adhesion glycoprotein expressed on the surface of cells of a mating type and consists of an anchorage subunit Aga1p and a receptor binding subunit Aga2p. Cell wall attachment of Aga2p is mediated through two disulfide bonds to Aga1p (Cappellaro, C., Baldermann, C., Rachel, R., and Tanner, W. (1994) EMBO J. 13, 4737-4744). We report here that purified Aga2p was unstable and had low molar specific activity relative to its receptor alpha-agglutinin. Aga2p co-expressed with a 149-residue fragment of Aga1p formed a disulfide-linked complex with specific activity 43-fold higher than Aga2p expressed alone. Circular dichroism of the complex revealed a mixed alpha/beta structure, whereas Aga2p alone had no periodic secondary structure. A 30-residue Cys-rich Aga1p fragment was partially active in stabilization of Aga2p activity. Mutation of either or both Aga2p cysteine residues eliminated stabilization of Aga2p. Thus the roles of Aga1p include both cell wall anchorage and cysteine-dependent conformational restriction of the binding subunit Aga2p. Mutagenesis of AGA2 identified only C-terminal residues of Aga2p as being essential for binding activity. Aga2p residues 45-72 are similar to sequences in soybean Nod genes, and include residues implicated in interactions with both Aga1p (including Cys(68)) and alpha-agglutinin.

The ALS gene family of Candida albicans

The ALS gene family of Candida albicans encodes large cell-surface glycoproteins that are implicated in the process of adhesion to host surfaces. ALS genes are also found in other Candida species that are isolated from cases of clinical disease. Genes in the ALS family are differentially regulated by physiologically relevant mechanisms. ALS genes exhibit several levels of variability including strain- and allele-specific size differences for the same gene, strain-specific differences in gene regulation, the absence of particular ALS genes in certain isolates, and additional ALS coding regions in others. The differential regulation and genetic variability of the ALS genes results in a diverse cell-surface Als protein profile that is also affected by growth conditions. The ALS genes are one example of a gene family associated with pathogenicity mechanisms in C. albicans and other Candida species.

Characterization of agglutinin-like sequence genes from non-albicans Candida and phylogenetic analysis of the ALS family

The ALS (agglutinin-like sequence) gene family of Candida albicans encodes cell-surface glycoproteins implicated in adhesion of the organism to host surfaces. Southern blot analysis with ALS-specific probes suggested the presence of ALS gene families in C. dubliniensis and C. tropicalis; three partial ALS genes were isolated from each organism. Northern blot analysis demonstrated that mechanisms governing expression of ALS genes in C. albicans and C. dubliniensis are different. Western blots with an anti-Als serum showed that cross-reactive proteins are linked by beta 1,6-glucan in the cell wall of each non-albicans Candida, suggesting similar cell wall architecture and conserved processing of Als proteins in these organisms. Although an ALS family is present in each organism, phylogenetic analysis of the C. albicans, C. dubliniensis, and C. tropicalis ALS genes indicated that, within each species, sequence diversification is extensive and unique ALS sequences have arisen. Phylogenetic analysis of the ALS and SAP (secreted aspartyl proteinase) families show that the ALS family is younger than the SAP family. ALS genes in C. albicans, C. dubliniensis, and C. tropicalis tend to be located on chromosomes that also encode genes from the SAP family, yet the two families have unexpectedly different evolutionary histories. Homologous recombination between the tandem repeat sequences present in ALS genes could explain the different histories for co-localized genes in a predominantly clonal organism like C. albicans.

The ALS5 gene of Candida albicans and analysis of the Als5p N-terminal domain

ALS genes of Candida albicans encode a family of cell-surface glycoproteins with a three-domain structure. Each Als protein has a relatively conserved N-terminal domain, a central domain consisting of a tandemly repeated motif, and a serine-threonine-rich C-terminal domain that is relatively variable across the family. The ALS family exhibits several types of variability that indicate the importance of considering strain and allelic differences when studying ALS genes and their encoded proteins. Analysis of ALS5 provided additional evidence of variability within the ALS family. Comparison of the ALS5 sequence from two strains indicated sequence differences larger than strain or allelic mismatches observed for other C. albicans genes. Screening a collection of commonly used C. albicans strains and clinical isolates indicated that ALS5 is not present in several of these strains, supporting the conclusion that the Als protein profile is variable among C. albicans isolates. Physical mapping of ALS5 showed that it is located close to ALS1 on chromosome 6. The N-terminal domain of Als5p was produced in Pichia pastoris to initiate structural analysis of this portion of the protein. The hydrophobic character of this portion of the protein was exploited in the purification scheme. Circular dichroism analysis of the purified, authenticated protein yielded a high content of antiparallel beta-sheet and little to no alpha-helical structure. These results are consistent with the conclusion that the N-terminal domain of Als5p has an immunoglobulin fold structure similar to that found in many cell adhesion molecules. Gene sequences of C. albicans ALS5 (Accession No. AF068866) and TPI1 (Accession No. AF124845) have been deposited in the GenBank database.

Establishment of a simple system to analyse the molecular interaction in the agglutination of Saccharomyces cerevisiae

Saccharomyces cerevisiae a-agglutinin, which is involved in mating and covalently anchoring to the cell wall, consists of two components, Aga1p and Aga2p, whose syntheses are individually regulated. To facilitate the analysis of the protein-protein interaction on agglutination between a- and alpha-agglutinins, the construction of a yeast strain (MATa) with the functional protein prepared by genetic fusion of Aga1p- and Aga2p-encoding genes and by the expression system using the UPR-ICL promoter derived from the n-alkane-assimilating yeast, Candida tropicalis, which is functional under the condition of lower glucose concentration was tried and the agglutination ability of the constructed strain was evaluated with a yeast strain (MATa) which expressed AGalpha1 encoding alpha-agglutinin under the control of the same promoter. The genes were integrated into the yeast chromosomes. Cell agglutination between both (MATa) strains was observed microscopically when these two strains were mix-cultured to a glucose-decreased concentration. The agglutination was further confirmed by the sedimentation test and by the quantification using a filter. These results proved that the constructed Aga1p-Aga2p fusion protein was enoughly functional for the interaction with the Agalpha1 protein, and that this phenomenon occurred dependent on glucose concentration, but independent of the peptide pheromones secreted by the cells of the opposite mating types. Using this system, the role of two disulphide linkages between Aga1p and Aga2p on the binding activity between Aga2p and Aga1p was first evaluated. Under the treatment by the SH-compound (dithiothreitol), in which Agalpha2p is easily released into the medium from the intact cell surface, the Aga1p and Aga2p fusion protein was a good tool to make clear the role of the disulphide linkages. As a result, the linkages had a significant effect on not only the assembly but also the binding activity. The novel and simple system described here may further facilitate the study of molecular interaction in agglutination.

The ALS6 and ALS7 genes of Candida albicans

ALS genes of Candida albicans encode a family of cell-surface glycoproteins that are composed of an N-terminal domain, a central domain of a tandemly repeated motif, and a relatively variable C-terminal domain. Although several ALS genes have been characterized, more ALS-like sequences are present in the C. albicans genome. Two short DNA sequences with similarity to the 5' domains of known ALS genes were detected among data from the C. albicans genome sequencing project. Probes developed from unique regions of these sequences were used to screen a genomic library from which two full-length genes, designated ALS6 and ALS7, were cloned. ALS6 and ALS7 encode features similar to other genes in the ALS family and map to chromosome 3, a chromosome previously not known to encode ALS sequences. ALS6 and ALS7 are present in all C. albicans strains examined. Additional analysis suggested that some C. albicans strains have another ALS gene with a 5' domain similar to that of ALS6. Characterization of ALS7 revealed a novel tandemly repeated sequence within the C-terminal domain. Unlike other ALS family tandem repeats, the newly characterized ALS7 repeat does not appear to define additional genes in the ALS family. However, our data and information from the C. albicans genome sequencing project suggest that there are additional ALS genes remaining to be characterized.

Genetic immobilization of proteins on the yeast cell surface

A genetic system has been exploited to immobilize proteins in their active and functional forms on the cell surface of yeast, Saccharomyces cerevisiae. DNAs encoding proteins with a secretion signal peptide were fused with the genes encoding yeast agglutinins, a- and alpha-type proteins involved in mating. The fusion gene was introduced into S. cerevisiae and expressed under the control of several promoters. Appearance of the fused proteins expressed on the cell surface was demonstrated biochemically and by immunofluorescence and immunoelectron microscopy techniques. Alpha-galactosidase from Cyamopsis tetragonoloba seeds, peptide libraries including scFv and variable regions of the T cell receptor from mammalian cells have been successfully immobilized on the yeast cell wall in the active form. Recently, surface-engineered yeasts have been constructed by immobilizing the enzymes and a functional protein, for example, green fluorescent protein (GFP) from Aequorea victoria. The yeasts were termed 'arming yeasts' with biocatalysts or functional proteins. Such arming cells displaying glucoamylase from Rhizopus oryzae and alpha-amylase from Bacillus stearothermophilus, or carboxymethylcellulase and beta-glucosidase from Aspergillus acleatus, could assimilate starch or cellooligosaccharides as the sole carbon source, although S. cerevisiae cannot intrinsically assimilate these substrates. GFP-arming cells can emit green fluorescence from the cell surface in response to the environmental conditions. The approach described in this review will enable us to endow living cells, including yeast cells, with novel additional abilities and to open new dimensions in the field of biotechnology.

Las21 participates in extracellular/cell surface phenomena in Saccharomyces cerevisiae

Las21 (Yj1062W) is a member of the major facilitator super family, possessing multimembrane spanning domains. The LAS21 gene was identified as a responsible gene for a Saccharomyces cerevisiae mutan which shows sensitivity to a local anestheticum, tetracaine. The null las21 mutant (las21 delta) is viable but shows temperature sensitive growth. We found, in addition to this phenotype, that the las21 delta strain shows a number of defects; mating deficiency, calcofluor resistance, and formation of Zymolyase sensitive spores. Temperature sensitive growth of the las21 delta mutant was found to be suppressed by 0.1 M MgSO4. Two multicopy suppressors were obtained. They are ECM33 (YBR078W) and PIR2/HSP150 (YJR159W) both have some roles in an extracellular function. The common features of the suppressors, genetic and physiological, of the las21 delta mutation suggest that Las21 participates in a global activity of extracellular phenomena. The las 21 phenotypes are consistent with the idea that Las21/Gpi7 acts in metabolism of glycosylphosphatidylinositol.

Adhesins in Candida albicans

The adherent properties of Candida albicans blastoconidia and germ tubes have long been appreciated, but little is known about the mechanisms responsible for adherence. Recently, three genes, ALA1, ALS1 and HWP1, encoding proteins with adherent properties and motifs consistent with linkage to the beta-1, 6-glucan of C. albicans cell walls have provided insight into the topology of protein adhesins. Hwp1, a developmentally regulated adhesin of germ tubes and hyphae, attaches to buccal epithelial cells by an unconventional, transglutaminase-mediated mechanism of adhesion.

The contribution of cell wall proteins to the organization of the yeast cell wall

Our knowledge of the yeast cell wall has increased rapidly in the past few years, allowing for the first time a description of its structure in molecular terms. Two types of cell wall proteins (CWPs) have been identified that are covalently linked to beta-glucan, namely GPI-CWPs and Pir-CWPs. Both define a characteristic supramolecular complex or structural unit. The GPI building block has the core structure GPI-CWP-->beta1,6-glucan-->beta1,3-glucan, which may become extended with one or more chitin chains. The Pir building block is less well characterized, but preliminary evidence points to the structure, Pir-CWP-->beta1,3-glucan, which probably also may become extended with one or more chitin chains. The molecular architecture of the cell wall is not fixed. The cell can make considerable adjustments to the composition and structure of its wall, for example, during the cell cycle or in response to environmental conditions such as nutrient and oxygen availability, temperature, and pH. When the cell wall is defective, dramatic changes can occur in its molecular architecture, pointing to the existence of cell wall repair mechanisms that compensate for cell damage. Finally, evidence is emerging that at least to a considerable extent the cell wall of Saccharomyces cerevisiae is representative for the cell wall of the Ascomycetes.

Assimilation of cellooligosaccharides by a cell surface-engineered yeast expressing beta-glucosidase and carboxymethylcellulase from aspergillus aculeatus

Since Saccharomyces cerevisiae lacks the cellulase complexes that hydrolyze cellulosic materials, which are abundant in the world, two types of hydrolytic enzymes involved in the degradation of cellulosic materials to glucose were genetically co-immobilized on its cell surface for direct utilization of cellulosic materials, one of the final goals of our studies. The genes encoding FI-carboxymethylcellulase (CMCase) and beta-glucosidase from the fungus Aspergillus aculeatus were individually fused with the gene encoding the C-terminal half (320 amino acid residues from the C terminus) of yeast alpha-agglutinin and introduced into S. cerevisiae. The delivery of CMCase and beta-glucosidase to the cell surface was carried out by the secretion signal sequence of the native signal sequence of CMCase and by the secretion signal sequence of glucoamylase from Rhizopus oryzae for beta-glucosidase, respectively. The genes were expressed by the glyceraldehyde-3-phosphate dehydrogenase promoter from S. cerevisiae. The CMCase and beta-glucosidase activities were detected in the cell pellet fraction, not in the culture supernatant. The display of CMCase and beta-glucosidase proteins on the cell surface was confirmed by immunofluorescence microscopy. The cells displaying these cellulases could grow on cellobiose or water-soluble cellooligosaccharides as the sole carbon source. The degradation and assimilation of cellooligosaccharides were confirmed by thin-layer chromatography. This result showed that the cell surface-engineered yeast with these enzymes can be endowed with the ability to assimilate cellooligosaccharides. This is the first step in the assimilation of cellulosic materials by S. cerevisiae expressing heterologous cellulase genes.

Amino acid sequence requirement for efficient incorporation of glycosylphosphatidylinositol-associated proteins into the cell wall of Saccharomyces cerevisiae

During cell wall biogenesis in Saccharomyces cerevisiae, some glycosylphosphatidylinositol (GPI)-attached proteins are detached from GPI moieties and bound to beta-1,6-glucan of the cell wall. The amino acid sequence requirement for the incorporation of GPI-attached proteins into the cell wall was studied by using reporter fusion proteins. Only the short omega-minus region composed of five amino acids, which is located upstream of the omega site for GPI attachment, determined the cellular localization of the GPI-associated proteins. Within the omega-minus region, amino acid residues at the omega-4 or -5 and omega-2 sites were important for the cell wall incorporation. Yap3p, a well characterized GPI-anchored plasma membrane aspartic protease, was localized in the cell wall when the omega-minus region was mutated to sequences containing Val or Ile at the omega-4 or -5 site and Val or Tyr at the omega-2 site.

Genetic organization and sequence analysis of the hypha-specific cell wall protein gene HWP1 of Candida albicans

A previously isolated partial cDNA encoding a cell wall protein antigen found on hyphal surfaces of the opportunistic fungal pathogen, Candida albicans (Staab et al., 1996) was used to clone the complete hyphal wall protein 1 gene (HWP1). Hyphal forms of C. albicans invade mucosal surfaces of immunocompromised patients such as those with AIDS. HWP1 consisted of an open reading frame predicting an acidic protein (pI of 3.37) with a calculated molecular size of 61,122. The antigenic domain was located in the N-terminal third of the protein. The remainder of the protein contained abundant hydroxy amino acids, and terminated with a string of 15 amino acids typical of sequences specifying post-translational modification with glycosylphosphatidylinositol (6PI). The analyses suggested that Hwp1 is a glucan-linked protein with serine/threonine-rich regions that are predicted to function in extending a ligand-binding domain into the extracellular space.

The Saccharomyces cerevisiae CCH1 gene is involved in calcium influx and mating

The yeast Saccharomyces cerevisiae gene CCH1 (ORF YGR217w) shows high homology with animal calcium channel alpha1-subunit genes. Knock-out mutants were constructed of Cch1 and of Mid1 which is known to mediate Ca2+ influx in response to the alpha-mating pheromone. Cch1 is involved in calcium influx and the late stage of the mating process. The cch1 mutant sensitivity against the alpha-mating pheromone can be reduced by the addition of extra calcium. The product of this gene is likely to interact with the MID1 gene product in Ca influx or its control.