Expression, cloning, and characterization of a Candida albicans gene, ALA1, that confers adherence properties upon Saccharomyces cerevisiae for extracellular matrix proteins

Genetic basis for probiotic yeast phenotypes revealed by nanopore sequencing

Probiotic yeasts are emerging as preventative and therapeutic solutions for disease. Often ingested via cultured foods and beverages, they can survive the harsh conditions of the gastrointestinal tract and adhere to it, where they provide nutrients and inhibit pathogens like Candida albicans. Yet, little is known of the genomic determinants of these beneficial traits. To this end, we have sequenced 2 food-derived probiotic yeast isolates that mitigate fungal infections. We find that the first strain, KTP, is a strain of Saccharomyces cerevisiae within a small clade that lacks any apparent ancestry from common European/wine S. cerevisiae strains. Significantly, we show that S. cerevisiae KTP genes involved in general stress, pH tolerance, and adherence are markedly different from S. cerevisiae S288C but are similar to the commercial probiotic yeast species S. boulardii. This suggests that even though S. cerevisiae KTP and S. boulardii are from different clades, they may achieve probiotic effect through similar genetic mechanisms. We find that the second strain, ApC, is a strain of Issatchenkia occidentalis, one of the few of this family of yeasts to be sequenced. Because of the dissimilarity of its genome structure and gene organization, we infer that I. occidentalis ApC likely achieves a probiotic effect through a different mechanism than the Saccharomyces strains. Therefore, this work establishes a strong genetic link among probiotic Saccharomycetes, advances the genomics of Issatchenkia yeasts, and indicates that probiotic activity is not monophyletic and complimentary mixtures of probiotics could enhance health benefits beyond a single species.

The Pga59 cell wall protein is an amyloid forming protein involved in adhesion and biofilm establishment in the pathogenic yeast Candida albicans

The human commensal fungus Candida albicans can attach to epithelia or indwelling medical devices and form biofilms, that are highly tolerant to antifungal drugs and can evade the immune response. The cell surface protein Pga59 has been shown to influence adhesion and biofilm formation. Here, we present evidence that Pga59 displays amyloid properties. Using electron microscopy, staining with an amyloid fibre-specific dye and X-ray diffraction experiments, we showed that the predicted amyloid-forming region of Pga59 is sufficient to build up an amyloid fibre in vitro and that recombinant Pga59 can also adopt a cross-β amyloid fibre architecture. Further, mutations impairing Pga59 amyloid assembly led to diminished adhesion to substrates and reduced biofilm production. Immunogold labelling on amyloid structures extracted from C. albicans revealed that Pga59 is used by the fungal cell to assemble amyloids within the cell wall in response to adhesion. Altogether, our results suggest that Pga59 amyloid properties are used by the fungal cell to mediate cell-substrate interactions and biofilm formation.

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.

A New Function for Amyloid-Like Interactions: Cross-Beta Aggregates of Adhesins form Cell-to-Cell Bonds

Amyloid structures assemble through a repeating type of bonding called "cross-β", in which identical sequences in many protein molecules form β-sheets that interdigitate through side chain interactions. We review the structural characteristics of such bonds. Single cell force microscopy (SCFM) shows that yeast expressing Als5 adhesin from Candida albicans demonstrate the empirical characteristics of cross-β interactions. These properties include affinity for amyloid-binding dyes, birefringence, critical concentration dependence, repeating structure, and inhibition by anti-amyloid agents. We present a model for how cross-β bonds form in trans between two adhering cells. These characteristics also apply to other fungal adhesins, so the mechanism appears to be an example of a new type of cell-cell adhesion.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

Comprehensive genetic analysis of adhesin proteins and their role in virulence of Candida albicans

Candida albicans is a microbial fungus that exists as a commensal member of the human microbiome and an opportunistic pathogen. Cell surface-associated adhesin proteins play a crucial role in C. albicans' ability to undergo cellular morphogenesis, develop robust biofilms, colonize, and cause infection in a host. However, a comprehensive analysis of the role and relationships between these adhesins has not been explored. We previously established a CRISPR-based platform for efficient generation of single- and double-gene deletions in C. albicans, which was used to construct a library of 144 mutants, comprising 12 unique adhesin genes deleted singly, and every possible combination of double deletions. Here, we exploit this adhesin mutant library to explore the role of adhesin proteins in C. albicans virulence. We perform a comprehensive, high-throughput screen of this library, using Caenorhabditis elegans as a simplified model host system, which identified mutants critical for virulence and significant genetic interactions. We perform follow-up analysis to assess the ability of high- and low-virulence strains to undergo cellular morphogenesis and form biofilms in vitro, as well as to colonize the C. elegans host. We further perform genetic interaction analysis to identify novel significant negative genetic interactions between adhesin mutants, whereby combinatorial perturbation of these genes significantly impairs virulence, more than expected based on virulence of the single mutant constituent strains. Together, this study yields important new insight into the role of adhesins, singly and in combinations, in mediating diverse facets of virulence of this critical fungal pathogen.

Als3-mediated attachment of enolase on the surface of Candida albicans cells regulates their interactions with host proteins

The multifunctional protein enolase has repeatedly been identified on the surface of numerous cell types, including a variety of pathogenic microorganisms. In Candida albicans-one of the most common fungal pathogens in humans-a surface-exposed enolase form has been previously demonstrated to play an important role in candidal pathogenicity. In our current study, the presence of enolase at the fungal cell surface under different growth conditions was examined, and a higher abundance of enolase at the surface of C. albicans hyphal forms compared to yeast-like cells was found. Affinity chromatography and chemical cross-linking indicated a member of the agglutinin-like sequence protein family-Als3-as an important potential partner required for the surface display of enolase. Analysis of Saccharomyces cerevisiae cells overexpressing Als3 with site-specific deletions showed that the Ig-like N-terminal region of Als3 (aa 166-225; aa 218-285; aa 270-305; aa 277-286) and the central repeat domain (aa 434-830) are essential for the interaction of this adhesin with enolase. In addition, binding between enolase and Als3 influenced subsequent docking of host plasma proteins-high molecular mass kininogen and plasminogen-on the candidal cell surface, thus supporting the hypothesis that C. albicans can modulate plasma proteolytic cascades to affect homeostasis within the host and propagate inflammation during infection.

Improved efficacy of antifungal drugs in combination with monoterpene phenols against Candida auris

Emergence of Candida auris has been described as a global health threat due to its ability to cause invasive infections with high mortality rate and multidrug resistance. Novel drugs and therapies are required to target this organism and its pathogenicity. Anti-virulence approach and combination therapy have been proposed as alternatives in recent years. This study evaluated the virulence factors in C. auris, combination antifungal activity of phenolic compounds with antifungal drugs and determined effect of the most active compound on positive pathogenicity markers of C. auris. Antifungal susceptibility profile of 25 clinical isolates of C. auris against antifungal agents as well as against phenolic compounds was obtained using CLSI guidelines. Combination of the most active phenolic compound with antifungal drugs was determined. Effect of carvacrol on the virulence factors was also studied. Carvacrol was the most active phenol with median MIC of 125 µg/ml and its combination with fluconazole, amphotericin B, nystatin and caspofungin resulted synergistic and additive effects in 68%, 64%, 96% and 28%, respectively. Combination also reduced the MIC values of the drugs. All test strains showed adherence ability to epithelial cells and 96% of strains produced proteinase. None of the strains produced hyphae and phospholipase. At low concentrations, carvacrol significantly inhibited the adherence ability and proteinase production (both p < 0.01). Carvacrol has antifungal and anti-virulence activity against C. auris. It also showed an enhanced antifungal activity in combination with antifungal agents. Therefore it has potential to be developed into a novel antifungal agent.

Advances in Plant-Derived Scaffold Proteins

Scaffold proteins form critical biomatrices that support cell adhesion and proliferation for regenerative medicine and drug screening. The increasing demand for such applications urges solutions for cost effective and sustainable supplies of hypoallergenic and biocompatible scaffold proteins. Here, we summarize recent efforts in obtaining plant-derived biosynthetic spider silk analogue and the extracellular matrix protein, collagen. Both proteins are composed of a large number of tandem block repeats, which makes production in bacterial hosts challenging. Furthermore, post-translational modification of collagen is essential for its function which requires co-transformation of multiple copies of human prolyl 4-hydroxylase. We discuss our perspectives on how the GAANTRY system could potentially assist the production of native-sized spider dragline silk proteins and prolyl hydroxylated collagen. The potential of recombinant scaffold proteins in drug delivery and drug discovery is also addressed.

From farm-scale to lab-scale: The characterization of engineered irrigation water distribution system biofilm models using an artificial freshwater source

Contaminants in freshwater environments, as well as the associated negative impacts on agricultural produce, have emerged as a critical theme of the water-energy-food nexus affecting food safety and irrigation management. Agricultural produce exposed to irrigation with questionable freshwater can internalize and concentrate pollutants. However, the potential risks posed by the ubiquitous presence of biofilms within irrigation water distribution systems (IWDS) remains overlooked, even though such biofilms may harbor and spread pathogenic, chemical, and other environmental pollutants. Our limited knowledge about the role and functional attributes of IWDS biofilms can be blamed mostly to experimental challenges encountered during attempted studies of these biofilms in their natural environments. Hence, a laboratory-based experimental system designed to simulate a freshwater environment was combined with a biofilm reactor capable of recreating the piping environments in water distribution systems. This experimental system was then tested to assess the robustness and repeatability of experimental early-stage biofilms with respect to physical structure and microbial community, using state-of-the-art confocal microscopy and next-generation sequencing, respectively. The results demonstrated the suitability of this laboratory-based experimental system for studying the impacts of selected pollutants on irrigation water distribution systems.

An Amyloid Core Sequence in the Major Candida albicans Adhesin Als1p Mediates Cell-Cell Adhesion

The human fungal commensal Candida albicans can become a serious opportunistic pathogen in immunocompromised hosts. The C. albicans cell adhesion protein Als1p is a highly expressed member of a large family of paralogous adhesins. Als1p can mediate binding to epithelial and endothelial cells, is upregulated in infections, and is important for biofilm formation. Als1p includes an amyloid-forming sequence at amino acids 325 to 331, identical to the sequence in the paralogs Als5p and Als3p. Therefore, we mutated Val326 to test whether this sequence is important for activity. Wild-type Als1p (Als1pWT) and Als1p with the V326N mutation (Als1pV326N) were expressed at similar levels in a Saccharomyces cerevisiae surface display model. Als1pV326N cells adhered to bovine serum albumin (BSA)-coated beads similarly to Als1pWT cells. However, cells displaying Als1pV326N showed visibly smaller aggregates and did not fluoresce in the presence of the amyloid-binding dye Thioflavin-T. A new analysis tool for single-molecule force spectroscopy-derived surface mapping showed that statistically significant force-dependent Als1p clustering occurred in Als1pWT cells but was absent in Als1pV326N cells. In single-cell force spectroscopy experiments, strong cell-cell adhesion was dependent on an intact amyloid core sequence on both interacting cells. Thus, the major adhesin Als1p interacts through amyloid-like β-aggregation to cluster adhesin molecules in cis on the cell surface as well as in trans to form cell-cell bonds.IMPORTANCE Microbial cell surface adhesins control essential processes such as adhesion, colonization, and biofilm formation. In the opportunistic fungal pathogen Candida albicans, the agglutinin-like sequence (ALS) gene family encodes eight cell surface glycoproteins that mediate adherence to biotic and abiotic surfaces and cell-cell aggregation. Als proteins are critical for commensalism and virulence. Their activities include attachment and invasion of endothelial and epithelial cells, morphogenesis, and formation of biofilms on host tissue and indwelling medical catheters. At the molecular level, Als5p-mediated cell-cell aggregation is dependent on the formation of amyloid-like nanodomains between Als5p-expressing cells. A single-site mutation to valine 326 abolishes cellular aggregation and amyloid formation. Our results show that the binding characteristics of Als1p follow a mechanistic model similar to Als5p, despite its differential expression and biological roles.

Fluidic Force Microscopy Demonstrates That Homophilic Adhesion by Candida albicans Als Proteins Is Mediated by Amyloid Bonds between Cells

The fungal pathogen Candida albicans frequently forms drug-resistant biofilms in hospital settings and in chronic disease patients. Cell adhesion and biofilm formation involve a family of cell surface Als (agglutinin-like sequence) proteins. It is now well documented that amyloid-like clusters of laterally arranged Als proteins activate cell-cell adhesion under mechanical stress, but whether amyloid-like bonds form between aggregating cells is not known. To address this issue, we measure the forces driving Als5-mediated intercellular adhesion using an innovative fluidic force microscopy platform. Strong cell-cell adhesion is dependent on expression of amyloid-forming Als5 at high cell surface density and is inhibited by a short antiamyloid peptide. Furthermore, there is greatly attenuated binding between cells expressing amyloid-forming Als5 and cells with a nonamyloid form of Als5. Thus, homophilic bonding between Als5 proteins on adhering cells is the major mode of fungal aggregation, rather than protein-ligand interactions. These results point to a model whereby amyloid-like β-sheet interactions play a dual role in cell-cell adhesion, that is, in formation of adhesin nanoclusters ( cis-interactions) and in homophilic bonding between amyloid sequences on opposing cells ( trans-interactions). Because potential amyloid-forming sequences are found in many microbial adhesins, we speculate that this novel mechanism of amyloid-based homophilic adhesion might be widespread and could represent an interesting target for treating biofilm-associated infections.

Candida albicans Interactions with Mucosal Surfaces during Health and Disease

Flexible adaptation to the host environment is a critical trait that underpins the success of numerous microbes. The polymorphic fungus Candida albicans has evolved to persist in the numerous challenging niches of the human body. The interaction of C. albicans with a mucosal surface is an essential prerequisite for fungal colonisation and epitomises the complex interface between microbe and host. C. albicans exhibits numerous adaptations to a healthy host that permit commensal colonisation of mucosal surfaces without provoking an overt immune response that may lead to clearance. Conversely, fungal adaptation to impaired immune fitness at mucosal surfaces enables pathogenic infiltration into underlying tissues, often with devastating consequences. This review will summarise our current understanding of the complex interactions that occur between C. albicans and the mucosal surfaces of the human body.

Serum Amyloid P Component Binds Fungal Surface Amyloid and Decreases Human Macrophage Phagocytosis and Secretion of Inflammatory Cytokines

In patients with invasive fungal diseases, there is often little cellular inflammatory response. We tested the idea that binding of the human constitutive plasma protein serum amyloid P component (SAP) (also called PTX2) to Candida albicans dampens the innate immune response to this fungus. Many pathogenic fungi have cell surface amyloid-like structures important for adhesion and biofilm formation. Human SAP bound to fungi that expressed functional cell surface amyloid, but SAP had minimal binding to fungi with reduced expression of cell surface amyloid. In the absence of SAP, phagocytosis of fungi by human macrophages was potentiated by expression of amyloid on the fungi. SAP binding to fungi inhibited their phagocytosis by macrophages. Macrophages pretreated with SAP displayed reduced fungal phagocytosis, reduced secretion of inflammatory cytokines (IFN-γ, IL-6, and TNF-α), and increased secretion of the anti-inflammatory cytokine IL-10. SAP bound to fungi or added to the medium upregulated the expression of the anti-inflammatory receptor CD206 on macrophages. These findings suggest that SAP bound to amyloid-like structures on fungal cells dampens the host cellular immune response in fungal diseases such as invasive candidiasis.IMPORTANCE Macrophages are a key part of our innate immune system and are responsible for recognizing invading microbes, ingesting them, and sending appropriate signals to other immune cells. We have found that human macrophages can recognize invading yeast pathogens that have a specific molecular pattern of proteins on their surfaces: these proteins have structures similar to the structures of amyloid aggregates in neurodegenerative diseases like Alzheimer's disease. However, this surface pattern also causes the fungi to bind a serum protein called serum amyloid P component (SAP). In turn, the SAP-coated yeasts are poorly recognized and seldom ingested by the macrophages, and the macrophages have a more tolerant and less inflammatory response in the presence of SAP. Therefore, we find that surface structures on the yeast can alter how the macrophages react to invading microbes.

What We Do Not Know about Fungal Cell Adhesion Molecules

There has been extensive research on structure and function of fungal cell adhesion molecules, but the most of the work has been about adhesins in Candida albicans and Saccharomyces cerevisiae. These yeasts are members of a single ascomycete order, and adhesion molecules from the six other fungal phyla are only sparsely described in the literature. In these other phyla, most of the research is at the cellular level, rather than at the molecular level, so there has been little characterization of the adhesion molecules themselves. A catalog of known adhesins shows some common features: high Ser/Thr content, tandem repeats, N- and O-glycosylations, GPI anchors, dibasic sequence motifs, and potential amyloid-forming sequences. However, none of these features is universal. Known ligands include proteins and glycans on homologous cells and host cells. Existing and novel tools can exploit the availability of genome sequences to identify and characterize new fungal adhesins. These include bioinformatics tools and well-established yeast surface display models, which could be coupled with an adhesion substrate array. Thus, new knowledge could be exploited to answer key questions in fungal ecology, animal and plant pathogenesis, and roles of biofilms in infection and biomass turnover.

Targeting the GPI biosynthetic pathway

The GPI (Glycosylphosphatidylinositol) biosynthetic pathway is a multistep conserved pathway in eukaryotes that culminates in the generation of GPI glycolipid which in turn anchors many proteins (GPI-APs) to the cell surface. In spite of the overall conservation of the pathway, there still exist subtle differences in the GPI pathway of mammals and other eukaryotes which holds a great promise so far as the development of drugs/inhibitors against specific targets in the GPI pathway of pathogens is concerned. Many of the GPI structures and their anchored proteins in pathogenic protozoans and fungi act as pathogenicity factors. Notable examples include GPI-anchored variant surface glycoprotein (VSG) in Trypanosoma brucei, GPI-anchored merozoite surface protein 1 (MSP1) and MSP2 in Plasmodium falciparum, protein-free GPI related molecules like lipophosphoglycans (LPGs) and glycoinositolphospholipids (GIPLs) in Leishmania spp., GPI-anchored Gal/GalNAc lectin and proteophosphoglycans in Entamoeba histolytica or the GPI-anchored mannoproteins in pathogenic fungi like Candida albicans. Research in this active area has already yielded encouraging results in Trypanosoma brucei by the development of parasite-specific inhibitors of GlcNCONH₂-β-PI, GlcNCONH₂-(2-O-octyl)-PI and salicylic hydroxamic acid (SHAM) targeting trypanosomal GlcNAc-PI de-N-acetylase as well as the development of antifungal inhibitors like BIQ/E1210/gepinacin/G365/G884 and YW3548/M743/M720 targeting the GPI specific fungal inositol acyltransferase (Gwt1) and the phosphoethanolamine transferase-I (Mcd4), respectively. These confirm the fact that the GPI pathway continues to be the focus of researchers, given its implications for the betterment of human life.

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.

Fungal Biofilms: Inside Out

We focus this article on turning a biofilm inside out. The "inside" of the biofilm comprises the individual biofilm-related phenotypes, their environmental drivers and genetic determinants, and the coordination of gene functions through transcriptional regulators. Investigators have viewed the inside of the biofilm through diverse approaches, and this article will attempt to capture the essence of many. The ultimate goal is to connect the inside to the "outside," which we view as biofilm structure, development, pharmacological attributes, and medical impact.

Fungal Biofilms: Inside Out

We focus this article on turning a biofilm inside out. The "inside" of the biofilm comprises the individual biofilm-related phenotypes, their environmental drivers and genetic determinants, and the coordination of gene functions through transcriptional regulators. Investigators have viewed the inside of the biofilm through diverse approaches, and this article will attempt to capture the essence of many. The ultimate goal is to connect the inside to the "outside," which we view as biofilm structure, development, pharmacological attributes, and medical impact.

The Pmt2p-Mediated Protein O-Mannosylation Is Required for Morphogenesis, Adhesive Properties, Cell Wall Integrity and Full Virulence of Magnaporthe oryzae

Protein O-mannosylation is a type of O-glycosylation that is characterized by the addition of mannose residues to target proteins, and is initially catalyzed by evolutionarily conserved protein O-mannosyltransferases (PMTs). In this study, three members of PMT were identified in Magnaporthe oryzae, and the pathogenic roles of MoPmt2, a member of PMT2 subfamily, were analyzed. We found that MoPmt2 is a homolog of Saccharomyces cerevisiae Pmt2 and could complement yeast Pmt2 function in resistance to CFW. Quantitative RT-PCR revealed that MoPmt2 is highly expressed during conidiation, and targeted disruption of MoPmt2 resulted in defects in conidiation and conidia morphology. The MoPmt2 mutants also showed a distinct reduction in fungal growth, which was associated with severe alterations in hyphal polarity. In addition, we found that the MoPmt2 mutants severely reduced virulence on both rice plants and barley leaves. The subsequent examination revealed that the fungal adhesion, conidial germination, CWI and invasive hyphae growth in host cells are responsible for defects on appressorium mediated penetration, and thus attenuated the pathogenicity of MoPmt2 mutants. Taken together, our results suggest that protein O-mannosyltransferase MoPmt2 plays essential roles in fungal growth and development, and is required for the full pathogenicity of M. oryzae.

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.

Candida albicans-epithelial interactions and pathogenicity mechanisms: scratching the surface

Until recently, epithelial cells have been a largely ignored component of host responses to microbes. However, this has been largely overturned over the last decade as an ever increasing number of studies have highlighted the key role that these cells play in many of our interactions with our microbiota and pathogens. Interactions of these cells with Candida albicans have been shown to be critical not just in host responses, but also in fungal cell responses, regulating fungal morphology and gene expression profile. In this review, we will explore the interactions between C. albicans and epithelial cells, and discuss how these interactions affect our relationship with this fungus.

The Human Disease-Associated Aβ Amyloid Core Sequence Forms Functional Amyloids in a Fungal Adhesin

There is increasing evidence that many amyloids in living cells have physiological functions. On the surfaces of fungal cells, amyloid core sequences in adhesins can aggregate into 100- to 1,000-nm-wide patches to form high-avidity adhesion nanodomains on the cell surface. The nanodomains form through interactions that have amyloid-like properties: binding of amyloid dyes, perturbation by antiamyloid agents, and interaction with homologous sequences. To test whether these functional interactions are mediated by typical amyloid interactions, we substituted an amyloid core sequence, LVFFA, from human Aβ protein for the native sequence IVIVA in the 1,419-residue Candida albicans adhesin Als5p. The chimeric protein formed cell surface nanodomains and mediated cellular aggregation. The native sequence and chimeric adhesins responded similarly to the amyloid dye thioflavin T and to amyloid perturbants. However, unlike the native protein, the nanodomains formed by the chimeric protein were not force activated and formed less-robust aggregates under flow. These results showed the similarity of amyloid interactions in the amyloid core sequences of native Als5p and Aβ, but they also highlighted emergent properties of the native sequence. Also, a peptide composed of the Aβ amyloid sequence flanked by amino acids from the adhesin formed two-dimensional sheets with sizes similar to the cell surface patches of the adhesins. These results inform an initial model for the structure of fungal cell surface amyloid nanodomains.

IMPORTANCE

Protein amyloid aggregates are markers of neurodegenerative diseases such as Alzheimer's and Parkinsonism. Nevertheless, there are also functional amyloids, including biofilm-associated amyloids in bacteria and fungi. In fungi, glycoprotein adhesins aggregate into cell surface patches through amyloid-like interactions, and the adhesin clustering strengthens cell-cell binding. These fungal surface amyloid nanodomains mediate biofilm persistence under flow, and they also moderate host inflammatory responses in fungal infections. To determine whether the amyloid-like properties of fungal surface nanodomains are sequence specific, we ask whether a disease-associated amyloid core sequence has properties equivalent to those of the native sequence in a fungal adhesin. A chimeric adhesin with an amyloid sequence from the Alzheimer's disease protein Aβ instead of its native sequence effectively clustered the adhesins on the cell surface, but it showed a different response to hydrodynamic shear. These results begin an analysis of the sequence dependence for newly discovered activities for fungal surface amyloid nanodomains.

Quantitative Analyses of Force-Induced Amyloid Formation in Candida albicans Als5p: Activation by Standard Laboratory Procedures

Candida albicans adhesins have amyloid-forming sequences. In Als5p, these amyloid sequences cluster cell surface adhesins to create high avidity surface adhesion nanodomains. Such nanodomains form after force is applied to the cell surface by atomic force microscopy or laminar flow. Here we report centrifuging and resuspending S. cerevisiae cells expressing Als5p led to 1.7-fold increase in initial rate of adhesion to ligand coated beads. Furthermore, mechanical stress from vortex-mixing of Als5p cells or C. albicans cells also induced additional formation of amyloid nanodomains and consequent activation of adhesion. Vortex-mixing for 60 seconds increased the initial rate of adhesion 1.6-fold. The effects of vortex-mixing were replicated in heat-killed cells as well. Activation was accompanied by increases in thioflavin T cell surface fluorescence measured by flow cytometry or by confocal microscopy. There was no adhesion activation in cells expressing amyloid-impaired Als5pV326N or in cells incubated with inhibitory concentrations of anti-amyloid dyes. Together these results demonstrated the activation of cell surface amyloid nanodomains in yeast expressing Als adhesins, and further delineate the forces that can activate adhesion in vivo. Consequently there is quantitative support for the hypothesis that amyloid forming adhesins act as both force sensors and effectors.

Candida species biofilm and Candida albicans ALS3 polymorphisms in clinical isolates

Over the last decades, there have been important changes in the epidemiology of Candida infections. In recent years, Candida species have emerged as important causes of invasive infections mainly among immunocompromised patients. This study analyzed Candida spp. isolates and compared the frequency and biofilm production of different species among the different sources of isolation: blood, urine, vulvovaginal secretions and peritoneal dialysis fluid. Biofilm production was quantified in 327 Candida isolates obtained from patients attended at a Brazilian tertiary public hospital (Botucatu, Sao Paulo). C. albicans ALS3 gene polymorphism was also evaluated by determining the number of repeated motifs in the central domain. Of the 198 total biofilm-positive isolates, 72 and 126 were considered as low and high biofilm producers, respectively. Biofilm production by C. albicans was significantly lower than that by non-albicans isolates and was most frequently observed in C. tropicalis. Biofilm production was more frequent among bloodstream isolates than other clinical sources, in urine, the isolates displayed a peculiar distribution by presenting two distinct peaks, one containing biofilm-negative isolates and the other containing isolates with intense biofilm production. The numbers of tandem-repeat copies per allele were not associated with biofilm production, suggesting the evolvement of other genetic determinants.

Functional regions of Candida albicans hyphal cell wall protein Als3 that determine interaction with the oral bacterium Streptococcus gordonii

The opportunistic pathogen Candida albicans colonizes the oral cavity and gastrointestinal tract. Adherence to host cells, extracellular matrix and salivary glycoproteins that coat oral surfaces, including prostheses, is an important prerequisite for colonization. In addition, interactions of C. albicans with commensal oral streptococci are suggested to promote retention and persistence of fungal cells in mixed-species communities. The hyphal filament specific cell wall protein Als3, a member of the Als protein family, is a major determinant in C. albicans adherence. Here, we utilized site-specific in-frame deletions within Als3 expressed on the surface of heterologous Saccharomyces cerevisiae to determine regions involved in interactions of Als3 with Streptococcus gordonii. N-terminal region amino acid residue deletions Δ166-225, Δ218-285, Δ270-305 and Δ277-286 were each effective in inhibiting binding of Strep. gordonii to Als3. In addition, these deletions differentially affected biofilm formation, hydrophobicity, and adherence to silicone and human tissue proteins. Deletion of the central repeat domain (Δ434-830) did not significantly affect interaction of Als3 with Strep. gordonii SspB protein, but affected other adherence properties and biofilm formation. Deletion of the amyloid-forming region (Δ325-331) did not affect interaction of Als3 with Strep. gordonii SspB adhesin, suggesting this interaction was amyloid-independent. These findings highlighted the essential function of the N-terminal domain of Als3 in mediating the interaction of C. albicans with S. gordonii, and suggested that amyloid formation is not essential for the inter-kingdom interaction.

Role of force-sensitive amyloid-like interactions in fungal catch bonding and biofilms

The Candida albicans Als adhesin Als5p has an amyloid-forming sequence that is required for aggregation and formation of model biofilms on polystyrene. Because amyloid formation can be triggered by force, we investigated whether laminar flow could activate amyloid formation and increase binding to surfaces. Shearing Saccharomyces cerevisiae cells expressing Als5p or C. albicans at 0.8 dyne/cm(2) increased the quantity and strength of cell-to-surface and cell-to-cell binding compared to that at 0.02 dyne/cm(2). Thioflavin T fluorescence showed that the laminar flow also induced adhesin aggregation into surface amyloid nanodomains in Als5p-expressing cells. Inhibitory concentrations of the amyloid dyes thioflavin S and Congo red or a sequence-specific anti-amyloid peptide decreased binding and biofilm formation under flow. Shear-induced binding also led to formation of robust biofilms. There was less shear-activated increase in adhesion, thioflavin fluorescence, and biofilm formation in cells expressing the amyloid-impaired V326N-substituted Als5p. Similarly, S. cerevisiae cells expressing Flo1p or Flo11p flocculins also showed shear-dependent binding, amyloid formation, biofilm formation, and inhibition by anti-amyloid compounds. Together, these results show that laminar flow activated amyloid formation and led to enhanced adhesion of yeast cells to surfaces and to biofilm formation.

Fungal invasion of epithelial cells

Interaction between host cells and invasive Candida plays a large role in the pathogenicity of Candida species. Fungal-induced endocytosis and active penetration are the two distinct, yet complementary invasion mechanisms of invasive candidiasis. Induced endocytosis is a microorganism-triggered, epithelial-driven, clathrin-mediated and actin-dependent process. During the fundamental pathological process of induced endocytosis, invasins (Als3 and Ssa1), which mediate the binding of host epithelial surface proteins, are expressed by Candida species on the hyphal surface. Sequentially, the interaction between invasins and host epithelial surface proteins stimulates the recruitment of clathrin, dynamin and cortactin to the sites where Candida enters epithelial cells, which in turn induce the actin cytoskeleton reorganization. Actin cytoskeleton provides the force required for fungal internalization. Parallely, active penetration of Candida can directly pass through epithelial cells possibly due to progressive elongation of hyphae and physical forces. Several molecules, such as secreted hydrolases and Als3, can affect the protective barrier of the epithelium and make Candida actively penetrate into epithelial cells through intercellular gaps of epithelial layers.

Between Amyloids and Aggregation Lies a Connection with Strength and Adhesion

We tell of a journey that led to discovery of amyloids formed by yeast cell adhesins and their importance in biofilms and host immunity. We begin with the identification of the adhesin functional amyloid-forming sequences that mediate fiber formation in vitro. Atomic force microscopy and confocal microscopy show 2-dimensional amyloid "nanodomains" on the surface of cells that are activated for adhesion. These nanodomains are arrays of adhesin molecules that bind multivalent ligands with high avidity. Nanodomains form when adhesin molecules are stretched in the AFM or under laminar flow. Treatment with antiamyloid perturbants or mutation of the amyloid sequence prevents adhesion nanodomain formation and activation. We are now discovering biological consequences. Adhesin nanodomains promote formation and maintenance of biofilms, which are microbial communities. Also, in abscesses within candidiasis patients, we find adhesin amyloids on the surface of the fungi. In both human infection and a Caenorhabditis elegans infection model, the presence of fungal surface amyloids elicits anti-inflammatory responses. Thus, this is a story of how fungal adhesins respond to extension forces through formation of cell surface amyloid nanodomains, with key consequences for biofilm formation and host responses.

Role for the fibrinogen-binding proteins coagulase and Efb in the Staphylococcus aureus-Candida interaction

Staphylococcus aureus and Candida species are increasingly coisolated from implant-associated polymicrobial infections creating an incremental health care problem. Synergistic effects between both genera seem to facilitate the formation of mixed S. aureus-Candida biofilms, which is thought to play a critical role in coinfections with these microorganisms. To identify and characterize S. aureus factors involved in the interaction with Candida species, we affinity-panned an S. aureus phage display library against Candida biofilms in the presence or absence of fibrinogen. Repeatedly isolated clones contained DNA fragments encoding portions of the S. aureus fibrinogen-binding proteins coagulase or Efb. The coagulase binds to prothrombin in a 1:1 ratio thereby inducing a conformational change and non-proteolytic activation of prothrombin, which in turn cleaves fibrinogen to fibrin. Efb has been known to inhibit opsonization. To study the role of coagulase and Efb in the S. aureus-Candida cross-kingdom interaction, we performed flow-cytometric phagocytosis assays. Preincubation with coagulase reduced the phagocytosis of Candida yeasts by granulocytes significantly and dose-dependently. By using confocal laser scanning microscopy, we demonstrated that the coagulase mediated the formation of fibrin surrounding the candidal cells. Furthermore, the addition of Efb significantly protected the yeasts against phagocytosis by granulocytes in a dose-dependent and saturable fashion. In conclusion, the inhibition of phagocytosis of Candida cells by coagulase and Efb via two distinct mechanisms suggests that S. aureus might be beneficial for Candida to persist as it helps Candida to circumvent the host immune system.

Does Candida albicans Als5p amyloid play a role in commensalism in Caenorhabditis elegans?

Candida albicans, a dimorphic fungus and an opportunistic pathogen, possesses a myriad of adherence factors, including members of the agglutinin-like sequence (Als) family of mannoproteins. The adhesin Als5p mediates adhesion to many substrates and is upregulated during commensal interactions but is downregulated during active C. albicans infections. An amyloid-forming core sequence at residues 325 to 331 is important for Als5p function, because a single-amino-acid substitution at position 326 (V326N) greatly reduces Als5p-mediated adherence. We evaluated the role of Als5p in host-microbe interactions by using Caenorhabditis elegans nematodes as a host model and feeding them Saccharomyces cerevisiae expressing Als5p on the surface. Als5p-expressing yeast had 8.5- and 3.5-fold-increased intestinal accumulation rates compared to Als5p-nonexpressing S. cerevisiae or yeast expressing amyloid-deficient Als5p(V326N), respectively. Surprisingly, this accumulation delayed S. cerevisiae-induced killing of C. elegans. The median survival time was nearly twice as long as that of nematodes fed nonexpressing or non-amyloid-forming Als5p(V326N)-expressing S. cerevisiae. Treatment with the amyloid-inhibiting dye Congo red or repression of Als5p expression abrogated the protective effect of Als5p. Furthermore, Als5p had no effect on oocyte quantity or quality, since nematodes fed either empty vector (EV)- or Als5p(V326N)-expressing S. cerevisiae had similar egg-laying and egg-hatching rates. This study is the first, to our knowledge, to show that expression of an amyloid-forming protein can attenuate pathogenicity in C. elegans.

Adhesins in human fungal pathogens: glue with plenty of stick

Understanding the pathogenesis of an infectious disease is critical for developing new methods to prevent infection and diagnose or cure disease. Adherence of microorganisms to host tissue is a prerequisite for tissue invasion and infection. Fungal cell wall adhesins involved in adherence to host tissue or abiotic medical devices are critical for colonization leading to invasion and damage of host tissue. Here, with a main focus on pathogenic Candida species, we summarize recent progress made in the field of adhesins in human fungal pathogens and underscore the importance of these proteins in establishment of fungal diseases.

Candida species: new insights into biofilm formation

Biofilms of Candida albicans, Candida parapsilosis, Candida glabrata and Candida tropicalis are associated with high indices of hospital morbidity and mortality. Major factors involved in the formation and growth of Candida biofilms are the chemical composition of the medical implant and the cell wall adhesins responsible for mediating Candida-Candida, Candida-human host cell and Candida-medical device adhesion. Strategies for elucidating the mechanisms that regulate the formation of Candida biofilms combine tools from biology, chemistry, nanoscience, material science and physics. This review proposes the use of new technologies, such as synchrotron radiation, to study the mechanisms of biofilm formation. In the future, this information is expected to facilitate the design of new materials and antifungal compounds that can eradicate nosocomial Candida infections due to biofilm formation on medical implants. This will reduce dissemination of candidiasis and hopefully improve the quality of life of patients.

The impact of protein glycosylation on Flo11-dependent adherence in Saccharomyces cerevisiae

Fungal cell adhesion molecules are critical for the attachment of cells to each other and to surfaces and in pathogens contribute to virulence. Fungal adhesins are typically heavily glycosylated. The impact of protein glycosylation on the function and regulation of adhesion glycoproteins is not clear. We examined the role of protein glycosylation on the adherence properties of the major adhesion molecule Muc1/Flo11 in the budding yeast Saccharomyces cerevisiae. Using a conditional mutant required for an early step in protein glycosylation, pmi40-101, we show that the glycosylation of Flo11 is required for invasive growth and biofilm/mat formation. Underglycosylated Flo11 was not defective in cell-surface localization or binding to wild-type cells in trans. However, wild-type Flo11 was defective for binding to the surface of cells undergoing a glycosylation stress. Shed Flo11 and other shed glycoproteins (Msb2 and Hkr1) were extremely stable with half-lives on the order of days. The glycosylation of Flo11 contributed to its stability. Moreover, the overall balance between Flo11 production, shedding, and turnover favored accumulation of the shed protein over time. Our findings may be applicable to fungal adhesion molecules in other species including pathogens.

New features of invasive candidiasis in humans: amyloid formation by fungi and deposition of serum amyloid P component by the host

BACKGROUND

Invasive candidiasis occurs in the gastrointestinal tract, especially in neutropenic patients. We were interested in determining whether invasive fungi formed amyloid in humans as they are known to do in vitro. We also sought to characterize the consequence(s) of such amyloid formation.

METHODS

Tissue from 25 autopsy patients with invasive candidiasis of the gastrointestinal tract was stained with amyloidophilic dyes and for the presence of serum amyloid P component (SAP). Confirmation of the interaction of SAP and Candida was demonstrated using Candida albicans and mutants for amyloid formation.

RESULTS

Amyloid was present on the cellular surface of fungi invading gut tissue. Moreover, SAP bound to the fungal cell walls, confirming the presence of amyloid. In vitro observations showed SAP bound avidly to fungi when amyloid formed in fungal cell walls. An unexpected result was the lack of host neutrophils in response to the invading fungi, not only in neutropenic patients but also in patients with normal or increased white blood counts.

CONCLUSIONS

We report the first demonstration of functional fungal amyloid in human tissue and the binding of SAP to invading fungi. It is postulated that fungal amyloid, SAP, or a complex of the proteins may inhibit the neutrophil response.

Human pathogens utilize host extracellular matrix proteins laminin and collagen for adhesion and invasion of the host

Laminin (Ln) and collagen are multifunctional glycoproteins that play an important role in cellular morphogenesis, cell signalling, tissue repair and cell migration. These proteins are ubiquitously present in tissues as a part of the basement membrane (BM), constitute a protective layer around blood capillaries and are included in the extracellular matrix (ECM). As a component of BMs, both Lns and collagen(s), thus function as major mechanical containment molecules that protect tissues from pathogens. Invasive pathogens breach the basal lamina and degrade ECM proteins of interstitial spaces and connective tissues using various ECM-degrading proteases or surface-bound plasminogen and matrix metalloproteinases recruited from the host. Most pathogens associated with the respiratory, gastrointestinal, or urogenital tracts, as well as with the central nervous system or the skin, have the capacity to bind and degrade Lns and collagen(s) in order to adhere to and invade host tissues. In this review, we focus on the adaptability of various pathogens to utilize these ECM proteins as enhancers for adhesion to host tissues or as a targets for degradation in order to breach the cellular barriers. The major pathogens discussed are Streptococcus, Staphylococcus, Pseudomonas, Salmonella, Yersinia, Treponema, Mycobacterium, Clostridium, Listeria, Porphyromonas and Haemophilus; Candida, Aspergillus, Pneumocystis, Cryptococcus and Coccidioides; Acanthamoeba, Trypanosoma and Trichomonas; retrovirus and papilloma virus.

LL37 and hBD-3 elevate the β-1,3-exoglucanase activity of Candida albicans Xog1p, resulting in reduced fungal adhesion to plastic

The opportunistic fungus Candida albicans causes oral thrush and vaginal candidiasis, as well as candidaemia in immunocompromised patients including those undergoing cancer chemotherapy, organ transplant and those with AIDS. We previously found that the AMPs (antimicrobial peptides) LL37 and hBD-3 (human β-defensin-3) inhibited C. albicans viability and its adhesion to plastic. For the present study, the mechanism by which LL37 and hBD-3 reduced C. albicans adhesion was investigated. After AMP treatment, C. albicans adhesion to plastic was reduced by up to ~60% and was dose-dependent. Our previous study indicated that LL37 might interact with the cell-wall β-1,3-exoglucanase Xog1p, which is involved in cell-wall β-glucan metabolism, and consequently the binding of LL37 or hBD-3 to Xog1p might cause the decrease in adhesion. For the present study, Xog1p(41-438)-6H, an N-terminally truncated, active, recombinant construct of Xog1p and Xog1p fragments were produced and used in pull-down assays and ELISA in vitro, which demonstrated that all constructs interacted with both AMPs. Enzymatic analyses showed that LL37 and hBD-3 enhanced the β-1,3-exoglucanase activity of Xog1p(41-438)-6H approximately 2-fold. Therefore elevated Xog1p activity might compromise cell-wall integrity and decrease C. albicans adhesion. To test this hypothesis, C. albicans was treated with 1.3 μM Xog1p(41-438)-6H and C. albicans adhesion to plastic decreased 47.7%. Taken together, the evidence suggests that Xog1p is one of the LL37/hBD-3 targets, and elevated β-1,3-exoglucanase activity reduces C. albicans adhesion to plastic.

Strengthening relationships: amyloids create adhesion nanodomains in yeasts

Budding yeasts adhere to biotic or abiotic surfaces and aggregate to form biofilms, using wall-anchored glycoprotein adhesins. The process is paradoxical: adhesins often show weak binding to specific ligands, yet mediate remarkably strong adherence. Single-molecule atomic force microscopy (AFM), genomics, biochemistry and cell biology have recently explained the puzzle, with Candida albicans Als adhesins as the paradigm. The strength of adhesion results partly from force-activated amyloid-like clustering of hundreds of adhesin molecules to form arrays of ordered multimeric binding sites. The various protein domains of eukaryotic adhesins cooperate to facilitate this fascinating new mechanism of activation.

Structural basis for the broad specificity to host-cell ligands by the pathogenic fungus Candida albicans

Candida albicans is the most prevalent fungal pathogen in humans and a major source of life-threatening nosocomial infections. The Als (agglutinin-like sequence) glycoproteins are an important virulence factor for this fungus and have been associated with binding of host-cell surface proteins and small peptides of random sequence, the formation of biofilms and amyloid fibers. High-resolution structures of N-terminal Als adhesins (NT-Als; up to 314 amino acids) show that ligand recognition relies on a motif capable of binding flexible C termini of peptides in extended conformation. Central to this mechanism is an invariant lysine that recognizes the C-terminal carboxylate of ligands at the end of a deep-binding cavity. In addition to several protein-peptide interactions, a network of water molecules runs parallel to one side of the ligand and contributes to the recognition of diverse peptide sequences. These data establish NT-Als adhesins as a separate family of peptide-binding proteins and an unexpected adhesion system for primary, widespread protein-protein interactions at the Candida/host-cell interface.

Expression, crystallization and preliminary X-ray data analysis of NT-Als9-2, a fungal adhesin from Candida albicans

Candida albicans is a common human fungal commensal that can also cause a range of infections from skin/mucosal `thrush' to severe systemic candidiasis. Adherence to host cells is one of the key determinants of Candida pathogenesis. The Als family of surface proteins has been implicated in adhesion of C. albicans, yet limited information has been published on the structure and mechanism of these fungal adhesins. The N-terminal region of these proteins has been shown to possess adhesive properties, making it a possible target for new therapeutic strategies. Recombinant NT-Als9-2 from C. albicans (residues 18-329) was overexpressed in Escherichia coli, purified and crystallized. Diffraction data were collected to 2.0 Å resolution. The crystals belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 34.73, b = 68.71, c = 120.03 Å, α = β = γ = 90° and one molecule in the asymmetric unit. Platinum-derivatized crystals belonged to the same space group, with similar unit-cell parameters, although they were not completely isomorphous.

A role for amyloid in cell aggregation and biofilm formation

Cell adhesion molecules in Saccharomyces cerevisiae and Candida albicans contain amyloid-forming sequences that are highly conserved. We have now used site-specific mutagenesis and specific peptide perturbants to explore amyloid-dependent activity in the Candida albicans adhesin Als5p. A V326N substitution in the amyloid-forming region conserved secondary structure and ligand binding, but abrogated formation of amyloid fibrils in soluble Als5p and reduced cell surface thioflavin T fluorescence. When displayed on the cell surface, Als5p with this substitution prevented formation of adhesion nanodomains and formation of large cellular aggregates and model biofilms. In addition, amyloid nanodomains were regulated by exogenous peptides. An amyloid-forming homologous peptide rescued aggregation and biofilm activity of Als5p^V326N cells, and V326N substitution peptide inhibited aggregation and biofilm activity in Als5p^WT cells. Therefore, specific site mutation, inhibition by anti-amyloid peturbants, and sequence-specificity of pro-amyloid and anti-amyloid peptides showed that amyloid formation is essential for nanodomain formation and activation.

Candida albicans Als3, a multifunctional adhesin and invasin

Candida albicans is part of the normal human flora, and it grows on mucosal surfaces in healthy individuals. In susceptible hosts, this organism can cause both mucosal and hematogenously disseminated disease. For C. albicans to persist in the host and induce disease, it must be able to adhere to biotic and abiotic surfaces, invade host cells, and obtain iron. The C. albicans hypha-specific surface protein Als3 is a member of the agglutinin-like sequence (Als) family of proteins and is important in all of these processes. Functioning as an adhesin, Als3 mediates attachment to epithelial cells, endothelial cells, and extracellular matrix proteins. It also plays an important role in biofilm formation on prosthetic surfaces, both alone and in mixed infection with Streptococcus gordonii. Als3 is one of two known C. albicans invasins. It binds to host cell receptors such as E-cadherin and N-cadherin and thereby induces host cells to endocytose the organism. Als3 also binds to host cell ferritin and enables C. albicans to utilize this protein as a source of iron. Because of its multiple functions and its high expression level in vivo, Als3 is a promising target for vaccines that induce protective cell-mediated and antibody responses. This review will summarize the multiple functions of this interesting and multifunctional protein.