Re-expression of 117 antigen, a cell surface glycoprotein of aggregating cells, during terminal differentiation of Dictyostelium discoideum prespore cells

Comparative analysis of spore coat formation, structure, and function in Dictyostelium

Dictyostelium produces spores at the end of its developmental cycle to propagate the lineage. The spore coat is an essential feature of spore biology contributing a semipermeable chemical and physical barrier to protect the enclosed amoeba. The coat is assembled from secreted proteins and a polysaccharide, and from cellulose produced at the cell surface. They are organized into a polarized molecular sandwich with proteins forming layers surrounding the microfibrillar cellulose core. Genetic and biochemical studies are beginning to provide insight into how the deliveries of protein and cellulose to the cell surface are coordinated and how cysteine-rich domains of the proteins interact to form the layers. A multidomain inner layer protein, SP85/PsB, seems to have a central role in regulating coat assembly and contributing to a core structural module that bridges proteins to cellulose. Coat formation and structure have many parallels in walls from plant, algal, yeast, protist, and animal cells.

The carboxyl terminus of Dictyostelium discoideum protein 1I encodes a functional glycosyl-phosphatidylinositol signal sequence

The 1I gene is expressed in the prespore cells of culminating Dictyostelium discoideum. The open reading frame of 1I cDNA encodes a protein of 155 amino acids with hydrophobic segments at both its NH(2)- and COOH-termini that are indicative of a glycosyl-phosphatidylinositol (GPI)-anchored protein. A hexaHis-tagged form of 1I expressed in D. discoideum cells appeared on Western blot analysis as a doublet of 27 and 24 kDa, with a minor polypeptide of 22 kDa. None of the polypeptides were released from the cell surface with bacterial phosphatidylinositol-specific phospholipase C, although all three were released upon nitrous acid treatment, indicating the presence of a phospholipase-resistant GPI anchor. Further evidence for the C-terminal sequence of 1I acting as a GPI attachment signal was obtained by replacing the GPI anchor signal sequence of porcine membrane dipeptidase with that from 1I. Two constructs of dipeptidase with the 1I GPI signal sequence were constructed, one of which included an additional six amino acids in the hydrophilic spacer. Both of the resultant constructs were targeted to the surface of COS cells and were GPI-anchored as shown by digestion with phospholipase C, indicating that the Dictyostelium GPI signal sequence is functional in mammalian cells. Site-specific antibodies recognising epitopes either side of the expected GPI anchor attachment site were used to determine the site of GPI anchor attachment in the constructs. These parallel approaches show that the C-terminal signal sequence of 1I can direct the addition of a GPI anchor.

Calcium uptake and gp80 messenger RNA destabilization follows cAMP receptor down regulation in Dictyostelium discoideum

The mechanism by which high concentrations of cAMP selectively destabilize the gp80 mRNA in Dictyostelium discoideum was investigated. This treatment which leads to down-regulation of the cAMP receptor was also found to cause an increase in calcium uptake. Given this observation, we sought a role for calcium as a second messenger in the degradation of the gp80 mRNA. Changes in the mRNA levels were examined after treating cells with compounds known to alter their intracellular Ca2+ concentrations. This included the use of A23187, Ca2+, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate HC1 (TMB-8), LiCl and 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate (ClPhS-Ado-3':5'-P). The sum of the data suggest that it is the cAMP-induced influx of Ca2+ across the plasma membrane, as apposed to a cAMP-mediated release of Ca2+ from intracellular stores, that initiates gp80 mRNA degradation. Treatment of cells with Concanavalin A (ConA) to induce cAMP receptor down-regulation, also causes a reduction in gp80 mRNA levels and an increase in calcium uptake.

The PsB glycoprotein complex is secreted as a preassembled precursor of the spore coat in Dictyostelium discoideum

The PsB glycoprotein in Dictyostelium discoideum is one of a diverse group of developmentally regulated, prespore-cell-specific proteins, that contain a common O-linked oligosaccharide. This post-translational modification is dependent on the wild-type modB allele. The PsB protein exists as part of a multiprotein complex of six different proteins, which have different post-translational modifications and are held together by both covalent and non-covalent interactions (Watson et al. (1993). J. Biol. Chem. 268, 22634–22641). In this study we have used microscopic and biochemical analyses to examine the cellular localization and function of the PsB complex during development. We found that the PsB complex first accumulates in prespore vesicles in slug cells and is secreted later during culmination and becomes localized to both the extracellular matrix of the apical spore mass of mature fruiting bodies and to the inner layer of the spore coat. The PsB associated with the spore coat is covalently bound by disulfide bridges. The PsB protein always exists in a multiprotein complex, but the composition of the PsB complex changes during secretion and spore maturation. Some of the PsB complex proteins have been identified as spore coat proteins. These data demonstrate that some of the proteins that form the spore coat exist as a preassembled precursor complex. The PsB complex is secreted in a developmentally regulated manner during the process of spore differentiation, at which time proteins of the complex, as well as additional spore coat proteins, become covalently associated in at least two forms of extracellular matrix: the interspore matrix and the spore coat. These and other studies show that proteins with modB dependent O-linked oligosaccharides are involved in a wide variety of processes underlying morphogenesis in this organism. These developmental processes are the direct result of cellular mechanisms regulating protein targeting, assembly and secretion, and the assembly of specific extracellular matrices.

A monoclonal antibody that interferes with the post-aggregation adhesion of Dictyostelium discoideum cells

A monoclonal antibody that interferes with the EDTA-resistant adhesion of Dictyostelium discoideum slug cells recognised a carbohydrate epitope on four major antigens (95, 90, 35 and 30 kDa) in slug cells. The 35 and 30 kDa antigens were specific for stalks and spores, respectively. The 30 kDa antigen was identified as the cell surface glycoprotein, PsA. Cyclic AMP, acting via cell surface receptors, induced only the 90 kDa slug cell antigen. Slug cell adhesion proteins may be involved in cell-sorting and the glycosylation of the 95 and 90 kDa antigens appeared to be abnormal in a mutant defective in cell-sorting. Previously, a 150 kDa glycoprotein has been strongly implicated in slug cell adhesion and the present work suggests that additional glycoprotein(s) are involved.

Post-translational modifications of the Dictyostelium discoideum glycoprotein PsA. Glycosylphosphatidylinositol membrane anchor and composition of O-linked oligosaccharides

Prespore-specific antigen (PsA) is a cell-surface glycoprotein isolated from Dictyostelium discoideum, which is post-translationally modified by addition of carbohydrate to threonine residues of the carboxy-terminal peptide domain, and a glycosylphosphatidylinositol (GPI) anchor which attaches the glycoprotein to the cell membrane. The GPI anchor was isolated by proteolytic cleavage of the protein, and the structure of the lipid and glycan portions of the anchor were determined. The lipid moiety of the anchor is an inositolphosphoceramide which contains C18:0 phytosphingosine as a long chain base, and a mixture of fatty acids with a C18:1 mono-unsaturated fatty acid as the major component. The purified GPI anchor was susceptible to digestion by a bacterial phosphatidylinositol-specific phospholipase-C enzyme. The glycan of the GPI anchor consisted of two molecular species present in the ratio 55:45, the structures of which were determined by exoglycosidase sequencing and found to be Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcNH2 and Man alpha 1-2Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcNH2. The glucosamine in both structures is glycosidically linked to the inositol ring of the inositolphosphoceramide. The GPI glycan structures are consistent with the conserved core structure of all characterised GPI anchors, and the structure of the D. discoideum GPI moiety has features in common with structures from yeast, protozoa and higher eukaryotes. Compositional analysis of the carbohydrate attached to threonine residues in the carboxy-terminal peptide domain is also presented. The oligosaccharides bind to wheat germ agglutinin, and contain glucosamine and fucose as the major constituents.

Pure populations of Dictyostelium discoideum prespore and prestalk cells obtained by flow cytometry have different redevelopment characteristics at their cell surfaces

The multicellular slug stage of Dictyostelium discoideum consists of two major differentiated cell types: prespore and prestalk cells, which become, respectively, the spores and the stalk of the fruiting body. It is known that these cells, although expressing cell-type-specific proteins, remain totipotent, and experimental disruption of slugs results in redifferentiation taking place. We looked at what happens to cell-type-specific surface molecules when a cell changes from one type of another. Using monoclonal antibodies and flow cytometry we were able for the first time to obtain pure populations of single cells of each cell type. These were analysed during redevelopment. The initial hypothesis was that a proportion of each cell type would redifferentiate to reestablish the original proportions. However, it was found that the two cell types responded quite differently. Whereas almost all prestalk cells retained their prestalk surface antigen, in contrast, all prespore cells redifferentiated. During this process redifferentiating prespore cells simultaneously expressed surface determinants of both cell types, an event not seen in normal development.

Cell adhesion in transformed D. discoideum cells: expression of gp80 and its biochemical characterization

Dictyostelium discoideum amoebae were transformed with an expression vector for the gp80, a protein believed to mediate EDTA-resistant cell adhesion in developmental cells. Vegetative cells, that do not normally contain gp80, expressed the protein and this expression was correlated with the formation of cell-cell adhesions. These contacts exhibited minimal EDTA-resistance. Biochemical analyses of the protein synthesized by vegetative cells suggested that it is identical to that produced by aggregation-competent cells, including the presence of a glycolipid anchor. Additional experiments indicated that the anchor was insensitive to hydrolysis by exogenous (glycosly)phosphatidylinositol-specific phospholipase C [G)PI-PLCs) but was sensitive to the endogenous anchor degrading enzyme. This enzyme, initially described in aggregating cells (da Silva and Klein, Exp. Cell Res., in press) was found to be present also in vegetative amoebae.

Developmental expression and characterization of the gene encoding spore coat protein SP60 in Dictyostelium discoideum

The complete coding sequence, upstream sequence and developmental expression of Dictyostelium discoideum AX2 spore coat protein gene SP60 is reported. The gene contains two exons, 154bp and 1121bp long, separated by a 119bp intron, and encodes a protein of 46,925 molecular weight plus a 23-amino-acid hydrophobic leader sequence. The N-terminus of the mature protein consists of four copies of a perfect hexapeptide repeat (GDWNNN). The central region is rich in cysteine residues, including four highly conserved cysteine-rich repeats with homology to 'EGF-like' repeats. The C-terminus is aspartate-rich and composed of multiple imperfect copies of a D(G/D)DYD repeat followed by several repeats of the tetrapeptide DNDW and derived sequences. A TATA box promoter motif juxtaposed to an oligo(dA) stretch lies 52bp upstream of the main transcriptional start site of the gene. Six AC-rich boxes occur in the region -327 to -556, all of which contain the consensus sequence CACAC. Two GC-rich boxes and a C-rich element (TTACCCCA) are also present upstream. Another open reading frame is positioned a short distance downstream of the SP60 gene in the opposite transcriptional orientation. Expression of the SP60 gene ceases upon disaggregation to single cells and cannot be restored by high levels of extracellular cAMP either alone or in combination with conditioned medium factors.

Formation of the Dictyostelium spore coat

The spore coat forms as a rigid extracellular wall around each spore cell during culmination. Coats purified from germinated spores contain multiple protein species and an approximately equal mass of polysaccharide, consisting mostly of cellulose and a galactose/N-acetylgalactosamine polysaccharide (GPS). All but the cellulose are prepackaged during prespore cell differentiation in a regulated secretory compartment, the prespore vesicle. The morphology of this compartment resembles an anastomosing, tubular network rather than a spherical vesicle. The molecules of the prespore vesicles are not uniformly mixed but are segregated into partially overlapping domains. Although lysosomal enzymes have been found in the prespore vesicle, this compartment does not function as a lysosome because it is not acidic, and a common antigen associated with acid hydrolases is found in another, acidic vesicle population. All the prespore vesicle profiles disappear at the time of appearance of their contents outside of the cell; this constitutes an early stage in spore coat formation, which can be detected both by microscopy and flow cytometry. As an electron-dense layer, the future outer layer of the coat, condenses, cellulose can be found and is located immediately beneath this outer layer. Certain proteins and the GPS become associated with either the outer or inner layers surrounding this middle cellulose layer. Assembly of the inner and outer layers occurs in part from a pool of glycoproteins that is shared between spores, and unincorporated molecules loosely reside in the interspore matrix, a location from which they can be easily washed away. When the glycosylation of several major protein species is disrupted by mutation, the coat is assembled, but differences are found in its porosity and the extractibility of certain proteins. In addition, the retention or loss of proteolytic fragments in the mutants indicates regions of spore coat proteins that are required for association with the coat. Comparative examination of the macrocyst demonstrates that patterns of molecular distributions are not conserved between the macrocyst and spore coats. Thus spore coat assembly is characterized by highly specific intermolecular interactions, leading to saturable associations of individual glycoproteins with specific layers and the exclusion of excess copies to the interspore space.