Cell surface anchorage and ligand-binding domains of the Saccharomyces cerevisiae cell adhesion protein alpha-agglutinin, a member of the immunoglobulin superfamily

Single-cell fluidic force microscopy reveals stress-dependent molecular interactions in yeast mating

Sexual agglutinins of the budding yeast Saccharomyces cerevisiae are proteins mediating cell aggregation during mating. Complementary agglutinins expressed by cells of opposite mating types "a" and "α" bind together to promote agglutination and facilitate fusion of haploid cells. By means of an innovative single-cell manipulation assay combining fluidic force microscopy with force spectroscopy, we unravel the strength of single specific bonds between a- and α-agglutinins (~100 pN) which require pheromone induction. Prolonged cell-cell contact strongly increases adhesion between mating cells, likely resulting from an increased expression of agglutinins. In addition, we highlight the critical role of disulfide bonds of the a-agglutinin and of histidine residue H₂₇₃ of α-agglutinin. Most interestingly, we find that mechanical tension enhances the interaction strength, pointing to a model where physical stress induces conformational changes in the agglutinins, from a weak-binding folded state, to a strong-binding extended state. Our single-cell technology shows promises for understanding and controlling the complex mechanism of yeast sexuality.

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.

Amyloid-Like β-Aggregates as Force-Sensitive Switches in Fungal Biofilms and Infections

Cellular aggregation is an essential step in the formation of biofilms, which promote fungal survival and persistence in hosts. In many of the known yeast cell adhesion proteins, there are amino acid sequences predicted to form amyloid-like β-aggregates. These sequences mediate amyloid formation in vitro. In vivo, these sequences mediate a phase transition from a disordered state to a partially ordered state to create patches of adhesins on the cell surface. These β-aggregated protein patches are called adhesin nanodomains, and their presence greatly increases and strengthens cell-cell interactions in fungal cell aggregation. Nanodomain formation is slow (with molecular response in minutes and the consequences being evident for hours), and strong interactions lead to enhanced biofilm formation. Unique among functional amyloids, fungal adhesin β-aggregation can be triggered by the application of physical shear force, leading to cellular responses to flow-induced stress and the formation of robust biofilms that persist under flow. Bioinformatics analysis suggests that this phenomenon may be widespread. Analysis of fungal abscesses shows the presence of surface amyloids in situ, a finding which supports the idea that phase changes to an amyloid-like state occur in vivo. The amyloid-coated fungi bind the damage-associated molecular pattern receptor serum amyloid P component, and there may be a consequential modulation of innate immune responses to the fungi. Structural data now suggest mechanisms for the force-mediated induction of the phase change. We summarize and discuss evidence that the sequences function as triggers for protein aggregation and subsequent cellular aggregation, both in vitro and in vivo.

Functional Amyloids in Reproduction

Amyloids are traditionally considered pathological protein aggregates that play causative roles in neurodegenerative disease, diabetes and prionopathies. However, increasing evidence indicates that in many biological systems nonpathological amyloids are formed for functional purposes. In this review, we will specifically describe amyloids that carry out biological roles in sexual reproduction including the processes of gametogenesis, germline specification, sperm maturation and fertilization. Several of these functional amyloids are evolutionarily conserved across several taxa, including human, emphasizing the critical role amyloids perform in reproduction. Evidence will also be presented suggesting that, if altered, some functional amyloids may become pathological.

Amyloid properties of the mouse egg zona pellucida

The zona pellucida (ZP) surrounding the oocyte is an extracellular fibrillar matrix that plays critical roles during fertilization including species-specific gamete recognition and protection from polyspermy. The mouse ZP is composed of three proteins, ZP1, ZP2, and ZP3, all of which have a ZP polymerization domain that directs protein fibril formation and assembly into the three-dimensional ZP matrix. Egg coats surrounding oocytes in nonmammalian vertebrates and in invertebrates are also fibrillar matrices and are composed of ZP domain-containing proteins suggesting the basic structure and function of the ZP/egg coat is highly conserved. However, sequence similarity between ZP domains is low across species and thus the mechanism for the conservation of ZP/egg coat structure and its function is not known. Using approaches classically used to identify amyloid including conformation-dependent antibodies and dyes, X-ray diffraction, and negative stain electron microscopy, our studies suggest the mouse ZP is a functional amyloid. Amyloids are cross-β sheet fibrillar structures that, while typically associated with neurodegenerative and prion diseases in mammals, can also carry out functional roles in normal cells without resulting pathology. An analysis of the ZP domain from mouse ZP3 and ZP3 homologs from five additional taxa using the algorithm AmylPred 2 to identify amyloidogenic sites, revealed in all taxa a remarkable conservation of regions that were predicted to form amyloid. This included a conserved amyloidogenic region that localized to a stretch of hydrophobic amino acids previously shown in mouse ZP3 to be essential for fibril assembly. Similarly, a domain in the yeast protein α-agglutinin/Sag 1p, that possesses ZP domain-like features and which is essential for mating, also had sites that were predicted to be amyloidogenic including a hydrophobic stretch that appeared analogous to the critical site in mouse ZP3. Together, these studies suggest that amyloidogenesis may be a conserved mechanism for ZP structure and function across billions of years of evolution.

Cell aggregations in yeasts and their applications

Yeasts can display four types of cellular aggregation: sexual, flocculation, biofilm formation, and filamentous growth. These cell aggregations arise, in some yeast strains, as a response to environmental or physiological changes. Sexual aggregation is part of the yeast mating process, representing the first step of meiotic recombination. The flocculation phenomenon is a calcium-dependent asexual reversible cellular aggregation that allows the yeast to withstand adverse conditions. Biofilm formation consists of multicellular aggregates that adhere to solid surfaces and are embedded in a protein matrix; this gives the yeast strain either the ability to colonize new environments or to survive harsh environmental conditions. Finally, the filamentous growth is the ability of some yeast strains to grow in filament forms. Filamentous growth can be attained by two different means, with the formation of either hyphae or pseudohyphae. Both hyphae and pseudohyphae arise when the yeast strain is under nutrient starvation conditions and they represent a means for the microbial strain to spread over a wide area to survey for food sources, without increasing its biomass. Additionally, this filamentous growth is also responsible for the invasive growth of some yeast.

Novel applications for glycosylphosphatidylinositol-anchored proteins in pharmaceutical and industrial biotechnology

Glycosylphosphatidylinositol (GPI)-anchored proteins have been regarded as typical cell surface proteins found in most eukaryotic cells from yeast to man. They are embedded in the outer plasma membrane leaflet via a carboxy-terminally linked complex glycolipid GPI structure. The amphiphilic nature of the GPI anchor, its compatibility with the function of the attached protein moiety and the capability of GPI-anchored proteins for spontaneous insertion into and transfer between artificial and cellular membranes initially suggested their potential for biotechnological applications. However, these expectations have been hardly fulfilled so far. Recent developments fuel novel hopes with regard to: (i) Automated online expression, extraction and purification of therapeutic proteins as GPI-anchored proteins based on their preferred accumulation in plasma membrane lipid rafts, (ii) multiplex custom-made protein chips based on GPI-anchored cell wall proteins in yeast, (iii) biomaterials and biosensors with films consisting of sets of distinct GPI-anchored binding-proteins or enzymes for sequential or combinatorial catalysis, and (iv) transport of therapeutic proteins across or into relevant tissue cells, e.g., enterocytes or adipocytes. Latter expectations are based on the demonstrated translocation of GPI-anchored proteins from plasma membrane lipid rafts to cytoplasmic lipid droplets and eventually further into microvesicles which upon release from donor cells transfer their GPI-anchored proteins to acceptor cells. The value of these technologies, which are all based on the interaction of GPI-anchored proteins with membranes and surfaces, for the engineering, production and targeted delivery of biomolecules for a huge variety of therapeutic and biotechnological purposes should become apparent in the near future.

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.

Cell wall-linked cryptococcal phospholipase B1 is a source of secreted enzyme and a determinant of cell wall integrity

Phospholipase B (Plb1) is secreted by pathogenic fungi and is a proven virulence determinant in Cryptococcus neoformans. Cell-associated Plb1 is presumptively involved in fungal membrane biogenesis and remodelling. We have also identified it in cryptococcal cell walls. Motif scanning programs predict that Plb1 is attached to cryptococcal membranes via a glycosylphosphatidylinositol (GPI) anchor, which could regulate Plb1 export and secretion. A functional GPI anchor was identified in cell-associated Plb1 by (G)PI-specific phospholipase C (PLC)-induced release of Plb1 from strain H99 membrane rafts and inhibition of GPI anchor synthesis by YW3548, which prevented Plb1 secretion and transport to membranes and cell walls. Plb1 containing beta-1,6-linked glucan was released from H99 (wild-type strain) cell walls by beta-1,3 glucanase, consistent with covalent attachment of Plb1 via beta-1,6-linked glucans to beta-1,3-linked glucan in the central scaffold of the wall. Naturally secreted Plb1 also contained beta-1,6-linked glucan, confirming that it originated from the cell wall. Plb1 maintains cell wall integrity because a H99 deletion mutant, DeltaPLB1, exhibited a morphological defect and was more susceptible than H99 to cell wall disruption by SDS and Congo red. Growth of DeltaPLB1 was unaffected by caffeine, excluding an effect of Plb1 on cell wall biogenesis-related signaling pathways. Environmental (heat) stress caused Plb1 accumulation in cell walls, with loss from membranes and reduced secretion, further supporting the importance of Plb1 in cell wall integrity. This is the first demonstration that Plb1 contributes to fungal survival by maintaining cell wall integrity and that the cell wall is a source of secreted enzyme.

Ancient Phylogenetic Beginnings of Immunoglobulin Hypermutation

Many structures and molecules closely related to those involved in the specific process of immunoglobulin (Ig) hypermutation existed before the appearance of primordial Ig genes. Consequently, these structures can be found even in animals and organisms distinct from vertebrates; likewise, homologues of hypermutation enzymes are present in a broad range of species, from bacteria to mammals. Our analysis, based predominantly on primary structure, demonstrates the existence of molecules similar to Ig domains, variable Ig domains (IGv), and antigen receptors (AR) in unicellular organisms, nonvertebrate metazoans, and nonvertebrate Coelomata, respectively. In addition, we deal here with some important structural properties of CDR1-like segments of the selected sponge adhesion molecule GCSAMS exhibiting chimerical Ig domain similarities, and demonstrate the occurrence of conserved regions corresponding to Ohno's modern intact primordial building block in the C-terminal part of IGv-related segments of nonvertebrate origin. The results of our analysis are also discussed with respect to the possible phylogeny of molecules preceding the hypothetical common antigen receptor ancestor.

The fission yeast Map4 protein is a novel adhesin required for mating

Cell adhesion is required for many cellular processes. In fungi, cell-cell contact during mating, flocculation or virulence is mediated by adhesins, which typically are glycosyl phosphatidyl inositol (GPI)-modified cell wall glycoproteins. Proteins with internal repeats (PIR) are surface proteins involved in the response to stress. In Schizosaccharomyces pombe no adhesins or PIR proteins have been described. Here we study the S. pombe Map4p, which defines a new class of surface protein that is not GPI-modified and has a serine/threonine rich domain and internal repeats that differ from those present in PIR proteins. Map4p is a mating type-specific adhesin required for mating in h(+) cells and enhances cell adhesion when overexpressed.

Cell wall construction inSaccharomyces cerevisiae

In this review, we discuss new insights in cell wall architecture and cell wall construction in the ascomycetous yeast Saccharomyces cerevisiae. Transcriptional profiling studies combined with biochemical work have provided ample evidence that the cell wall is a highly adaptable organelle. In particular, the protein population that is anchored to the stress-bearing polysaccharides of the cell wall, and forms the interface with the outside world, is highly diverse. This diversity is believed to play an important role in adaptation of the cell to environmental conditions, in growth mode and in survival. Cell wall construction is tightly controlled and strictly coordinated with progression of the cell cycle. This is reflected in the usage of specific cell wall proteins during consecutive phases of the cell cycle and in the recent discovery of a cell wall integrity checkpoint. When the cell is challenged with stress conditions that affect the cell wall, a specific transcriptional response is observed that includes the general stress response, the cell wall integrity pathway and the calcineurin pathway. This salvage mechanism includes increased expression of putative cell wall assemblases and some potential cross-linking cell wall proteins, and crucial changes in cell wall architecture. We discuss some more enzymes involved in cell wall construction and also potential inhibitors of these enzymes. Finally, we use both biochemical and genomic data to infer that the architectural principles used by S. cerevisiae to build its cell wall are also used by many other ascomycetous yeasts and also by some mycelial ascomycetous fungi.

Construction and real-time RT-PCR validation of Candida albicans PALS-GFP reporter strains and their use in flow cytometry analysis of ALS gene expression in budding and filamenting cells

The gene encoding yeast-enhanced green fluorescent protein (GFP) was placed under control of ALS gene promoters in Candida albicans. The PALS-GFP reporter strains were validated using various techniques including a new real-time RT-PCR assay to quantify ALS gene expression. The PALS-GFP reporter strains were grown in media that promoted yeast or germ tube forms, and the resulting fluorescence was measured by flow cytometry. In addition to results that indicate differences in ALS gene expression due to growth medium, growth stage and developmental programme, new data show large differences in transcriptional level among the ALS genes. Expression of ALS1 was associated with transfer of the PALS1-GFP strain to fresh growth medium. ALS3 expression increased markedly when germ tubes were visible microscopically and ALS7 expression exhibited a transient peak between 2 and 3 h following inoculation into fresh YPD medium. Transcription from the ALS1 and ALS3 promoters was strongest among those tested and contrasted markedly with the weaker promoter strength at the ALS5, ALS6, ALS7 and ALS9 loci. These weaker transcriptional responses were also observed using real-time RT-PCR measurements on wild-type C. albicans cells. Assuming a positive correlation between transcriptional level and protein production, these results suggest that some Als proteins are abundant on the C. albicans cell surface while others are produced at a much lower level.

Expression of transglutaminase substrate activity on Candida albicans germ tubes through a coiled, disulfide-bonded N-terminal domain of Hwp1 requires C-terminal glycosylphosphatidylinositol modification

By serving as a microbial substrate for epithelial cell transglutaminase, Hwp1 (Hyphal wall protein 1) of Candida albicans participates in cross-links with proteins on the mammalian mucosa. Biophysical properties of the transglutaminase substrate domain were explored using a recombinant protein representative of the N-terminal domain of Hwp1 and were similar to other transglutaminase substrates, the small proline-rich proteins of cornified envelopes found in stratified squamous epithelia. Recombinant Hwp1 lacks alpha and beta structures by circular dichroism and likely exists as a disulfide-cross-linked coiled-coil. The transglutaminase substrate property prompted a unique approach for investigating the features of surface Hwp1 on germ tubes. A lysine analog, 5-(biotinamido)pentylamine, was cross-linked to germ tubes catalyzed by transglutaminase 2 prior to cell fractionation, immunoprecipitation, and detection with streptavidin conjugates. The majority of the transglutaminase-modifiable Hwp1 was covalently attached to the beta-glucan of hyphae by the C terminus of Hwp1 via a glycosylphosphatidylinositol remnant anchor. A putative precursor of cell wall forms of Hwp1 was identified in the cell extract and in the culture medium. Hwp1 was modified by relatively short N-linked glycans, and the molecular size of the protein was reduced by hypomannosylation when expressed in O-glycosylation mutant strains. Hwp1 combines features of mammalian transglutaminase substrate proteins with characteristics of fungal cell wall proteins to form an unconventional adhesin at the hyphal wall of C. albicans.

Functional and structural diversity in the Als protein family of Candida albicans

The human fungal pathogen Candida albicans colonizes and invades a wide range of host tissues. Adherence to host constituents plays an important role in this process. Two members of the C. albicans Als protein family (Als1p and Als5p) have been found to mediate adherence; however, the functions of other members of this family are unknown. In this study, members of the ALS gene family were cloned and expressed in Saccharomyces cerevisiae to characterize their individual functions. Distinct Als proteins conferred distinct adherence profiles to diverse host substrates. Using chimeric Als5p-Als6p constructs, the regions mediating substrate-specific adherence were localized to the N-terminal domains in Als proteins. Interestingly, a subset of Als proteins also mediated endothelial cell invasion, a previously unknown function of this family. Consistent with these results, homology modeling revealed that Als members contain anti-parallel beta-sheet motifs interposed by extended regions, homologous to adhesins or invasins of the immunoglobulin superfamily. This finding was confirmed using circular dichroism and Fourier transform infrared spectrometric analysis of the N-terminal domain of Als1p. Specific regions of amino acid hypervariability were found among the N-terminal domains of Als proteins, and energy-based models predicted similarities and differences in the N-terminal domains that probably govern the diverse function of Als family members. Collectively, these results indicate that the structural and functional diversity within the Als family provides C. albicans with an array of cell wall proteins capable of recognizing and interacting with a wide range of host constituents during infection.

A Sensitive Predictor for Potential GPI Lipid Modification Sites in Fungal Protein Sequences and its Application to Genome-wide Studies for Aspergillus nidulans, Candida albicans Neurospora crassa, Saccharomyces cerevisiae and Schizosaccharomyces pombe

The fungal transamidase complex that executes glycosylphosphatidylinositol (GPI) lipid anchoring of precursor proteins has overlapping but distinct sequence specificity compared with the animal system. Therefore, a taxon-specific prediction tool for the recognition of the C-terminal signal in fungal sequences is necessary. We have collected a learning set of fungal precursor protein sequences from the literature and fungal proteomes. Although the general four segment scheme of the recognition signal is maintained also in fungal precursors, there are taxon specificities in details. A fungal big-Pi predictor has been developed for the assessment of query sequence concordance with fungi-specific recognition signal requirements. The sensitivity of this predictor is close to 90%. The rate of false positive prediction is in the range of 0.1%. The fungal big-Pi tool successfully predicts the Gas1 mutation series described by C. Nuoffer and co-workers, and recognizes that the human PLAP C terminus is not a target for the fungal transamidase complex. Lists of potentially GPI lipid anchored proteins for five fungal proteomes have been generated and the hits have been functionally classified. The fungal big-Pi prediction WWW server as well as precursor lists are available at

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.

Modular domain structure in the Candida glabrata adhesin Epa1p, a beta1,6 glucan-cross-linked cell wall protein

The yeast pathogen Candida glabrata adheres avidly to cultured human epithelial cells. This interaction depends on the expression of EPA1, which encodes a lectin belonging to a large family of GPI-anchored glucan-cross-linked cell wall proteins (GPI-CWPs) found in diverse fungal species. To understand the relationship between different domains of EPA1 and its function, we have mapped functional domains of Epa1p and analysed their contribution to Epa1p function. We found that the N-terminal third of the protein contains the ligand-binding domain, and that the GPI anchor is essential both for cross-linking in the cell wall and for Epa1p-mediated adherence. We also found that the C-terminal Ser/Thr-rich domain, characteristic of many GPI-CWPs, was absolutely essential for function. Although Epa1p derivatives lacking the Ser/Thr domain were expressed abundantly in the cell wall, they were localized to internal layers of the cell wall; such constructs were unable to mediate adherence. The outer layer of the yeast cell wall is known to act as a permeability barrier; we found that the C-terminal Ser/Thr-rich region was absolutely required to project the N-terminal domain of Epa1p through this permeability barrier and into the external environment. Thus, the Ser/Thr-rich domain of Epa1p and, presumably, of other related GPI-CWPs serves an essential structural role in localization of the protein at the external surface of the yeast cell where it can interact with its ligand. In conclusion, Epa1p has a modular structure, with each domain serving a distinct and essential role in the function of the adhesin.

Environmentally induced reversible conformational switching in the yeast cell adhesion protein alpha-agglutinin

The yeast cell adhesion protein alpha-agglutinin is expressed on the surface of a free-living organism and is subjected to a variety of environmental conditions. Circular dichroism (CD) spectroscopy shows that the binding region of alpha-agglutinin has a beta-sheet-rich structure, with only approximately 2% alpha-helix under native conditions (15-40 degrees C at pH 5.5). This region is predicted to fold into three immunoglobulin-like domains, and models are consistent with the CD spectra as well as with peptide mapping and site-specific mutagenesis. However, secondary structure prediction algorithms show that segments comprising approximately 17% of the residues have high alpha-helical and low beta-sheet potential. Two model peptides of such segments had helical tendencies, and one of these peptides showed pH-dependent conformational switching. Similarly, CD spectroscopy of the binding region of alpha-agglutinin showed reversible conversion from beta-rich to mixed alpha/beta structure at elevated temperatures or when the pH was changed. The reversibility of these changes implied that there is a small energy difference between the all-beta and the alpha/beta states. Similar changes followed cleavage of peptide or disulfide bonds. Together, these observations imply that short sequences of high helical propensity are constrained to a beta-rich state by covalent and local charge interactions under native conditions, but form helices under non-native conditions.

Proteome analysis of Aspergillus fumigatus identifies glycosylphosphatidylinositol-anchored proteins associated to the cell wall biosynthesis

Previous studies in Aspergillus fumigatus (Mouyna I., Fontaine T., Vai M., Monod M., Fonzi W. A., Diaquin M., Popolo L., Hartland R. P., Latgé J.-P, J. Biol. Chem. 2000, 275, 14882-14889) have shown that a glucanosyltransferase playing an important role in fungal cell wall biosynthesis is glycosylphosphatidylinositol (GPI) anchored to the membrane. To identify other GPI-anchored proteins putatively involved in cell wall biogenesis, a proteomic analysis has been undertaken in A. fumigatus and the protein data were matched with the yeast genomic data. GPI-anchored proteins of A. fumigatus were released from membrane preparation by an endogenous GPI-phospholipase C, purified by liquid chromatography and separated by two-dimensional electrophoresis. They were characterized by their peptide mass fingerprint through matrix-assisted laser desorption/ionization-time of flight-(MALDI-TOF)-mass spectrometry and by internal amino acid sequencing. Nine GPI-anchored proteins were identified in A. fumigatus. Five of them were homologs of putatively GPI-anchored yeast proteins (Csa1p, Crh1p, Crh2p, Ecm33p, Gas1p) of unknown function but shown by gene disruption analysis to play a role in cell wall morphogenesis. In addition, a comparative study performed with chitin synthase and glucanosyl transferase mutants of A. fumigatus showed that a modification of the growth phenotype seen in these mutants was associated to an alteration of the pattern of GPI-anchored proteins. These results suggest that GPI-anchored proteins identified in this study are involved in A. fumigatus cell wall organization.

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.

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.

Molecular breeding of yeast with higher metal-adsorption capacity by expression of histidine-repeat insertion in the protein anchored to the cell wall

A fusion protein of hexa-histidine repeat (His) and glycosylphosphatidylinositol (GPI)-anchor region of Saccharomyces cerevisiae Cwp1 with Aspergillus oryzae Taka-amylase A (TAA) was expressed on the yeast cell surface. The expressed fusion protein (TAA-His-Cwp1) was localized on the cell wall and demonstrated amylolytic activity. In comparison with the TAA-Cwp1 expressing strain, these cells exhibited 1.6- to 2.8-fold higher adsorbing capacity for Cu(2+), Ni(2+), and Zn(2+).

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.

beta-1,6-Glucan synthesis in Saccharomyces cerevisiae

beta-1,6-Glucan is an essential fungal-specific component of the Saccharomyces cerevisiae cell wall that interconnects all other wall components into a lattice. Considerable biochemical and genetic effort has been directed at the identification and characterization of the steps involved in its biosynthesis. Structural studies show that the polymer plays a central role in wall structure, attaching mannoproteins via their glycosylphosphatidylinositol (GPI) glycan remnant to beta-1,3-glucan and chitin. Genetic approaches have identified genes that upon disruption result in beta-1,6-glucan defects of varying severity, often with reduced growth or lethality. These gene products have been localized throughout the secretory pathway and at the cell surface, suggesting a possible biosynthetic route. Current structural and genetic data have therefore allowed the development of models to predict biosynthetic events. Based on knowledge of beta-1,3-glucan and chitin synthesis, it is likely that the bulk of beta-1,6-glucan polymer synthesis occurs at the cell surface, but requires key prior intracellular events. However, the activity of most of the identified gene products remain unknown, making it unclear to what extent and how directly they contribute to the synthesis of this polymer. With the recent availability of new tools, reagents and methods (including genomics), the field is poised for a convergence of biochemical and genetic methods to identify and characterize the biochemical steps in the synthesis of this polymer.

Two endoplasmic reticulum (ER) membrane proteins that facilitate ER-to-Golgi transport of glycosylphosphatidylinositol-anchored proteins

Many eukaryotic cell surface proteins are anchored in the lipid bilayer through glycosylphosphatidylinositol (GPI). GPI anchors are covalently attached in the endoplasmic reticulum (ER). The modified proteins are then transported through the secretory pathway to the cell surface. We have identified two genes in Saccharomyces cerevisiae, LAG1 and a novel gene termed DGT1 (for "delayed GPI-anchored protein transport"), encoding structurally related proteins with multiple membrane-spanning domains. Both proteins are localized to the ER, as demonstrated by immunofluorescence microscopy. Deletion of either gene caused no detectable phenotype, whereas lag1Delta dgt1Delta cells displayed growth defects and a significant delay in ER-to-Golgi transport of GPI-anchored proteins, suggesting that LAG1 and DGT1 encode functionally redundant or overlapping proteins. The rate of GPI anchor attachment was not affected, nor was the transport rate of several non-GPI-anchored proteins. Consistent with a role of Lag1p and Dgt1p in GPI-anchored protein transport, lag1Delta dgt1Delta cells deposit abnormal, multilayered cell walls. Both proteins have significant sequence similarity to TRAM, a mammalian membrane protein thought to be involved in protein translocation across the ER membrane. In vivo translocation studies, however, did not detect any defects in protein translocation in lag1Delta dgt1Delta cells, suggesting that neither yeast gene plays a role in this process. Instead, we propose that Lag1p and Dgt1p facilitate efficient ER-to-Golgi transport of GPI-anchored proteins.

The Gas1 glycoprotein, a putative wall polymer cross-linker

The yeast cell wall, which for years has been regarded as a static cellular component, has been revealed to be dynamic in its structure and composition and complex in its enzymatic activity. The S. cerevisiae cell wall is composed of beta-1,3/beta-1,6-glucans, mannoproteins, and chitin, which are assembled into an extracellular matrix essential for maintenance of cell integrity. Gas1p, a glycoprotein anchored to the outer leaflet of the plasma membrane through a glycosylphosphatidylinositol, plays a key role in cell wall assembly. Loss of Gas1p leads to several morphogenetic defects and to a decrease in the amount of cross-links between the cell wall glucans. These defects in turn trigger a compensatory response that guarantees cell viability. Several Gas1p homologs have been isolated from Candida species and S. pombe. The Gas1p family also includes two plant proteins with endo-beta-1,3-glucanase activity. Sequence comparisons reveal that Gas1p family proteins have a modular organization of domains. The genetic and molecular analyses reviewed here suggest that Gas1p could play a role as a polymer cross-linker, presumably by catalyzing a transglycosylation reaction.

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.

The carboxyl terminus of Pneumocystis carinii glycoprotein A encodes a functional glycosylphosphatidylinositol signal sequence

Pneumocystis carinii pneumonia is a hallmark disease associated with AIDS. An abundant glycoprotein, termed gpA, on the surface of P. carinii is considered an important factor in host-parasite interactions. The primary structure of ferret P. carinii gpA contains a carboxyl-terminal sequence characteristic of a signal for glycosylphosphatidylinositol (GPI) anchors. Here we report the capacity for this gpA carboxyl sequence to direct attachment of a secreted protein, human growth hormone (hGH), to the membranes of COS cells. A control fusion protein (hGHDAF37) was obtained which, under the direction of the GPI signal from decay accelerating factor, directs hGH cell surface expression. A construct (phGH2-1A30) was created similar to hGHDAF37 by fusing hGH to the putative GPI signal sequence encoded in the terminal 30 residues from a ferret P. carinii gpA cDNA clone. By indirect immunofluorescent staining, hGH was detected on the surface of COS cells transfected with phGH2-1A30; this surface location was confirmed by confocal laser cytometry. Metabolic labeling with [3H]ethanolamine and subsequent immunopurification of hGH from cells transfected with phGH2-1A30 confirmed that a lipid moiety characteristic of a conventional GPI anchor was linked covalently to hGH, and cell surface hGH2-1A30 fusion protein was sensitive to enzymatic cleavage by phosphatidylinositol-phospholipase C. Furthermore, hGH2-1A30 recombinant protein cofractionated with 5'-nucleotidase, a classical GPI-anchored membrane marker. Together, these results indicate that the carboxyl-terminal residues of ferret P. carinii gpA constitute a biologically functional GPI consensus domain, thus providing a potential mechanism for antigenic variation of P. carinii gpA during P. carinii pneumonia.

Involvement of protein N-glycosyl chain glucosylation and processing in the biosynthesis of cell wall beta-1,6-glucan of Saccharomyces cerevisiae

beta-1,6-Glucan plays a key structural role in the yeast cell wall. Of the genes involved in its biosynthesis, the activity of Cwh41p is known, i.e., the glucosidase I enzyme of protein N-chain glucose processing. We therefore examined the effects of N-chain glucosylation and processing mutants on beta-1,6-glucan biosynthesis and show that incomplete N-chain glucose processing results in a loss of beta-1,6-glucan, demonstrating a relationship between N-chain glucosylation/processing and beta-1,6-glucan biosynthesis. To explore the involvement of other N-chain-dependent events with beta-1,6-glucan synthesis, we investigated the Saccharomyces cerevisiae KRE5 and CNE1 genes, which encode homologs of the "quality control" components UDP-Glc:glycoprotein glucosyltransferase and calnexin, respectively. We show that the essential activity of Kre5p is separate from its possible role as a UDP-Glc:glycoprotein glucosyltransferase. We also observe a approximately 30% decrease in beta-1,6-glucan upon disruption of the CNE1 gene, a phenotype that is additive with other beta-1,6-glucan synthetic mutants. Analysis of the cell wall anchorage of the mannoprotein alpha-agglutinin suggests the existence of two beta-1,6-glucan biosynthetic pathways, one N-chain dependent, the other involving protein glycosylphosphatidylinositol modification.

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.

Pmt1 mannosyl transferase is involved in cell wall incorporation of several proteins in Saccharomyces cerevisiae

We constructed hybrid proteins containing a plant alpha-galactosidase fused to various C-terminal moieties of the hypoxic Srp1p; this allowed us to identify a cell wall-bound form of Srp1p. We showed that the last 30 amino acids of Srp1p, but not the last 16, contain sufficient information to signal glycosyl-phosphatidylinositol anchor attachment and subsequent cell wall anchorage. The cell wall-bound form was shown to be linked by means of a beta1,6-glucose-containing side-chain. Pmt1p enzyme is known as a protein-O-mannosyltransferase that initiates the O-glycosidic chains on proteins. We found that a pmt1 deletion mutant was highly sensitive to zymolyase and that in this strain the alpha-galactosidase-Srp1 fusion proteins, an alpha-galactosidase-Sed1 hybrid protein and an alpha-galactosidase-alpha-agglutinin hybrid protein were absent from both the membrane and the cell wall fractions. However, the plasma membrane protein Gas1p still receives its glycosyl-phosphatidylinositol anchor in pmt1 cells, and in this mutant strain an alpha-galactosidase-Cwp2 fusion protein was found linked to the cell wall but devoid of beta1,6-glucan side-chain, indicating an alternative mechanism of cell wall anchorage.

In silicio identification of glycosyl-phosphatidylinositol-anchored plasma-membrane and cell wall proteins of Saccharomyces cerevisiae

Use of the Von Heijne algorithm allowed the identification of 686 open reading frames (ORFs) in the genome of Saccharomyces cerevisiae that encode proteins with a potential N-terminal signal sequence for entering the secretory pathway. On further analysis, 51 of these proteins contain a potential glycosyl-phosphatidylinositol (GPI)-attachment signal. Seven additional ORFs were found to belong to this group. Upon examination of the possible GPI-attachment sites, it was found that in yeast the most probable amino acids for GPI-attachment as asparagine and glycine. In yeast, GPI-proteins are found at the cell surface, either attached to the plasma-membrane or as an intrinsic part of the cell wall. It was noted that plasma-membrane GPI-proteins possess a dibasic residue motif just before their predicted GPI-attachment site. Based on this, and on homologies between proteins, families of plasma-membrane and cell wall proteins were assigned, revealing 20 potential plasma-membrane and 38 potential cell wall proteins. For members of three plasma-membrane protein families, a function has been described. On the other hand, most of the cell wall proteins seem to be structural components of the wall, responsive to different growth conditions. The GPI-attachment site of yeast slightly differs from mammalian cells. This might be of use in the development of anti-fungal drugs.

The main protein of the aggregation factor responsible for species-specific cell adhesion in the marine sponge Microciona prolifera is highly polymorphic

Species-specific cell recognition in sponges, the oldest living metazoans, is based on a proteoglycan-like aggregation factor. We have screened individual sponge cDNA libraries, identifying multiple related forms for the aggregation factor core protein (MAFp3). Northern blots show the presence in several human tissues of transcripts strongly binding a MAFp3-specific probe. The open reading frame for MAFp3 is not interrupted in the 5' direction, revealing variable protein sequences that contain numerous introns equally spaced. We have studied tissue histocompatibility within a sponge population, finding 100% correlation between rejection behavior and the individual-specific restriction fragment length polymorphism pattern using aggregation factor-related probes. PCR amplifications with specific primers showed that at least some of the MAFp3 forms are allelic and distribute in the population used. A pronounced polymorphism is also observed when analyzing purified aggregation factor in polyacrylamide gels. Protease digestion of the polymorphic glycosaminoglycan-containing bands indicates that glycans are also responsible for the variability. The data presented reveal a high polymorphism of aggregation factor components, which matches the elevated sponge alloincompatibility, suggesting an involvement of the cell adhesion system in sponge allogeneic reactions.

Identification of a mannoprotein present in the inner layer of the cell wall of Saccharomyces cerevisiae

Cell wall extracts from the double-mutant mnn1 mnn9 strain were used as the immunogen to obtain a monoclonal antibody (MAb), SAC A6, that recognizes a specific mannoprotein--which we have named Icwp--in the walls of cells of Saccharomyces cerevisiae. Icwp runs as a polydisperse band of over 180 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of Zymolyase extracts of cell walls, although an analysis of the secretory pattern of the mannoprotein shows that at the level of secretory vesicles, it behaves like a discrete band of 140 kDa. Immunofluorescence analysis with the MAb showed that Icwp lies at the inner layer of the cell wall, being accessible to the antibody only after the outer layer of mannoproteins is disturbed by treatment with tunicamycin. The screening of a lambda gt11 expression library enabled us to identify the open reading frame (ORF) coding for Icwp. ICWP (EMBL accession number YLR391w, frame +3) codes for 238 amino acids, of which over 40% are serine or threonine, and contains a putative N-glycosylation site and a putative glycosylphosphatidylinositol attachment signal. Both disruption and overexpression of the ORF caused increased sensitivities to calcofluor white and Congo red, while the disruption caused an increased sensitivity to Zymolyase digestion, suggesting for Icwp a structural role in association with glucan.

Restrictive glycosylphosphatidylinositol anchor synthesis in cwh6/gpi3 yeast cells causes aberrant biogenesis of cell wall proteins

We previously reported that the defects in the Saccharomyces cerevisiae cwh6 Calcofluor white-hypersensitive cell wall mutant are caused by a mutation in SPT14/GPI3, a gene involved in glycosylphosphatidylinositol (GPI) anchor biosynthesis. Here we describe the effect of cwh6/spt14/gpi3 on the biogenesis of cell wall proteins. It was found that the release of precursors of cell wall proteins from the endoplasmic reticulum (ER) was retarded. This was accompanied by proliferation of ER structures. The majority of the cell wall protein precursors that eventually left the ER were not covalently incorporated into the cell wall but were secreted into the growth medium. Despite the inefficient incorporation of cell wall proteins, there was no net effect on the protein level in the cell wall. It is postulated that the availability of GPI-dependent cell wall proteins determines the rate of cell wall construction and limits growth rate.

The retention mechanism of cell wall proteins in Saccharomyces cerevisiae. Wall-bound Cwp2p is beta-1,6-glucosylated

It has been proposed that the cell wall proteins of Saccharomyces cerevisiae are anchored by means of a beta-1,6-glucose-containing side chain. Recently, we have identified three cell wall mannoproteins. Two of these mannoproteins are recognized in their cell wall bound form by an antiserum raised against beta-1,6-glucan but the third, Cwp2p, is not. This could indicate the existence of alternative retention mechanisms for cell wall proteins. Western analysis of a fusion protein consisting of Cwp2p and the reporter enzyme alpha-galactosidase revealed that this protein is glycosyl phosphatidylinositol-anchored in the intracellular precursor form and is recognized by an anti beta-1,6-glucan antiserum in the cell wall bound form. The cell wall bound forms of fusion proteins consisting of the anchor regions of Sed1p or Flo1p and alpha-galactosidase were also recognized by an anti beta-1,6-glucan antiserum. This is consistent with the existence of a general anchoring mechanism of proteins to the cell wall by means of a beta-1,6-glucose-containing carbohydrate chain. Western analysis of a yeast strain producing c-myc epitope tagged Cwp2p revealed that this protein is only detectable if fatty acid chains are present on the protein, indicating that the lack of recognition of Cwp2p by an anti beta-1,6-glucan antiserum is caused by a blotting artefact of the mature protein.

FLO11, a yeast gene related to the STA genes, encodes a novel cell surface flocculin

We report the characterization of a gene encoding a novel flocculin related to the STA genes of yeast, which encode secreted glucoamylase. The STA genes comprise sequences that are homologous to the sporulation-specific glucoamylase SGA and to two other sequences, S2 and S1. We find that S2 and S1 are part of a single gene which we have named FLO11. The sequence of FLO11 reveals a 4,104-bp open reading frame on chromosome IX whose predicted product is similar in overall structure to the class of yeast serine/threonine-rich GPI-anchored cell wall proteins. An amino-terminal domain containing a signal sequence and a carboxy-terminal domain with homology to GPI (glycosyl-phosphatidyl-inositol) anchor-containing proteins are separated by a central domain containing a highly repeated threonine- and serine-rich sequence. Yeast cells that express FLO11 aggregate in the calcium-dependent process of flocculation. Flocculation is abolished when FLO11 is disrupted. The product of STA1 also is shown to have flocculating activity. When a green fluorescent protein fusion of FLO11 was expressed from the FLO11 promoter on a single-copy plasmid, fluorescence was observed in vivo at the periphery of cells. We propose that FLO11 encodes a flocculin because of its demonstrated role in flocculation, its structural similarity to other members of the FLO gene family, and the cell surface location of its product. FLO11 gene sequences are present in all yeast strains tested, including all standard laboratory strains, unlike the STA genes which are present only in the variant strain Saccharomyces cerevisiae var. diastaticus. FLO11 differs from all other yeast flocculins in that it is located near a centromere rather than a telomere, and its expression is regulated by mating type. Repression of FLO11-dependent flocculation in diploids is conferred by the mating-type repressor al/alpha2.

Gpi1, a Saccharomyces cerevisiae protein that participates in the first step in glycosylphosphatidylinositol anchor synthesis

The temperature-sensitive Saccharomyces cerevisiae gpi1 mutant is blocked in [3H]inositol incorporation into protein and defective in the synthesis of N-acetylglucosaminylphosphatidylinositol, the first step in glycosylphosphatidylinositol (GPI) anchor assembly (Leidich, S. D., Drapp, D. A., and Orlean, P. (1994) J. Biol. Chem. 269, 10193-10196). The GPI1 gene, which encodes a 609-amino acid membrane protein, was cloned by complementation of the temperature sensitivity of gpi1 and corrects the mutant's [3H]inositol labeling and enzymatic defects. Disruption of GPI1 yields viable haploid cells that are temperature-sensitive for growth, for [3H]inositol incorporation into protein, and for GPI anchor-dependent processing of the Gas1/Ggp1 protein and that lack in vitro N-acetylglucosaminylphosphatidylinositol synthetic activity. The Gpi1 protein thus participates in GPI synthesis and is required for growth at 37 degrees C. When grown at a semipermissive temperature of 30 degrees C, gpi1 cells and gpi1::URA3 disruptants form large, round, multiply budded cells with a separation defect. Homozygous gpi1/gpi1, gpi1::URA3/gpi1::URA3, gpi2/gpi2, and gpi3/gpi3 diploids undergo meiosis, but are defective in ascospore wall maturation for they fail to give the fluorescence due to the dityrosine-containing layer in the ascospore wall. These findings indicate that GPIs have key roles in the morphogenesis and development of S. cerevisiae.