Cell adhesion proteins in the nonvertebrate eukaryotes

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.

Conserved WCPL and CX4C domains mediate several mating adhesin interactions in Saccharomyces cerevisiae

Several adhesins are induced by pheromones during mating in Saccharomyces cerevisiae, including Aga1p, Aga2p, Sag1p (Agalpha1p), and Fig2p. These four proteins all participate in or influence a well-studied agglutinin interaction mediated by Aga1p-Aga2p complexes and Sag1p; however, they also play redundant and essential roles in mating via an unknown mechanism. Aga1p and Fig2p both contain repeated, conserved WCPL and CX(4)C domains. This study was directed toward understanding the mechanism underlying the collective requirement of agglutinins and Fig2p for mating. Apart from the well-known agglutinin interaction between Aga2p and Sag1p, three more pairs of interactions in cells of opposite mating type were revealed by this study, including bilateral heterotypic interactions between Aga1p and Fig2p and a homotypic interaction between Fig2p and Fig2p. These four pairs of adhesin interactions are collectively required for maximum mating efficiency and normal zygote morphogenesis. GPI-less, epitope-tagged forms of Aga1p and Fig2p can be co-immunoprecipitated from the culture medium of mating cells in a manner dependent on the WCPL and CX(4)C domains in the R1 repeat of Aga1p. Using site-directed mutagenesis, the conserved residues in Aga1p that interact with Fig2p were identified. Aga1p is involved in two distinct adhesive functions that are independent of each other, which raises the possibility for combinatorial interactions of this protein with its different adhesion receptors, Sag1 and Fig2p, a property of many higher eukaryotic adhesins.

Interaction of alpha-agglutinin and a-agglutinin, Saccharomyces cerevisiae sexual cell adhesion molecules

alpha-Agglutinin and a-agglutinin are complementary cell adhesion glycoproteins active during mating in the yeast Saccharomyces cerevisiae. They bind with high affinity and high specificity: cells of opposite mating types are irreversibly bound by a few pairs of agglutinins. Equilibrium and surface plasmon resonance kinetic analyses showed that the purified binding region of alpha-agglutinin interacted similarly with purified a-agglutinin and with a-agglutinin expressed on cell surfaces. At 20 degrees C, the K(D) for the interaction was 2 x 10(-9) to 5 x 10(-9) M. This high affinity was a result of a very low dissociation rate ( approximately 2.6 x 10(-4) s(-1)) coupled with a low association rate (= 5 x 10(4) M(-1) s(-1)). Circular-dichroism spectroscopy showed that binding of the proteins was accompanied by measurable changes in secondary structure. Furthermore, when binding was assessed at 10 degrees C, the association kinetics were sigmoidal, with a very low initial rate. An induced-fit model of binding with substantial apposition of hydrophobic surfaces on the two ligands can explain the observed affinity, kinetics, and specificity and the conformational effects of the binding reaction.

Receptor protein-tyrosine phosphatases: origin of domains (catalytic domain, Ig-related domain, fibronectin type III module) based on the sequence of the sponge Geodia cydonium

Reversible tyrosine phosphorylation of proteins is one of the major regulatory physiological events in response to cell-cell- and cell-matrix contact in Metazoa. Previously it was documented that the tyrosine phosphorylating enzymes, the tyrosine kinases (TKs), are autapomorphic characters of Metazoa, including sponges. In this paper the tyrosine dephosphorylating enzymes, the protein-tyrosine phosphatases (PTPs), are studied which can be grouped into two subfamilies, the soluble PTPs and the receptor PTPs (RPTPs). PTPs are characterized by one PTPase domain which interestingly comprises sequence similarity to yeast PTPs. In contrast to the PTPs, the RPTPs - which have been found only in Metazoa - are provided with two PTPase domains. To study the evolution of the RPTPs the full-length size RPTP was cloned from the marine demosponge Geodia cydonium, the phylogenetic oldest metazoan taxon. The 3253 bp long sequence has a putative open reading frame coding for a 999 aa long RPTP which is characterized by two fibronectin (type III; FN-III) domains in the extracellular portion, one intracellular immunoglobulin (Ig)-related domain, and two PTPase domains. Phylogenetic analysis revealed that the sponge FN-III domains form the basis of the metazoan FN-III domain with the common metazoan ancestor. The Ig-related, typical metazoan, module is classified to the disulphide lacking Ig members and represents the phylogenetic earliest member of this group. The beta-sheet propensity was calculated and the characteristic amino acids are present in the seven beta-sheets. The analysis of the two PTPase domains of the sponge RPTP demonstrates that the first domain is closely related to the PTPase domains present in the soluble PTPs, while the second PTPase domain is only distantly related to them. By constructing a rooted phylogenetic cladogram it became overt that the duplication of the PTPase domains must have occurred already in yeast. This interesting finding indicates that two conserved PTPase domains originated from a common ancestor in yeast while the evolutionary novelties, the FN-III domains and the Ig-related module, were added during the transition to the Metazoa. Hence, the tyrosine dephosphorylating enzyme, RPTP, is an example for a modular protein which is composed of ancient modules (PTPase domain[s]) and two metazoan novelties, while the tyrosine phosphorylating enzymes, the TKs, evolved only in Metazoa.

Monoclonal antibody MS13 identifies a plasmatocyte membrane protein and inhibits encapsulation and spreading reactions of Manduca sexta hemocytes

Lepidopterans generally can successfully defend themselves against a variety of parasites or parasitoids. One mechanism they use is to encapsulate the invader in many layers of hemocytes. For encapsulation to occur, the hemocytes must attach to the foreign material, spread, and adhere to each other. The molecules that mediate these processes are not known. One method to identify proteins potentially necessary for adhesion, spreading, and, thus, encapsulation is to use monoclonal antibodies that interfere with these functions. In this paper, we report that a monoclonal antibody against Manduca sexta plasmatocytes effectively inhibited encapsulation of synthetic beads in vitro and in vivo. Furthermore, it inhibited plasmatocyte spreading in vitro. Other anti-hemocyte antibodies did not have these effects. The plasmatocyte-specific monoclonal antibody, mAb MS13, recognized a protein of approximately 90,000 daltons as indicated by Western blot analysis of hemocyte lysate proteins. The epitope recognized by mAb MS13 was present on the exterior surface of plasmatocytes. Using indirect immunohistochemistry with hemocyte-specific antibodies, we also determined that during encapsulation plasmatocytes were the first cells bound to latex beads and later layers consisted of both plasmatocytes and granular cells. Arch.

Towards an understanding of the molecular basis of immune responses in sponges: the marine demosponge Geodia cydonium as a model

The phylogenetic position of the phylum Porifera (sponges) is near the base of the kingdom Metazoa. During the last few years, not only rRNA sequences but, more importantly, cDNA/genes that code for proteins have been isolated and characterized from sponges, in particular from the marine demosponge Geodia cydonium. The analysis of the deduced amino acid sequences of these proteins allowed a molecular biological approach to the question of the monophyly of the Metazoa. Molecules of the extracellular matrix/basal lamina, with the integrin receptor, fibronectin, and galectin as prominent examples, and of cell-surface receptors (tyrosine kinase receptor), elements of sensory systems (crystallin, metabotropic glutamate receptor) as well as homologs/modules of an immune system (immunoglobulin-like molecules, scavenger receptor cysteine-rich [SRCR]- and short consensus repeats [SCR]-repeats), classify the Porifera as true Metazoa. As living fossils, provided with simple, primordial molecules allowing cell-cell and cell-matrix adhesion as well as processes of signal transduction as known in a more complex manner from higher Metazoa, sponges also show pecularities not known in later phyla. In this paper, the adhesion molecules presumably involved in the sponge immune system are reviewed; these are the basic adhesion molecules (galectin, integrin, fibronectin, and collagen) and especially the highly polymorphic adhesion molecules, the receptor tyrosine kinase as well as the polypeptides comprising scavenger receptor cysteine-rich (SRCR) and short consensus repeats (SCR) modules. In addition, it is reported that in the model sponge system of G. cydonium, allogeneic rejection involves an upregulation of phenylalanine hydroxylase, an enzyme initiating the pathway to melanin synthesis.

Accumulation in marine sponge grafts of the mRNA encoding the main proteins of the cell adhesion system

Specific cell adhesion in the marine sponge Microciona prolifera is mediated by an extracellular aggregation factor complex, whose main protein component, termed MAFp3, is highly polymorphic. We have now identified MAFp4, an approximately 400-kDa protein, from the aggregation factor that is translated from the same mRNA as MAFp3. The existence of multiple potential sites for N-glycosylation and calcium binding suggests a direct involvement of MAFp4 in the species-specific aggregation of sponge cells. The deduced partial polypeptide consists of a 16-fold reiterated motif that shows significant similarity to a repeat in an endoglucanase from the symbiontic bacterium Azorhizobium caulinodans and to the intracellular loop of mammalian Na+-Ca2+ exchangers. Restriction fragment length polymorphism analysis indicated that the genomic variability of MAFp4 is high and comparable to that of MAFp3. Their combined polymorphism correlates with allogeneic responses studied in a population of 23 sponge individuals. Peptide mass fingerprinting of tryptic digests of the polymorphic MAFp3 bands observed on polyacrylamide gels after chemical deglycosylation of the Microciona aggregation factor revealed that the variability detected on Southern blots at least partially reflects the individual variability of aggregation factor protein components. Polyclonal antibodies raised against MAFp3 strongly cross-reacted with a 68-kDa protein localized in sponge cell membranes. Immunohistochemical use of the anti-MAFp3 antibodies strongly stained a cell layer along the line of contact in allogeneic grafts. We show that the transcription level of the MAFp3/MAFp4 mRNA in sponge allo- and isografts is clearly increased in comparison with non-grafted tissue. These data are discussed with respect to a possible evolutionary relationship between cell adhesion and histocompatibility systems.

The putative sponge aggregation receptor. Isolation and characterization of a molecule composed of scavenger receptor cysteine-rich domains and short consensus repeats

Porifera (sponges) are the oldest extant metazoan phylum. Dissociated sponge cells serve as a classic system to study processes of cell reaggregation. The reaggregation of dissociated cells is mediated by an extracellularly localized aggregation factor (AF), based on heterophilic interactions of the third order; the AF bridges two cells by ligating a cell-surface-bound aggregation receptor (AR). In the present study we report cloning, expression and immunohistochemical localization of a polypeptide from the marine sponge Geodia cydonium, which very likely represents the AR. The presumed AR gene gives rise to at least three forms of alternatively spliced transcripts of 6.5, 4.9 and 3.9 kb, as detected by northern blotting. Two cDNA clones corresponding to the shorter forms were already reported earlier; here we present an analysis of the largest. All three putative polypeptides feature scavenger receptor cysteine-rich (SRCR) domains. The largest form, SRCR-SCR-Car, is a cell-surface receptor of molecular mass 220 kDa, which is assumed to be the cell-adhesion receptor AR; the second form, SRCR-Re, is also a putative receptor of 166 kDa, while the third form, SRCR-Mo, is a soluble molecule of 129 kDa. The SRCR-SCR-Car molecule consists of fourteen SRCR domains, six short consensus repeats (SCRs), a C-terminal transmembrane domain and a cytoplasmic tail; its fourteenth SRCR domain features an Arg-Gly-Asp tripeptide. To obtain monoclonal antibodies, a 170-amino-acid-long polypeptide that is found in all three forms of the SRCR-containing proteins was expressed in E. coli. In a western blot of sponge cells lysate the monoclonal antibody raised against the recombinant polypeptide recognized two major immuno-reacting polypeptides (220 and 117 kDa) and two minor bands (36 and 32 kDa). The antibody was found to react with antigen(s) predominantly localized on the plasma membranes of cells, especially those of spherulous cells. In a functional assay Fab' fragments of the antibodies suppressed AF-mediated cell-cell reaggregation. Additionally, a recombinant SRCR-soluble fragment effectively inhibited AF-mediated cell-cell reaggregation. We conclude that the 220 kDa SRCR-containing protein of the sponge G. cydonium is very likely the AR.

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

Adherence of Candida albicans to host tissues is a necessary step for maintenance of its commensal status and is likely a necessary step in the pathogenesis of candidiasis. The extracellular matrix (ECM) proteins are some of the host tissue and plasma proteins to which C. albicans adheres through adhesins located on the fungal cell surface. To isolate genes encoding ECM adhesins, an assay was developed based on the ability of yeast cells to adhere to magnetic beads coated with the ECM protein fibronectin, type IV collagen, or laminin. A C. albicans genomic library was constructed by cloning XbaI-partially-digested and size-selected fragments into pAUR112, an Escherichia coli-yeast low-copy-number shuttle vector. The C. albicans library was transformed into Saccharomyces cerevisiae YPH 499, and clones capable of adherence were selected by using ECM protein-coated magnetic beads. A plasmid containing an approximately 8-kb insert was isolated from 29 adherent clones. These clones exhibited adherence to all ECM protein-coated magnetic beads and to human buccal epithelial cells. The ALA1 gene (for agglutinin-like adhesin) was localized by subcloning it into a 5-kb XbaI fragment which retained the adherence phenotype in both orientations. The complete DNA sequence of the 5-kb insert was determined, and an open reading frame (ORF) encoding 1,419 amino acid residues was identified. Deletions from the 5' and 3' ends extending into the DNA sequence encoding the 1,419-amino-acid ORF product inactivated the adherence phenotype, suggesting that it is the coding region of the ALA1 gene. A database search identified ALA1 to be similar to the C. albicans ALS1 (for agglutinin-like sequence 1) protein and the S. cerevisiae agglutinin protein (AG alpha1), although the homology at the primary amino acid sequence level is limited to the first half of each of these proteins. ALA1 contains a central domain of six tandem repeats of 36 amino acids. We discuss the significance of various predicted ALA1 structural motifs and their relationships to function in the adherence process.

A novel member of an ancient superfamily: sponge (Geodia cydonium, Porifera) putative protein that features scavenger receptor cysteine-rich repeats

Proteins featuring scavenger receptor cysteine-rich (SRCR) domains are prominent receptors known from vertebrates and from one phylum of invertebrates, the echinoderms. In the present study we report the first putative SRCR protein from the marine sponge Geodia cydonium (Porifera), a member of the lowest phylum of contemporary Metazoans. Two forms of SRCR molecules were characterized, which apparently represent alternative splicing of the same transcript. The long putative SRCR protein, of 1536 aa, features twelve SRCR repeats, a C-terminal transmembrane domain and a cytoplasmic tail. The sequence of the short form is identical with the long form except that it lacks a coding region near the C terminus, thus the 1195 aa deduced protein consists of only the first ten SRCR domains and the last 26 C-terminal aa residues, without the transmembrane domain. Homology searches revealed that the sponge putative SRCR protein shares with bovine T-cell antigen WC1 29.2% identity in 1054 aa overlap, 33.9% identity in 475 aa overlap with sea urchin speract and 56% identity in 110 aa overlap with macrophage scavenger receptor type I. Based upon the number and location of the conserved Cys residues, the sponge SRCR domain repeats were classified as belonging to group A of the SRCR superfamily. With twelve SRCR repeats, one more than those in any of the previously described SRCR proteins, and several membrane-bound and soluble forms, it seems that the most primitive known member of this family may be the structurally most complex one among SRCR containing proteins.

Polymorphism in the immunoglobulin-like domains of the receptor tyrosine kinase from the sponge Geodia cydonium

Sponges [Porifera] are the phylogenetically oldest phylum of the Metazoa. They are provided with both cellular and humoral allorecognition systems. The underlying molecules are not yet known. To study allorecognition in sponges we first determined the frequency of graft rejection in a natural population of the marine sponge Geodia cydonium. We then determined, for the first time at the molecular level, the degree of sequence polymorphism in segments of one molecule which may be related to sponge allorecognition and host defense: the Ig-like domains from the receptor tyrosine kinase [RTK]. Thirty six pairs of auto- and allografts were assayed, either by parabiotic attachment or insertion of grafts. All of the autografts fused, while only two allografts fused and 34 pairs were incompatible. Rejection among the parabiotic allografts was characterized by the formation of a collagenous barrier, while the allografts that were inserted into the host underwent destruction. At the molecular level we first cloned to completion the 5'-end of sponge RTK, which displays a Pro-Ser-Thr-rich sequence; this is thought to act as a module of cell adhesion proteins. Then we analyzed RT-PCR products of amplification across the two Ig-like domains of RTK (about 500 bp), from two pairs of fusing sponges and one pair of rejecting sponges. High levels of polymorphism were recorded, including 18 nucleotide-substitution positions and a tri-nucleotide deletion, which translate into 13 polymorphic amino acid positions. Two of the six sponges were scored as heterozygotes. Among 9 informative polymorphic sites that were tested for linkage disequilibrium, 11 pairwise comparisons were found to be significant, implying the possibility of distinguishable alleles in this locus. To the best of our knowledge this is the first report of polymorphism in Ig-like domains of a receptor from invertebrates that may be associated with allorecognition. This data attests also that fusion in sponges is not confined to genetically identical individuals.