The N-terminal domain of the Flo11 protein from Saccharomyces cerevisiae is an adhesin without mannose-binding activity

The expression of the Flo11 flocculin in Saccharomyces cerevisiae offers the cell a wide range of phenotypes, depending on the strain and the environmental conditions. The most important are pseudohyphae development, invasive growth and flocculation. The mechanism of cellular adhesion mediated by Flo11p is not well understood. Therefore, the N-terminal domain of Flo11p was purified and studied. Although its amino acid sequence shows less similarity with the other flocculins, Flo11p belongs to the flocculin family. However, the N-terminal domain contains the 'Flo11-domain' (PF10181), but not the mannose-binding PA14 domain, which is present in the other flocculins (Flo1p, Flo5p, Flo9p and Flo10p). Structural and binding properties of the N-terminal domain of Flo11p were studied. It is shown that this domain is O-glycosylated and is structurally composed mainly of β-sheets, which is typical for the members of the flocculin family. Furthermore, fluorescence spectroscopy binding studies revealed that N-Flo11p does not bind mannose, which is in contrast to the other Flo proteins. However, surface plasmon resonance analysis showed that N-Flo11p self-interacts and explains the cell-cell interaction capacity of FLO11-expressing cells.     

Deletion of Intragenic Tandem Repeats in Unit C of FLO1 of Saccharomyces cerevisiae Increases the Conformational Stability of Flocculin under Acidic and Alkaline Conditions

Flocculation is an attractive property for Saccaromyces cerevisiae, which plays important roles in fermentation industry and environmental remediation. The process of flocculation is mediated by a family of cell surface flocculins. As one member of flocculins, Flo1 is characterized by four families of repeats (designated as repeat units A, B, C and D) in the central domain. It is generally accepted that variation of repeat unit A in length in Flo1 influences the degree of flocculation or specificity for sugar recognization. However, no reports were observed for other repeat units. Here, we compared the flocculation ability and its sensitivity to environmental factors between yeast strain YSF1 carrying the intact FLO1 gene and yeast strains carrying the derived forms of FLO1 with partial or complete deletion of repeats in unit C. No obvious differences in flocculation ability and specificity of carbohydrate recognition were observed among these yeast strains, which indicates the truncated flocculins can stride across the cell wall and cluster the N-terminal domain on the surface of yeast cells as the intact Flo1 thereby improving intercellular binding. However, yeast strains with the truncated flocculins required more mannose to inhibit completely the flocculation, displayed broad tolerance of flocculation to pH fluctuation, and the fewer the repeats in unit C, the stronger adaptability of flocculation to pH change, which was not relevant to the position of deletion. This suggests that more stable active conformation is obtained for flocculin by deletion the repeat unit C in the central domain of Flo1, which was validated further by the higher hydrophobicity on the surface of cells of YSF1c with complete deletion of unit C under neutral and alkaline conditions and the stabilization of GFP conformation by fusion with flocculin with complete deletion of unit C in the central domain.     

Characteristics of Flo11-dependent flocculation in Saccharomyces cerevisiae

The FLO11-encoded flocculin is required for a variety of important phenotypes in Saccharomyces cerevisiae, including flocculation, adhesion to agar and plastic, invasive growth, pseudohyphae formation and biofilm development. We present evidence that Flo11p belongs to the Flo1-type class of flocculins rather than to the NewFlo class. Both Flo1-type and NewFlo yeast flocculation are inhibited by mannose. NewFlo flocculation, however, is also inhibited by several other carbohydrates including glucose, maltose and sucrose. These differences have in at least one case been shown to reflect differences in the structure of the carbohydrate-binding site of the flocculins. We report that Flo11p-dependent flocculation is inhibited by mannose, but not by glucose, maltose or sucrose. Furthermore, Flo11p contains a peptide sequence highly similar to one that has been shown to characterise Flo1-type flocculins. Further characterisation of the properties of Flo11p-dependent flocculation revealed that it is dependent on calcium, occurs only at cell densities greater than 1 x 10(8) ml(-1), and only occurs at acidic pH.     

高效絮凝素毕赤酵母表面展示系统的构建

为了获得高效的脂肪酶毕赤酵母表面展示系统,利用来自酿酒酵母絮凝素蛋白Flo1的N端874个氨基酸残基(FS)和C端的1101个氨基酸残基(FL)作为锚定蛋白分别构建了2套载体系统.带有前肽的米黑根毛霉脂肪酶(ProRML)克隆到构建的2套展示载体中,使米黑根毛霉脂肪酶(RML)分别以N端锚定或C端锚定的方式实现在毕赤酵母细胞表面的展示.利用RMLC端的Flag标签,通过流式细胞术和激光扫描共聚焦显微镜检测2套系统中RML在酵母表面的展示情况.研究发现,N端锚定于酵母表面的展示酶FSR以pNPC为底物时,水解活力达到了105.3U/g,大约为C端锚定的展示酶FLR活力的2倍.同时FSR比FLR具有更宽的温度、pH作用范围和更好的热稳定性.与游离酶和固定化酶相比,展示酶FSR也表现出更为优良的热稳定性.结果提示,基于Flo1N端锚定的展示系统更适合展示活性中心近C端的脂肪酶,推动了展示酶的进一步研究和开发.     

The N-terminal carbohydrate recognition site of the cation-independent mannose 6-phosphate receptor

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR) plays a critical role in the trafficking of newly synthesized mannose 6-phosphate-containing acid hydrolases to the lysosome. The receptor contains two high affinity carbohydrate recognition sites within its 15-domain extracytoplasmic region, with essential residues for carbohydrate recognition located in domain 3 and domain 9. Previous studies have shown that these two sites are distinct with respect to carbohydrate specificity. In addition, expression of truncated forms of the CI-MPR demonstrated that domain 9 can be expressed as an isolated domain, retaining high affinity (Kd approximately 1 nm) carbohydrate binding, whereas expression of domain 3 alone resulted in a protein capable of only low affinity binding (Kd approximately 1 microm) toward a lysosomal enzyme. In the current report the crystal structure of the N-terminal 432 residues of the CI-MPR, encompassing domains 1-3, was solved in the presence of bound mannose 6-phosphate. The structure reveals the unique architecture of this carbohydrate binding pocket and provides insight into the ability of this site to recognize a variety of mannose-containing sugars.     

IL-2, a lectin with specificity for high mannose glycopeptides

Utilizing a solid phase binding assay, we have demonstrated that rIL-2 binds with high affinity to the human urinary glycoprotein uromodulin. This binding is specifically inhibited by the saccharides diacetylchitobiose and Man(alpha 1-3)(Man(alpha 1-6]Man-O-methyl and by the high mannose glycopeptides Man5GlcNAc2-R and Man6GlcNAc2-R, but not by Man9GlcNAc2-R. rIL-2 also binds OVA, a glycoprotein which contains approximately 50% high mannose chains at a single glycosylation site, and to yeast mannan. This binding is inhibited by the same battery of saccharides which inhibit the binding to uromodulin. The conclusion that rIL-2 is a lectin is further supported by the observation that the sequence of IL-2 shares 27% homology with a 33-residue sequence of the carbohydrate-binding domain of human mannose-binding protein. The potential physiologic relevance of the carbohydrate binding activity is further elucidated by studies which show that 1) binding of soluble rIL-2 to immobilized uromodulin is enhanced at a pH of 4 to5 in the presence of divalent cations, and 2) neither uromodulin nor the high mannose glycopeptide Man5GlcNAc2Asn blocks the binding of rIL-2 to the IL-2R. Thus the carbohydrate-binding site of rIL-2 is distinct from the cell surface receptor-binding site, and might function preferentially in acidic microenvironments.     

Construction of yeast strains with high cell surface lipase activity by using novel display systems based on the Flo1p flocculation functional domain

We constructed a novel cell-surface display system, using as a new type of cell-wall anchor 3,297 or 4,341 bp of the 3' region of the FLO1 gene (FS or FL gene, respectively), which encodes the flocculation functional domain of Flo1p. In this system, the N terminus of the target protein was fused to the FS or FL protein and the fusion proteins were expressed under the control of the inducible promoter UPR-ICL (5' upstream region of the isocitrate lyase of Candida tropicalis). Using this new system, recombinant lipase with a pro sequence from Rhizopus oryzae (rProROL), which has its active site near the C terminus, was displayed on the cell surface. Cell-surface display of the FSProROL and FLProROL fusion proteins was confirmed by immunofluorescence microscopy and immunoblotting. Lipase activity reached 145 IU/liter (61.3 IU/g [dry cell weight]) on the surface of the yeast cells, which successfully catalyzed the methanolysis reaction. Using these whole-cell biocatalysts, methylesters synthesized from triglyceride and methanol reached 78.3% after 72 h of reaction. To our knowledge, this is the first example of cell-surface display of lipase with high activity. Interestingly, the yeast cells displaying the FLProROL protein showed strong flocculation, even though the glycosylphosphatidylinositol anchor attachment signal and cell-membrane-anchoring region of Flo1p had been deleted from this gene. The cell-surface display system based on FL thus endows the yeast strain with both novel enzyme display and strong flocculation ability.     

Region of Flo1 Proteins Responsible for Sugar Recognition

Yeast flocculation is a phenomenon which is believed to result from an interaction between a lectin-like protein and a mannose chain located on the yeast cell surface. The FLO1 gene, which encodes a cell wall protein, is considered to play an important role in yeast flocculation, which is inhibited by mannose but not by glucose (mannose-specific flocculation). A new homologue of FLO1, named Lg-FLO1, was isolated from a flocculent bottom-fermenting yeast strain in which flocculation is inhibited by both mannose and glucose (mannose/glucose-specific flocculation). In order to confirm that both FLO1 and Lg-FLO1 are involved in the yeast flocculation phenomenon, the FLO1 gene in the mannose-specific flocculation strain was replaced by the Lg-FLO1 gene. The transformant in which the Lg-FLO1 gene was incorporated showed the same flocculation phenotype as the mannose/glucose-specific flocculation strain, suggesting that the FLO1 and Lg-FLO1 genes encode mannose-specific and mannose/glucose-specific lectin-like proteins, respectively. Moreover, the sugar recognition sites for these sugars were identified by expressing chimeric FLO1 and Lg-FLO1 genes. It was found that the region from amino acid 196 to amino acid 240 of both gene products is important for flocculation phenotypes. Further mutational analysis of this region suggested that Thr-202 in the Lg-Flo1 protein and Trp-228 in the Flo1 protein are involved in sugar recognition.     

Localization and cell surface anchoring of the Saccharomyces cerevisiae flocculation protein Flo1p.

The Saccharomyces cerevisiae FLO1 gene encodes a large 1,536-amino-acid serine- and threonine-rich protein involved in flocculation. We have assessed the localization of Flo1p by immunoelectron microscopy, and in this study we show that this protein is located in the external mannoprotein layer of the cell wall, at the plasma membrane level and in the periplasm. The protein was also visualized in the endoplasmic reticulum and in the nuclear envelope, indicating that it was secreted through the secretory pathway. The protein was detected by Western blotting in cell wall extracts as a high-molecular-mass (>200 kDa) polydisperse material obviously as a result of extensive N and probably O glycosylation. Flo1p was extracted from cell walls in large amounts by boiling in sodium dodecyl sulfate, suggesting that it is noncovalently anchored to the cell wall network. The membranous forms of Flo1p were shown to be solubilized by phosphatidylinositol-phospholipase C treatment, suggesting that Flo1p is glycosyl phosphatidylinositol (GPI) anchored to this organelle. The expression of truncated forms with the hydrophobic C-terminal domain deleted led to the secretion of the protein in the culture medium. The hydrophobic C terminus, which is a putative GPI anchoring domain, is therefore necessary for the attachment of Flo1p in the cell wall. Deletion analysis also revealed that the N-terminal domain of Flo1p was essential for cellular aggregation. On the whole, our data indicate that Flo1p is a true cell wall protein which plays a direct role in cell-cell interaction.     

Expression and characterization of the flocculin Flo11/Muc1, a Saccharomyces cerevisiae mannoprotein with homotypic properties of adhesion

The Flo11/Muc1 flocculin has diverse phenotypic effects. Saccharomyces cerevisiae cells of strain background Σ1278b require Flo11p to form pseudohyphae, invade agar, adhere to plastic, and develop biofilms, but they do not flocculate. We show that S. cerevisiae var. diastaticus strains, on the other hand, exhibit Flo11-dependent flocculation and biofilm formation but do not invade agar or form pseudohyphae. In order to study the nature of the Flo11p proteins produced by these two types of strains, we examined secreted Flo11p, encoded by a plasmid-borne gene, in which the glycosylphosphatidylinositol anchor sequences had been replaced by a histidine tag. A protein of approximately 196 kDa was secreted from both strains, which upon purification and concentration, aggregated into a form with a very high molecular mass. When secreted Flo11p was covalently attached to microscopic beads, it conferred the ability to specifically bind to S. cerevisiae var. diastaticus cells, which flocculate, but not to Σ1278b cells, which do not flocculate. This was true for the 196-kDa form as well as the high-molecular-weight form of Flo11p, regardless of the strain source. The coated beads bound to S. cerevisiae var. diastaticus cells expressing FLO11 and failed to bind to cells with a deletion of FLO11, demonstrating a homotypic adhesive mechanism. Flo11p was shown to be a mannoprotein. Bead-to-cell adhesion was inhibited by mannose, which also inhibits Flo11-dependent flocculation in vivo, further suggesting that this in vitro system is a useful model for the study of fungal adhesion.     

Flocculation protein structure and cell–cell adhesion mechanism in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Cell–cell adhesion occurs in a broad spectrum of biological processes, of which yeast flocculation is an area of interest for evolutionary scientists to brewers and winemakers. The flocculation mechanism is based on a lectin-carbohydrate interaction but is not yet fully understood, although the first model dates back to the 1950s. This review will update the current understanding of the complex mechanism behind yeast flocculation. Moreover, modern technologies to measure the forces involved in single carbohydrate-lectin interactions, are discussed. The Flo1 protein has been extensively described as the protein responsible for strong flocculation. Recently, more research has been directed to the detailed analysis of this flocculin. Due to the advances in the field of bioinformatics, more information about Flo1p could be obtained via structurally or functionally related proteins. Here, we review the current knowledge of the Flo1 protein, with a strong emphasis towards its structure.     

酵线菌絮凝的分型及其生理生化特性的研究

通过对４１０余株酵母菌进行絮凝测定,从中筛选到５株强絮凝菌。依据不同糖对其絮凝水平的抑制,将５株强絮凝菌分为Ｆｌｏ１型和ＮｅｗＦｌｏ型。对这两种絮凝型菌株的相关生理生化特性进行了研究。结果表明,Ｆｌｏ１型菌絮凝只受甘露糖抑制,它对高温（７０℃）、蛋白酶Ｅ、胰蛋白酶敏感,而对蛋白酶Ｋ、糜蛋白酶、Ｃａ＾２＋、ｐＨ有一定受性。ＮｅｗＦｌｏ型菌絮凝受甘露糖等多种糖抑制,它对高温（７０℃）、各种蛋白酶、Ｃａ     

Construction of Yeast Strains with High Cell Surface Lipase Activity by Using Novel Display Systems Based on the Flo1p Flocculation Functional Domain

We constructed a novel cell-surface display system, using as a new type of cell-wall anchor 3,297 or 4,341 bp of the 3′ region of the FLO1 gene (FS or FL gene, respectively), which encodes the flocculation functional domain of Flo1p. In this system, the N terminus of the target protein was fused to the FS or FL protein and the fusion proteins were expressed under the control of the inducible promoter UPR-ICL (5′ upstream region of the isocitrate lyase of Candida tropicalis). Using this new system, recombinant lipase with a pro sequence from Rhizopus oryzae (rProROL), which has its active site near the C terminus, was displayed on the cell surface. Cell-surface display of the FSProROL and FLProROL fusion proteins was confirmed by immunofluorescence microscopy and immunoblotting. Lipase activity reached 145 IU/liter (61.3 IU/g [dry cell weight]) on the surface of the yeast cells, which successfully catalyzed the methanolysis reaction. Using these whole-cell biocatalysts, methylesters synthesized from triglyceride and methanol reached 78.3% after 72 h of reaction. To our knowledge, this is the first example of cell-surface display of lipase with high activity. Interestingly, the yeast cells displaying the FLProROL protein showed strong flocculation, even though the glycosylphosphatidylinositol anchor attachment signal and cell-membrane-anchoring region of Flo1p had been deleted from this gene. The cell-surface display system based on FL thus endows the yeast strain with both novel enzyme display and strong flocculation ability.     

Human ZG16p recognizes pathogenic fungi through non-self polyvalent mannose in the digestive system

Human zymogen granule protein 16 (ZG16p) contains a Jacalin-like lectin domain, although its glycan-binding properties are not fully understood. Here, we screened the glycan-binding specificity of ZG16p by recently developed glycoconjugate microarray. ZG16p appeared to exhibit selective binding to α- and β-linked mannose-polyacrylamide-biotin probes. In more quantitative analysis using frontal affinity chromatography, dissociation constants to two types of polyvalent mannose, i.e. high-density mannose and yeast mannan, were determined to be 1.3 and 1.7?μM, respectively. Mutation of the evolutionarily conserved amino acid Asp151, which is involved in sugar binding among the Jacalin-related lectins (JRLs), abolished binding activity to mannose. By immunohistochemical staining, ZG16p was specifically detected in mucus-secreting cells of the digestive system such as serosanguineous acinar cells of the parotid gland, acinar cells of the pancreas and goblet cells of the intestine. Finally, we showed that ZG16p recognizes pathogenic Candida and Malassezia species in a polyvalent mannose-dependent manner. We propose that ZG16p is a novel member of mannose-specific JRLs, which recognizes pathogenic fungi through non-self polyvalent mannose in the digestive system.     

Deletion of tandem repeats causes flocculation phenotype conversion from Flo1 to NewFlo in Saccharomyces cerevisiae

A gene, FLONS, conferring NewFlo-type flocculation ability in yeast was cloned. The 3,396-bp ORF encoded a peptide of 1,132 amino acids with high identity to Flo1 protein. Aligned with the FLO1 gene, two repeated regions (675 and 540 bp) were lost in the middle of FLONS, revealing that this gene was a derived form of the FLO1 gene. The missing repeated sequence contained three highly homologous repeat units. Although the flocculation phenotype of the transformant YTS-S with the FLONS gene was inhibited by both mannose and glucose, it exhibited some distinguished physiological characteristics from the reported typical NewFlo-type flocculation during detailed investigation. The deletion of repeats was suspected to cause conversion of the flocculation phenotype from Flo1 to NewFlo, suggesting that intragenic tandem repeats generated functional variability in Flo1 protein.     

Essential Roles of Cell Surface Protein and Carbohydrate Components in Flocculation of a Brewer’s Yeast

Co-flocculation between cells of beer yeast IFO 2018, a flocculent strain, and non-flocculent strains was investigated by means of a chemical modification method. Treatment with periodate deprived non-flocculent cells, but not flocculent cells, of the ability to co-flocculate. Treatment with mercaptoethanol or photo-irradiation in the presence of methylene blue deprived flocculent cells, but not non-flocculent cells, of the co-flocculating ability. Mercaptoethanol-treated or photoirradiated flocculent cells (beer yeast IFO 2018) co-flocculated with periodate-treated flocculent cells, but periodate-treated cells subsequently subjected to mercaptoethanol treatment or photoirradiation neither flocculated by themselves nor co-flocculated with other cells. Thus, it is likely that both protein and carbohydrate components of the yeast cell surface play important roles in the mutual recognition and intercellular interaction involved in flocculation. It is strongly suggested that the essential carbohydrate which is widely distributed among Saccharomyces species is the mannan fraction on the cell wall, and that a flocculent yeast strain produces surface protein component(s) which recognize and bind the mannan component of adjacent cells.     

The carbohydrate-recognition domain of Dectin-2 is a C-type lectin with specificity for high mannose

We examined the carbohydrate-binding potential of the C-type lectin-like receptor Dectin-2 (Clecf4n). The carbohydrate-recognition domain (CRD) of Dectin-2 exhibited cation-dependent mannose/fucose-like lectin activity, with an IC(50) for mannose of approximately 20 mM compared to an IC(50) of 1.5 mM for the macrophage mannose receptor when assayed by similar methodology. The extracellular domain of Dectin-2 exhibited binding to live Candida albicans and the Saccharomyces-derived particle zymosan. This binding was completely abrogated by cation chelation and was competed by yeast mannans. We compared the lectin activity of Dectin-2 with that of two other C-type lectin receptors (mannose receptor and SIGNR1) known to bind fungal mannans. Both mannose receptor and SIGNR1 were able to bind bacterial capsular polysaccharides derived from Streptococcus pneumoniae, but interestingly they exhibited distinct binding profiles. The Dectin-2 CRD exhibited only weak interactions to some of these capsular polysaccharides, indicative of different structural or affinity requirements for binding, when compared with the other two lectins. Glycan array analysis of the carbohydrate recognition by Dectin-2 indicated specific recognition of high-mannose structures (Man(9)GlcNAc(2)). The differences in the specificity of these three mannose-specific lectins indicate that mannose recognition is mediated by distinct receptors, with unique specificity, that are expressed by discrete subpopulations of cells, and this further highlights the complex nature of carbohydrate recognition by immune cells.     

Modulation of drug sensitivity in yeast cells by the ATP-binding domain of human DNA topoisomerase IIalpha

Epipodophyllotoxins are effective antitumour drugs that trap eukaryotic DNA topoisomerase II in a covalent complex with DNA. Based on DNA cleavage assays, the mode of interaction of these drugs was proposed to involve amino acid residues of the catalytic site. An in vitro binding study, however, revealed two potential binding sites for etoposide within human DNA topoisomerase IIα (htopoIIα), one in the catalytic core of the enzyme and one in the ATP-binding N-terminal domain. Here we have tested how N-terminal mutations that reduce the affinity of the site for etoposide or ATP affect the sensitivity of yeast cells to etoposide. Surprisingly, when introduced into full-length enzymes, mutations that lower the drug binding capacity of the N-terminal domain in vitro render yeast more sensitive to epipodophyllotoxins. Consistently, when the htopoIIα N-terminal domain alone is overexpressed in the presence of yeast topoII, cells become more resistant to etoposide. Point mutations that weaken etoposide binding eliminate this resistance phenotype. We argue that the N-terminal ATP-binding pocket competes with the active site of the holoenzyme for binding etoposide both in cis and in trans with different outcomes, suggesting that each topoisomerase II monomer has two non-equivalent drug-binding sites.