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.

Immunological evidence of a P-glycoprotein in the microorganism Dictyostelium

Starting from the observation of an efficient resistance to the vital dye Hoechst 33342, we postulated the presence of a P-glycoprotein in Dictyostelium discoïdeum cells. This hypothesis is supported by the present data obtained by immunoblotting, using the murine JSB-1 monoclonal antibody. Two components of 170 and 115 kDa were recognized in lysates and crude plasma membrane of Dictyostelium cells grown in the presence of Hoechst 33342. Control experiments conducted without Hoechst 33342 demonstrated in these cells a constitutive expression of the P-glycoprotein, which was compared to its anthracycline-induced expression in a resistant human leukemic cell line, K562.

The 80L5C4 epitope overlaps with the homophilic binding site of the cell adhesion molecule gp80 of Dictyostelium

The monoclonal antibody (mAb) 80L5C4 is a potent inhibitor of the cell adhesion molecule gp80 of Dictyostelium discoideum. To map the exact location of the epitope recognized by mAb 80L5C4, overlapping hexapeptides were synthesized on plastic pins and the binding p6 mAb 80L5C4 to these peptides was monitored by enzyme-linked immunosorbent assay. The 80L5C4 epitope is mapped to a single hexapeptide sequence GYKLNV, which shares five amino acid residues with the octapeptide sequence YKLNVNDS involved in gp80 homophilic binding. Analogue studies indicate that the hydrophobic residues within this sequence are crucial for antigen recognition.

Heterogeneity of microtubule organizing center components as revealed by monoclonal antibodies to mammalian centrosomes and to nucleus-associated bodies from dictyostelium

The molecular composition of two morphologically distinct microtubule-organizing centers (MTOCs) was compared by probing with monoclonal antibodies raised against (i) nucleus-associated bodies (NABs) isolated in a complex with nuclei from the cellular slime mold Dictyostelium discoideum and (ii) mammalian mitotic spindles isolated from Chinese hamster ovary (CHO) cells. The staining patterns observed by immunofluorescence microscopy in whole CHO cells and Dictyostelium amoebae showed that the distribution of thirteen MTOC antigens is heterogeneous. Not all antibodies recognized the MTOC in both interphase and mitosis. Most of the anti-MTOC antibodies cross-reacted with other cellular organelles such as nuclei, Golgi apparatus-like aggregates and cytoskeletal elements. Two antibodies, CHO3 and AX3, recognized phosphorylated epitopes present in both mammalian centrosomes and Dictyostelium NABs. On immunoblots, most of the antibodies showed multiple bands, often of high molecular weight, indicating that the antigenic determinants are shared among different molecules. One antibody inhibited the regrowth of microtubules onto centrosomes in vitro after addition of exogenous tubulin to detergent-lysed CHO cells on coverslips; this antibody binds to an antigen(s) that might be essential for the microtubule-nucleating activity of centrosomes. These observations demonstrate that molecular components in different MTOCs exhibit a variety of distinct subcellular localizations and functional properties, and that some antigenic molecules have been conserved among morphologically distinct MTOCs.

Immunodominant carbohydrate determinants in the multicellular stages of Dictyostelium discoideum

Two families of glycoprotein are defined in Dictyostelium discoideum by the presence of different glycoconjugates, both of which are highly immunogenic in mice. The previously described monoclonal antibodies MUD50 and MUD62 recognize the glycoconjugates and identify the respective glycoprotein families. Both types of glycosylation occur on vegetative and developmentally regulated glycoproteins. The immunodominant components of both families are reportedly O-linked sugars, but Western blots do not identify any glycoprotein that has both O-glycans, suggesting that there are two independently processed types of O-linked glycosylation in D. discoideum. The synthesis of the two O-glycan families is affected by glycosylation-defective mutations. Strains with a mutation at the modB locus lack one of these glycosylation types (that recognized by MUD50) and this mutation alters the size of two minor glycoproteins in the second family. Two new mutants, HU2470 (mod-352) and HU2471 (mod-353), lack the epitope recognized by MUD62. The two mutations map to different chromosomes. The mod-353 mutation also affects the size of PsA, a cell surface glycoprotein carrying the modB-dependent O-glycan.

Identification of a homologue for annexin VII (synexin) in Dictyostelium discoideum

Immunological and biochemical data have been used to show that the slime mold Dictyostelium discoideum expresses a Ca2+/phospholipid-binding protein related to vertebrate annexins. The Dictyostelium protein (apparent molecular mass 46 kDa) is recognized by an antibody directed against an annexin consensus peptide and exhibits the properties characteristic for annexins, i.e. it interacts in a Ca2(+)-dependent manner with negatively charged phospholipids. Limited proteolysis converts the 46-kDa protein into a 32-kDa derivative which retains the Ca2+/phospholipid-binding properties of the 46-kDa polypeptide. Partial protein sequence data identify the Dictyostelium protein as the typical annexin and indicate that the 46-kDa protein is an annexin VII (synexin) homologue. The identification of an annexin in a simple eucaryote should lead to the introduction of genetic approaches to analyze the physiological role of the annexins.

Activation of mechanical responses in leukocytes

Different kinds of leukocytes undergo cytoskeleton-dependent mechanical responses associated with their specific physiological functions. We have investigated cellular stiffening of several types of leukocytes using a method which measures the force resisting cellular indentation. We have found that lymphocytes stiffen in response to crosslinking cell surface antigens in a process associated with the much studied capping and patching processes. Further studies of myosin-deficient mutants of the ameba Dictyostelium discoideum suggest that this stiffening process results from a myosin dependent contractile process. Rat basophilic leukemia cells and pancreatic islet cells stiffen when triggered to secrete. The function of these cytoskeleton dependent processes is now unknown, but, at least in the islet cells, may be related to a regulation of the rate of secretion. Primary neutrophils stiffen in response to the chemotactic agent, fMet-Leu-Phe. This stiffening may be responsible for retention of these cells in the pulmonary microcirculation during response to inflammation. These observations pose the challenge of determining the structural basis, mechanism, and physiological function of each of these cellular responses.

A stalk-specific wheat germ agglutinin binding protein, wst34, in Dictyostelium discoideum can be detected with antiserum raised against Dictyostelium mucoroides stalk

A new stalk-specific wheat germ agglutinin (WGA) binding protein, wst34, has been identified in Dictyostelium discoideum and purified by the use of preparative sodium dodecyl sulfate - polyacrylamide gel electrophoresis and a WGA-affinity column. In normal development, wst34 appears during culmination and is maintained in stalk cells. It has a molecular mass of 34 kilodaltons and a pI value of 5.5-6.5. A polyclonal antiserum raised against stalk cell proteins of Dictyostelium mucoroides recognizes wst34 in western blots of D. discoideum proteins.

An anticarbohydrate monoclonal antibody inhibits cell-cell adhesion in many species of Dictyostelium but not of Polysphondylium

The anticarbohydrate monoclonal antibody d-41 inhibits the adhesion of aggregating cells, as measured by an in vitro assay, in every species of Dictyostelium tested but in none of the species from the genus Polysphondylium. Although d-41 binds significantly to the surface of cells from both genera, the ability to inhibit adhesion correlates with the binding of the antibody to a few, mostly developmentally regulated, membrane-associated proteins in each of the species affected. Previous work in D. discoideum and D. purpureum have shown that the major d-41-b binding proteins from these species at this time in development are directly involved in the adhesion process. Therefore, the presence of the epitope on these proteins in the other species of Dictyostelium implicates them in the adhesion mechanism. The function of the carbohydrates containing the epitope is yet to be determined.

Biochemical and genetic analysis of an antigenic determinant found on N-linked oligosaccharides in Dictyostelium

Dictyostelium discoideum synthesizes many highly immunogenic carbohydrates of unknown structure and function. We have used monoclonal antibodies prepared against one of these called CA1 to investigate its structure and the consequences of its loss. CA1 is preferentially expressed on lysosomal enzymes as a specific arrangement of mannose-6-SO4 residues on N-linked oligosaccharides. Mutant strains HL241 and HL243 do not express CA1, and synthesize a truncated lipid-linked oligosaccharide (LLO) precursor that lacks the critical mannose residues needed for expression. The lesion appears to result from the loss of mannosyl transferase activity involved in LLO biosynthesis. The truncated LLO is poorly transferred to an artificial peptide acceptor in a cell-free N-glycosylation assay, and this appears to result from improper topological localization of the LLO or to a lower affinity of the LLO for the oligosaccharyl transferase. Although both mutants share these lesions, they are biochemically and genetically distinct. Only HL243 is lower in N-glycosylation in intact cells, and this is not a result of an altered structure of the LLO. There are other differences between the strains. HL241 can form fruiting bodies at a slower rate than normal while HL243 cannot aggregate. Genetic analysis of defects shows that the CA1 lesion in HL241 is recessive, while the lesion in both CA1 and in development are dominant and co-segregate in HL243 and are, therefore, likely to be in the same gene. Lysosomal enzyme targeting is normal but enzyme processing proceeds at a 2-3 fold slower rate in HL241 and HL243 compared to wild-type. Strain HL244 does not express CA1 since it completely lacks protein sulfation, but lysosomal enzyme targeting and processing proceeds at a normal rate, showing that sulfate is not essential for these processes. Alterations in oligosaccharide structure can have individualized effects on the biosynthesis of lysosomal enzymes. The results presented here illustrate how this approach can be used to study both the structure and function of carbohydrate epitopes.

Changes in cell surface glycoproteins during Dictyostelium development analysed using monoclonal antibodies

We have produced a series of monoclonal antibodies that recognize carbohydrate epitopes on cell surface glycoproteins of developing amoebae of Dictyostelium discoideum. The antibodies were found to have differential specificity for amoebae at different stages of development and were classified into types A to E on the basis of their temporal pattern of reactivity with the developing amoebal cell surface. Evidence from Western Blots and digestion of the glycoproteins with alkaline phosphatase were consistent with previous reports that the cell surface glycoproteins are extensively processed during development, leading at 16 h of development to the exposure of a highly antigenic core recognized by antibodies in group E. The nature of this core structure is indicated by the finding that antibodies in group E were found also to bind with high avidity to the plant glycoprotein horse radish peroxidase.

Phagocytosis in Dictyostelium discoideum is inhibited by antibodies directed primarily against common carbohydrate epitopes of a major cell-surface plasma membrane glycoprotein

Using a water-soluble, reversible biotinylating reagent, we retrieved three surface-exposed proteins from a complex mixture of crude membrane proteins. The compound, sulfosuccinimidyl 2-(biotinamido)ethyl-1-3'-dithiopropionate (sulfo-NHS-SS-biotin), which has a cleavable disulfide bond, was used to label Dictyostelium discoideum amebae. Cells were lysed and a crude membrane preparation was isolated and solubilized with Triton X-100. Biotinylated molecules were bound to immobilized streptavidin and then eluted from the affinity matrix with dithiothreitol. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that out of the original complex mixture of detergent-solubilized membrane proteins, three major species at 130, 100, and 77 kDa were specifically bound and eluted with thiol reagents. These three proteins were glycoproteins (gp) since they bound concanavalin A. As demonstrated by one-dimensional peptide mapping, the retrieved gp130 and gp100 also were present in specialized plasma membrane subdomains called contact regions which are regions of cell-cell cohesion isolated from aggregated, developed amebae. This finding provides preliminary evidence that the two proteins may be involved in cell-cell interactions during both the vegetative and aggregation stages of the D. discoideum life cycle. The retrieved gp130 species has a relative mobility on SDS-gels similar to that of gp126, a surface-exposed glycoprotein. gp126 has been suggested to play roles both as a phagocytosis receptor and as a cohesion molecule (C.M. Chadwick, J.E. Ellison, and D.R. Garrod, (1984) Nature (London) 307, 646). To test if the retrieved gp130 was the same as gp126, a polyclonal antiserum was raised against gel-purified, endoglycosidase F-treated gp130. The immune serum recognized epitopes, apparently carbohydrates, present on many D. discoideum membrane proteins. Univalent IgG fragments from this antiserum inhibited phagocytosis, suggesting that anti-carbohydrate activity was responsible for the functional inhibition of phagocytosis.

Evidence that a glycolipid tail anchors antigen 117 to the plasma membrane of Dictyostelium discoideum cells

We describe the biochemical features of the putative cell cohesion molecule antigen 117, indicating that it is anchored to the plasma membrane by a glycolipid tail. Antigen 117 can be radiolabeled with [3H]myristate, [3H]palmitate, and [14C]ethanolamine. The fatty acid label is removed by periodate oxidation and nitrous acid deamination, indicating that the fatty acid is attached to the protein by a structure containing carbohydrate and an unsubstituted glucosamine. As cells develop aggregation competence, the antigen is released from the cell surface in a soluble form that can still be radiolabeled with [14C]ethanolamine but not with [3H]myristate or [3H]palmitate. The molecular weight of the released antigen is similar to that found in the plasma membrane, but it preferentially partitions in Triton X-114 as a hydrophilic, as opposed to a hydrophobic, protein. Plasma membranes contain the enzyme activity responsible for the release of the antigen in a soluble form.

Characterization of an antigenically related family of cell-type specific proteins implicated in slug migration in Dictyostelium discoideum

The monoclonal antibody MUD50 recognizes a group of developmentally regulated proteins, which are almost exclusively expressed by prespore cells in developing aggregates of Dictyostelium discoideum. Some of these antigens are integrally associated with the cell membrane, as assessed by physical and detergent-fractionation procedures. The MUD50-reactive proteins are glycosylated and some are phosphorylated. Post-translational modification is the common antigenic feature that is recognized by the MUD50 antibody in these cell-type-specific proteins. A glycosylation-defective mutant, DL118, (modB) does not express the MUD50 epitope, but does express the MUD52 epitope, which is found on a different group of glycoproteins. Therefore, we conclude that MUD50 recognizes a particular carbohydrate epitope on a restricted group of proteins. These proteins are structurally diverse, but are apparently involved in the maintenance of structure and movement of the multicellular D. discoideum slug.

The expression of glycoproteins in the extracellular matrix of the cellular slime mold Dictyostelium discoideum

In this report we examine the accumulation of glycoconjugates in the extracellular medium and insoluble matrices surrounding developing cells of the cellular slime mold Dictyostelium discoideum. Conditions were employed which permitted advanced development (slug stage and beyond) in suspension culture. Under these conditions, up to one-third of the total culture protein appeared as non-sedimentable, extracellular material over the course of 48 h of incubation. Most of the secreted molecules expressed carbohydrate antigens (glycoantigens) as detected by Western blotting, using a panel of six monoclonal antibodies. Since the glycoantigens are secreted, immunoelectron microscopy was used to localize the glycoantigens in the extracellular matrices surrounding normally developing cells, including the slime sheath, stalk tube, inner spore coat, outer spore coat, and intercellular fluid between spores. Each glycoantigen had a characteristic distribution, and each extracellular matrix space contained a unique combination of glycoantigens. Thus, although each of these matrices (except inter-spore fluid) contains cellulose as a primary component, they could be distinguished on the basis of their glycoantigen and, by inference, glycoprotein compositions. Furthermore, there were differences between anterior and posterior regions of both slime sheaths and stalk tubes. These observations show that secretion as detected in suspension culture occurs under normal conditions as a part of the process of depositing extracellular matrices around the cells. The distributions show that the cell aggregate positionally regulates the expression and deposition of secretory glycoproteins; the resultant patterns of expression of unique protein-linked carbohydrate structures imply a functional role in matrix organization and possibly cell activity which can now be explored.

Detergent treatment of Dictyostelium discoideum cells allows examination of internal cell type-specific antigens by flow cytometry

Monoclonal antibodies are used extensively in flow cytometry to identify subpopulations of cells differing in surface antigens. Conventional studies on living cells do not allow analysis of internal antigens, because antibody molecules do not pass through an intact plasma membrane. It is important for developmental studies on Dictyostelium discoideum that not only surface but also internal antigens be analysed. Here techniques are reported that make possible such studies by permeabilising cells with mild detergent treatments using digitonin. Flow cytometer profiles of unfixed cells show that antigens recognised by two monoclonal antibodies, MUD102 and MUD3, are found inside subpopulations of cells in the D. discoideum slug. Double-labelling experiments were carried out to demonstrate that the antigens recognised by these antibodies are present inside prespore but not prestalk cells. The detergent treatment leads to loss of forward-angle light scatter, but 90 degrees light scatter of cells is not greatly affected. While fixed cells sometimes gave satisfactory results, internal labelling did not reliably demonstrate the two subpopulations observed with unfixed cells.

Characterization and distribution of multiple antigens on N-linked oligosaccharides of Dictyostelium discoideum proteins

Monoclonal antibodies were prepared against a mixture of purified lysosomal enzymes from Dictyostelium discoideum. Three classes of antibodies were found which recognized distinct antigenic determinants on N-linked oligosaccharides of multiple proteins. The structure of the determinants was studied by competition assays using monosaccharides and oligosaccharide/glycopeptide fractions prepared from one Dictyostelium lysosomal enzyme or other sources. The results of these studies suggest that one class of antibody recognizes an epitope containing residues of Man-6-SO4, another recognizes a domain containing a modified GlcNAc, and the third class recognizes an undefined determinant that involves the oligosaccharide. The three determinants are found on multiple overlapping, but nonidentical sets of glycoproteins. The ability to produce monoclonal antibodies against unusual N-linked oligosaccharides offers a powerful tool which can be used to investigate the occurrence, structure, biosynthesis, and the biological roles of these highly immunogenic saccharides.

Glycoantigen expression is regulated both temporally and spatially during development in the cellular slime molds Dictyostelium discoideum and D. mucoroides

Six monoclonal antibodies were isolated which react with common antigens shared by multiple glycoconjugate species in the cellular slime mold Dictyostelium discoideum. Based on competition of antibody binding by glycopeptides and simple sugars, and inhibition of antibody binding by antigen pretreatment with Na periodate, it is argued that at least five of the six antibodies recognize epitopes which contain carbohydrate. These epitopes are consequently referred to as glycoantigens (GAs). Three of the GAs are expressed during growth and throughout the developmental cycle, but are eventually enriched in prestalk and stalk cells. The remaining three are expressed only during and/or after aggregation and are exclusively expressed or highly enriched in prespore cells and spores. These conclusions are derived from Western blot immunoanalysis of purified cell types, immunofluorescence, and EM immunocytochemistry. The two GAs found only in prespore cells appear to be exclusively enclosed within prespore vesicles. The third GA of this type, which is only enriched in prespore cells compared to prestalk cells, is also found in other vesicle types as well as on the cell surface. Two of the GAs enriched in prestalk cells are initially found in all cells of the slug. They are undetectable in spores and prominent in stalk cells. The third GA, though found in the interiors of both prestalk and prespore cells, is enriched on the cell surface of prestalk cells. The chief characteristics of expression of four of these GAs are conserved in the related species D. mucoroides. This species is characterized by continuous transdifferentiation of prespore cells into prestalk cells. This shows that the prespore cells maintain specific mechanisms for turning over their cell type specific GAs and that prestalk cells express a specific mechanism for inducing at least one of their cell-type specific GAs. These observations identify specific carbohydrate structures (as GAs) whose synthesis, subsequent localization and turnover are developmentally regulated. The exclusive association of two GAs with prespore vesicles identifies these GAs as markers for this organelle and raises questions regarding the functional significance of this association. The restricted cell surface localization of the other four GAs, together with data from cell adhesion studies, suggest the possibility of a potential role for these GAs in intercellular recognition leading to cell sorting.

Regulation of stalk and spore antigen expression in monolayer cultures of Dictyostelium discoideum by pH

The terminal differentiation of Dictyostelium discoideum cells plated as monolayers with cyclic AMP is dramatically affected by developmental buffer conditions. High pH and addition of weak bases induces spore differentiation while low pH and weak acids favour stalk cell formation. In order to analyse the timing and nature of this regulation we have raised and characterized an anti-stalk serum which we have used together with an anti-spore serum to monitor developmental progression in the monolayer system and to detect the phenotypic effects of pH at earlier stages of development. The stalk serum detects both polysaccharide and protein antigens expressed during the terminal stages of normal development. In monolayer culture, the stalk-specific protein antigen appears precociously, while the timing of prespore vacuole appearance is unaffected. Expression of stalk polysaccharide antigens in monolayer cultures occurs as early as 12 h and is localized in a single subset of cells or region of extracellular space within the small cell clumps that form. The effects of pH (and acid/base) on these phenotype-specific antigens can be detected early in development, shortly after their first appearance. In monolayers of wild-type V12 M2 cells, the low pH regimes appear to act more by suppressing the spore than enhancing the stalk pathway, while the high pH regimes both suppress stalk and enhance spore antigen expression. In monolayers of the sporogenous mutant HM29, low pH regimes both enhance stalk antigen and suppress spore antigen expression. These results show that extracellular pH regulates phenotypic expression during a large part of the differentiation process and is not simply restricted to terminal cytodifferentiation.

Rapid isolation of monoclonal antibodies specific for cell surface differentiation antigens

Two immunization procedures were compared for their ability to yield monoclonal antibodies that react with plasma membrane-bound differentiation antigens of Dictyostelium. In the first method, hybridomas prepared from BALB/c mice immunized with aggregating amoebae produced monoclonal antibodies that recognized antigens present on both growing and aggregating Dictyostelium amoebae. None of the monoclonal antibodies reacted with only the injected aggregation-stage cell type. In contrast, monoclonal antibodies that reacted with differentiation antigens were easily obtained by primary immunization of BALB/c mice with living aggregation-stage cells, followed by secondary immunization with a preparation of plasma membrane from aggregating cells or intact aggregating cells mixed with polyclonal BALB/c antiserum raised against undifferentiated cells. By this method, approximately 20% of all anti-Dictyostelium monoclonal antibodies obtained in a fusion are specific for differentiation antigens. The properties and developmental regulation of several of these antigens are described. The possible uses of this immunological method to detect unique determinants on other kinds of cells and the likely immune mechanisms that make it successful are discussed.

The use of tolerization in the production of monoclonal antibodies against minor antigenic determinants

An initial attempt to prepare monoclonal antibodies specific for the Dictyostelium discoideum lysosomal enzyme beta-glucosidase was unsuccessful. All of the antibodies resulting from this fusion recognized an extremely immunogenic epitope that is present on all of the lysosomal enzymes of Dictyostelium. In two succeeding fusions, changes in the immunization schedule intended to increase the immune response to enzyme-specific epitopes were not entirely successful. Although nine hybridomas producing antibodies specific for beta-glucosidase resulted from these two fusions, most (70%) of the cell lines isolated secrete antibodies that recognize the shared, immunodominant epitope. Moreover, the nine beta-glucosidase-specific antibodies proved to be of limited utility since none recognize the native enzyme. Therefore, we attempted to tolerize a BALB/c mouse to the common epitope by injecting the lysosomal enzyme, N-acetylglucosaminidase, within 40 h after birth. As an adult, this animal was immunized with beta-glucosidase. Fusion of the spleen cells from this mouse with myeloma cells resulted in the isolation of nine hybridoma lines that produce antibodies specific for beta-glucosidase. No antibodies reactive with the common epitope were detected. These results suggest that tolerization may provide a means whereby an undesired class of antibody-producing cell lines can be selectively eliminated from the products of a fusion.

Immunological homologies between ribosomal proteins amongst lower eukaryotes

Polyclonal antibodies were raised against the purified ribosomal proteins L1 and L2, the 5S rRNA binding protein L3, all from Saccharomyces cerevisiae, and against L1 and L2 from Schizosaccharomyces pombe (numbering according to Otaka and Osawa 1981; Otaka et al. 1983, respectively). For clarity prefixes Sc and Sp have been added to the numbering of proteins derived from S. cerevisiae and S. pombe, respectively. Ribosomal proteins from these yeasts and from Kluyveromyces marxianus, Rhodotorula glutinis, the slime mold Dictyostelium discoideum and the protozoan Tetrahymena thermophila were checked for antigenic cross-reactivity by the immunoblot technique. Anti-ScL1 bound to the largest ribosomal proteins of all organisms but not with equal strength. A fast migrating protein band from R. glutinis was also reactive. Anti-ScL2 reacted strongly with L2 or analogous proteins derived exclusively from the yeasts. Anti-ScL3 cross-reacted only with one protein band from K. marxianus, whereas anti-SpL1 cross-reacted with L1 or its analogues from the other organisms, but also with proteins of lower molecular weight. In S. cerevisiae, these proteins are located exclusively on the small ribosomal subunit. L2 or analogous ribosomal proteins of all organisms were recognized by anti-SpL2 but additionally the ribosomal protein YL28 of S. cerevisiae and fast migrating proteins of T. thermophila exhibited anti-SpL2 binding.

Monoclonal antibodies against Dictyostelium plasma membranes: their binding to simple sugars

Monoclonal antibodies raised against purified membranes from Dictyostelium discoideum were classified according to three criteria: type of antigen as revealed in immunoblots, developmental regulation of the target antigens, and location of the antigens on the cell surface. Some antibodies reacted with myosin, two with glycolipids. One group of antibodies bound to the protein moiety of the contact site A glycoprotein, whereas another group reacted with carbohydrate epitopes that the contact site A glycoprotein shared with a few other membrane glycoproteins. Binding of the latter antibodies to their antigens was either specifically blocked by N-acetylglucosamine or by maltose as well as methyl-alpha-mannoside and N-acetylglucosamine. These anti-carbohydrate antibodies bound specifically to agarose beads derivatized with some sugars. These results and competition studies with several carbohydrates suggest that the epitope recognized by the antibodies contains as major components N-acetylglucosamine, maltose and alpha-mannose residues. One monoclonal antibody, which reacts with N-acetylglucosamine, was used for affinity purification of the contact site A glycoprotein from a crude membrane extract. N-acetylglucosamine was used as a mild eluent of the antigen from the antibody column. No detergents were added during the entire purification procedure.