Developmentally regulated proteins of the plasma membrane of Dictyostelium discoideum. The carbohydrate-binding protein

Cloning of cDNA for the contact site A glycoprotein of Dictyostelium discoideum

A cell surface glycoprotein of Dictyostelium discoideum with M(r) 80,000 (gp80) has been shown to mediate the formation of the developmentally acquired EDTA-resistant cell-cell binding sites termed contact sites A. We have isolated cDNA clones encoding gp80 by immunological screening of an expression library prepared in Escherichia coli. Double-stranded cDNA was prepared from poly(A)(+) RNA isolated from cells at 8 hr of development and cloned into the bacteriophage expression vector lambdagt11. Two recombinant phages containing cDNA inserts of 1.2 and 0.8 kilobases were isolated and shown to contain sequences coding for gp80 by the immunoselect assay. Partial DNA sequence analysis also confirmed that one of these cDNA clones, lambdaDdgp80c-19, contained the coding sequence for the amino terminus of gp80. DNA-RNA hybridization showed that the insert of lambdaDdgp80c-19 hybridized to a single mRNA transcript of approximately 2.0 kilobases. gp80 mRNA became detectable after 6 hr of development, reached its maximum level at 9 hr, and dropped to a negligible level by 15 hr. This pattern of mRNA accumulation corresponded closely to that of gp80 synthesis in D. discoideum cells, suggesting that gp80 expression is regulated at the transcriptional level.

Specific cell-cell contacts are essential for induction of gene expression during differentiation of Dictyostelium discoideum

Postaggregation Dictyostelium discoideum cells contain 2000-3000 mRNA species that are absent from pre-aggregation cells. These aggregation-dependent sequences compose 30% of the mass of the late mRNA and represent the transcription products of an additional 11% of the single-copy genome. By analysis of mutants that are blocked at different stages of differentiation, we show that induction of expression of these genes is correlated with the formation of tight cell-cell contacts that resist EDTA. In particular, mutants that exhibit chemotaxis and aggregate to form loose mounds but do not form cell-cell contacts that resist EDTA fail to induce these late mRNA and protein species. By contrast, mutants that form normal contacts but progress no further through development do express the late mRNA species. Thus, interactions at the cell surface are involved in developmental induction of a large group of coregulated mRNAs. We have employed two independent assays for these developmentally regulated mRNAs: hybridization of gel-separated RNAs to cloned nuclear DNAs and hybridization of mRNA to a cDNA probe specific for the population of 2000-3000 regulated sequences.

Serologically distinguishable alterations in the molecular specificity of cell cohesion during morphogenesis in Dictyostelium discoideum

Cells of the mutant strain JC-5 of the slime mold Dictyostelium discoideum exhibit at a specific, late developmental stage a temperature-sensitive morphogenetic defect associated with the loss of cell cohension. We show that at the restrictive temperature, the loss of cohesion and attendant dispersal of multicellular aggregates is associated with the disappearance or sequestration of a plasma membrane-bound moiety capable of reacting with and, hence, absorbing cohesion-blocking Fab. At the permissive temperature, the maintenance of cohesiveness past the critical stage or the recovery of lost cohesiveness is correlated with the presence or reappearance of the Fab-reactive moiety.This moiety is absent or sterically incapable of reaction with Fab preparations at an earlier developmental stage in either mutant or wild-type cells-i.e., at a time when they have just entered into multicellular aggregates. Conversely, a serologically distinguishable membrane-bound moiety present in the early mutant or wild-type cells, whose reaction with homologous Fab also precludes their cohesion, is absent or serologically unreactive in either mutant or wild-type cells that are at comparable late developmental stages. We conclude that the cohesive moiety responsible for initiation of cell aggregates is supplanted by or transformed into a serologically distinct, cohesive complex responsible for the maintenance of the aggregate's integrity through the later stages of development.

Transduction of the chemotactic signal to the actin cytoskeleton of Dictyostelium discoideum

Dictyostelium discoideum amebae chemotax toward folate during vegetative growth and toward extracellular cAMP during the aggregation phase that follows starvation. Stimulation of starving amebae with extracellular cAMP leads to both actin polymerization and pseudopod extension (Hall et al., 1988, J. Cell. Biochem. 37, 285-299). We have identified an actin nucleation activity (NA) from starving amebae that is regulated by cAMP receptors and controls actin polymerization (Hall et al., 1989, J. Cell Biol., in press). We show here that NA from vegetative cells is also regulated by chemotactic receptors for folate. Our studies indicate that NA is an essential effector in control of the actin cytoskeleton by chemotactic receptors. Guided by a recently proposed model for signal transduction from the cAMP receptor (Snaar-Jagalska et al., 1988, Dev. Genet. 9, 215-225), we investigated which of three signaling pathways activates the NA effector. Treatment of whole cells with a commercial pertussis toxin preparation (PT) inhibited cAMP-stimulated NA. However, endotoxin contamination of the PT appears to account for this effect. The synag7 mutation and caffeine treatment do not inhibit activation of NA by cAMP. Thus, neither activation of adenylate cyclase nor a G protein sensitive to PT treatment of whole cells is necessary for the NA response. Actin nucleation activity stimulated with folate is normal in vegetative fgdA cells. However, cAMP suppresses rather than activates NA in starving fgdA cells. This indicates that the components of the actin nucleation effector are present and that a pathway regulating the inhibitor(s) of nucleation remains functional in starving fgdA cells. The locus of the fgdA defect, a G protein implicated in phospholipase C activation, is directly or indirectly responsible for transduction of the stimulatory chemotactic signal from cAMP receptors to the nucleation effector in Dictyostelium.

Identification of an octapeptide involved in homophilic interaction of the cell adhesion molecule gp80 of dictyostelium discoideum

During development of Dictyostelium discoideum, a surface glycoprotein of Mr 80,000 (gp80) is known to mediate EDTA-resistant cell-cell adhesion via homophilic interaction. Antibodies directed against a 13 amino acid sequence (13-mer) near the NH2 terminus of the protein were found to inhibit cell reassociation. This 13-mer also inhibited gp80-cell interaction and gp80-gp80 interaction. The cell binding site was mapped to the octapeptide sequence YKLNVNDS by using shorter peptide sequences to inhibit gp80 interaction. High salt concentrations inhibited homophilic interactions of both the 13-mer and gp80, suggesting that ionic interactions are involved in the forward binding reaction. Since disruption of homophilic interactions between the bound molecules required the presence of Triton X-100, hydrophobic interactions may occur after the initial ionic binding.

Codon usage and gene expression level in Dictyostelium discoideum: highly expressed genes do 'prefer' optimal codons

Codon usage patterns in the slime mould Dictyostelium discoideum have been re-examined (a total of 58 genes have been analysed). Considering the extreme A + T-richness of this genome (G + C = 22%), there is a surprising degree of codon usage variation among genes. For example, G + C content at silent sites varies from less than 10% to greater than 30%. It was previously suggested [Warrick, H.M. and Spudich, J.A. (1988) Nucleic Acids Res. 16: 6617-6635] that highly expressed genes contain fewer 'optimal' codons than genes expressed at lower levels. However, it appears that the optimal codons were misidentified. Multivariate statistical analysis shows that the greatest variation among genes is in relative usage of a particular subset of codons (about one per amino acid), many of which are C-ending. We have identified these as optimal codons, since (i) their frequency is positively correlated with gene expression level, and (ii) there is a strong mutation bias in this genome towards A and T nucleotides. Thus, codon usage in D. discoideum can be explained by a balance between the forces of mutational bias and translational selection.

Mediation of cell-cell adhesion by the altered contact site A glycoprotein expressed in modB mutants of Dictyostelium discoideum

In Dictyostelium discoideum, a surface glycoprotein with Mr 80,000 (gp80) has been found to mediate the EDTA-resistant contact sites A at the aggregation stage of development. To evaluate the role of the carbohydrate moiety in cell-cell adhesion, we have examined the accumulation and activity of an altered gp80 molecule in two glycosylation (modB) mutants. Both mutants synthesize an altered gp80 of lower molecular size. This modB-gp80 can be detected by the monoclonal antibody 80L5C4, which is capable of blocking cell-cell adhesion (C. -H. Siu, T. Y. Lam, and A. Choi, (1985) J. Biol. Chem. 260, 16,030-16,036). The mutant cells exhibit both EDTA-sensitive and EDTA-resistant types of cell-cell binding, though to a lesser extent than that of the parental strain, and the EDTA-resistant binding sites are blocked in the presence of 80L5C4 Fab. Mutant cells can also bind Covaspheres conjugated with gp80. These results suggest that the modB-gp80 protein still retains the domain essential for its cell binding activity and the carbohydrate moiety affected by the modB mutation is not directly involved in cell-cell adhesion.

Codon preference in Dictyostelium discoideum

Dictyostelium discoideum is of increasing interest as a model eukaryotic cell because its many attributes have recently been expanded to include improved genetic and biochemical manipulability. The ability to transform Dictyostelium using drug resistance as a selectable marker (1) and to gene target by high frequency homologous integration (2) makes this organism particularly useful for molecular genetic approaches to cell structure and function. Given this background, it becomes important to analyze the codon preference used in this organism. Dictyostelium displays a strong and unique overall codon preference. This preference varies between different coding regions and even varies between coding regions from the same gene family. The degree of codon preference may be correlated with expression levels but not with the developmental time of expression of the gene product. The strong codon preference can be applied to identify coding regions in Dictyostelium DNA and aid in the design of oligonucleotide probes for cloning Dictyostelium genes.

Expression of the contact site A glycoprotein in Dictyostelium discoideum: quantitation and developmental regulation

We have previously reported that a monoclonal antibody directed specifically against a surface glycoprotein of Mr 80,000 (gp80) inhibits the EDTA-resistant contact sites A of Dictyostelium discoideum (Siu, C.-H., and Choi, A.H.C. (1985) J. Biol. Chem. 260, 16030-16036). In this report, we describe an assay using this monoclonal antibody to quantitate the amount of gp80 expressed at different developmental stages. Under normal conditions, gp80 is detectable after 6 h of development and it rapidly accumulates between 6 and 10 h, corresponding to the time when cells acquire their EDTA-resistant binding sites. At the peak level, there are 1.5.10(5) gp80 molecules per cell. More than 90% of the cellular gp80 is located on the cell surface. When cells are given exogenous pulses of cAMP, a precocious and enhanced expression of gp80 is induced. At the peak level, the cAMP-pulsed cells accumulate five times more gp80 than the non-pulsed cells. This is preceeded by an equally rapid accumulation of gp80 transcripts, suggesting that cAMP regulates gp80 synthesis at the transcriptional level.

Density-dependent induction of discoidin-I synthesis in exponentially growing cells of Dictyostelium discoideum

The synthesis of the lectin, discoidin I, by vegetative cells of Dictyostelium discoideum (strain NC4) was monitored using immunoblot analysis and indirect immunofluorescence. Suspension cultures were used, so that the D. discoideum cell density and the concentration of bacteria could be controlled. Discoidin-I production was found to be a function of the relative densities of D. discoideum cells and food bacteria. Synthesis was initiated in exponentially growing D. discoideum cells approximately three generations before depletion of the food supply. In the growth medium of cells producing discoidin I, a soluble activity was detected that caused low-density cells to begin discoidin-I synthesis. This activity was not dialyzable and was destroyed by heat. A similar activity was produced by AX3 cells during axenic growth. Density-dependent induction of other 'early developmental' proteins was also detected in wild-type cells. These findings suggest that the expression of several 'early developmental' genes is regulated by a mechanism that measures cell density relative to food supply, not by starvation per se.

Nucleotide sequence of the myxobacterial hemagglutinin gene contains four homologous domains

Myxococcus xanthus, a Gram-negative bacterium, has a complex life cycle that includes fruiting-body formation, a primitive form of multicellular development. Myxobacterial hemagglutinin (MBHA) is a lectin that is induced during the aggregation phase of fruiting-body formation. We have cloned the gene for MBHA and determined its sequence by the dideoxy chain-termination technique. The sequence data show the probable sites for translational initiation and termination and suggest that MBHA does not contain a cleaved leader signal peptide. The DNA sequence shows four strong internal homologies. The deduced amino acid sequence shows that the protein (Mr 27,920) consists of four highly conserved domains each consisting of 67 amino acids. Thus MBHA is physically multivalent in structure, a requirement for all hemagglutinins.

Analysis of gene expression in rapidly developing mutants of Dictyostelium discoideum

Developmentally regulated gene expression has been analyzed in the wild-type D. discoideum strain NC-4 and a series of temporally deranged mutants. The mutants include representatives from each class of rapid development mutation, Fr17(rdeA-) and HT506(rdeC-), and strain HIfm-1, which appears to be defective in the timing of events early in development. We have monitored four prespore-specific genes, three of which show coordinate expression in the wild type. The coordination is maintained in each of the mutant strains though the specific expression pattern varied from strain to strain. Likewise, a series of prestalk-specific genes have been analyzed. They also show coordinated expression in the wild type and in all of the mutants. The timing of expression, however, is different between the prestalk-specific and the prespore-specific with the overall pattern of expression being unique for each strain examined. These results confirm our previous suggestion that the major classes of prestalk- and prespore-specific genes are coordinately regulated and show that a great deal of tolerance is allowed in the timing of specific gene expression as it relates to terminal differentiation. In addition we have analyzed the expression of actin, discoidin I, and I42. These genes, or gene families, are preferentially expressed in either vegetatively growing cells or in cells during the early stages of development. As with the cell-type-specific genes, the pattern of expression of the three early gene classes is unique for each strain examined.

Phenocopy of discoidin I-minus mutants by antisense transformation in Dictyostelium

Using an antisense construct of the discoidin gene transfected into Dictyostelium, we have repressed the expression of the three endogenous discoidin genes. Transformants exhibit a greater than 90% reduction in accumulated discoidin mRNA and protein. Nuclear run-on assays show that both the endogenous and the antisense genes are transcribed. Since only minor amounts of endogenous gene transcripts and none from the antisense gene can be detected on blots, we suggest that hybrids are formed within the nucleus and are rapidly degraded. Discoidin is believed to play a role in cell-substratum interaction and exhibits homologies to fibronectin. Discoidin-minus mutants exhibit the developmental phenotype of not streaming on a plastic surface. Antisense transformants show a similar phenotype and are thus phenocopies of these mutants.

Rapid, high-yield purification of cell surface membrane using colloidal magnetite coated with polyvinylamine: sedimentation versus magnetic isolation

A new technique for the magnetic isolation of external plasma membrane from Dictyostelium discoideum is described and compared to a previously published procedure employing sedimentation of silica-coated plasma membrane. The magnetic isolation technique involves coating intact cells with a polyvinylamine-magnetite colloid and overcoating with polyacrylate to form a dense pellicle. The magnetite pellicle totally coated the cells and was not internalized. Coated cells were lysed and membrane fragments retrieved from the cell homogenate using a diverging field electromagnet. The membrane obtained in such a manner was analyzed for marker enzyme activity and cell surface label. The plasma membrane was obtained in high yield (42%) with an average purification of 8-fold. The polyvinylamine-magnetite pellicle shielded the external plasma membrane face to proteolysis by papain and pronase. It also acted as a barrier to alpha-methylmannoside in concanavalin A-carbohydrate competition studies.

Overproduction of discoidin I by a temperature-sensitive motility mutant of Dictyostelium discoideum

Dictyostelium discoideum MC2 is a temperature-sensitive motility mutant of AX3. Mutant cells are incapable of growth, phagocytosis, and migration under restrictive conditions (Kayman et al., J. Cell Biol. 92:705-711, 1982). We show here that at the restrictive temperature MC2 cells grown axenically or on bacteria synthesized excessive quantities of the lectin discoidin I. By two-dimensional gel electrophoresis and peptide mapping, the proteins overproduced by MC2 cells were indistinguishable from discoidin I synthesized at lower levels in AX3 cells. At least two of the three species of discoidin I were overproduced. This protein family constituted 9% of the total protein in cells that were incubated overnight at 27 degrees C in axenic medium. Although MC2 cells are defective in nutrient uptake under restrictive conditions, the overproduction of discoidin I did not appear to be part of a pleiotropic response to starvation. We propose that transcription of the coordinately regulated discoidin I genes is altered in mutant cells. This alteration may be related to the motility defects manifested by MC2.

Overproduction of discoidin I by a temperature-sensitive motility mutant of Dictyostelium discoideum

Dictyostelium discoideum MC2 is a temperature-sensitive motility mutant of AX3. Mutant cells are incapable of growth, phagocytosis, and migration under restrictive conditions (Kayman et al., J. Cell Biol. 92:705-711, 1982). We show here that at the restrictive temperature MC2 cells grown axenically or on bacteria synthesized excessive quantities of the lectin discoidin I. By two-dimensional gel electrophoresis and peptide mapping, the proteins overproduced by MC2 cells were indistinguishable from discoidin I synthesized at lower levels in AX3 cells. At least two of the three species of discoidin I were overproduced. This protein family constituted 9% of the total protein in cells that were incubated overnight at 27 degrees C in axenic medium. Although MC2 cells are defective in nutrient uptake under restrictive conditions, the overproduction of discoidin I did not appear to be part of a pleiotropic response to starvation. We propose that transcription of the coordinately regulated discoidin I genes is altered in mutant cells. This alteration may be related to the motility defects manifested by MC2.

Protein synthesis during development and differentiation in the cellular slime mould Dictyostelium discoideum

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Soluble lectins: a new class of extracellular proteins

Soluble lectins of cellular slime molds and vertebrates are present at extracellular sites in the developing or adult tissues that make them. Some lectins are concentrated around cell groups, as in extracellular matrix or elastic fibers. Others are at the interface between cells and the external environment, as in mucin or slime. Specific glycoproteins, proteoglycans, or polysaccharides that bind these endogenous lectins may also be present at these sites. Interactions between the lectins and glycoconjugates appear to play a role in shaping extracellular environments.

The role of membrane lectins in Dictyostelium discoideum aggregation as ascertained by specific univalent antibodies against discoidin I

Antibodies against pure discoidin I have been used as a tool to ascertain the role of this lectin in aggregation of Dictyostelium discoideum. Discoidin I is widely expressed over the cell surface of aggregation-competent AX-2 cells, as ascertained by indirect immunofluorescence with specific (antidiscoidin I) antibodies. Univalent antidiscoidin I antibodies (Fab fragments) inhibit the aggregation-specific intercellular adhesion of D discoideum AX-2 cells in an in vitro assay. This inhibition depends on antibody concentration and cell density; a 50% inhibition of cell aggregation was obtained at antidiscoidin I Fab concentration of 4.5 mg/ml and 1 X 10(6) cells/ml. Aggregation and morphogenesis on solid support is also effectively inhibited when AX-2 cells are starved in the presence of antidiscoidin I Fab fragments. The inhibition of morphogenesis is also dose dependent and more effective than in the in vitro assay. No inhibition of aggregation either in the in vitro assay or on morphogenesis on solid support was observed with preimmune Fab fragments at any of the concentrations tested (up to 9.6 mg/ml).

Involvement of a cell-surface glycoprotein in the cell-sorting process of Dictyostelium discoideum

Cell sorting among prespore cells and prestalk cells in Dictyostelium discoideum was studied by using fluorochrome-labeled cells in an in vitro assay. Labeled prestalk cells first formed randomly mixed aggregates with unlabeled prespore cells. Then cells began to sort out from each other. About 3-4 hr later, prestalk cells became clustered at one pole of the aggregate. Aggregates deposited on an agar surface underwent morphogenesis and formed migrating slugs within 3 hr. The addition of Fab fragments directed against a cell-surface glycoprotein of Mr 150,000 (gp 150) to the cell mixture completely inhibited the cell-sorting phenomenon. Morphogenesis of such aggregates on agar was also delayed by 5 hr. However, inclusion of Fab fragments directed against the endogenous lectins, the contact site A glycoprotein, or vegetative cells had no detectable effect on cell sorting or morphogenesis of these reconstituted aggregates.

Developmental regulation of Dictyostelium discoideum plasma membrane proteins

Developmental changes in the plasma membrane proteins of Dictyostelium discoideum have been studied using metabolic labeling with [35S]methionine and two-dimensional electrophoresis. Pulse labeling for 1 h at the early interphase, late interphase, aggregation, and tip formation stages of development showed that the profile of newly synthesized plasma membrane proteins changed dramatically over this interval. Only 14% of the polypeptide species were synthesized at all four stages at detectable levels; 86% of the species changed over this developmental interval according to the criterion that they were synthesized at some but not all of the four stages tested. Long-term labeling during vegetative growth followed by initiation of development showed that the "steady-state" levels of the plasma membrane proteins changed very little over the same period. The only changes were in minor species (33% overall change). Similar analyses of whole cell proteins showed 27 and 20% change, respectively. Cell surface radioiodination revealed 52 external proteins in the plasma membrane. Comparison with the uniform methionine labeling results showed that these proteins were, with one notable exception, minor membrane components. In these external proteins, also, developmental changes were limited and were observed in the less abundant species. These results demonstrate the existence of two general classes of plasma membrane proteins. The first is a population of high-abundance proteins that are present in vegetative cells and are largely conserved through development. These possibly serve "housekeeping" functions common to all stages. The second class consists of low-abundance species that are expressed in a highly stage-specific manner and which presumably participate in developmentally important functions.

Failure to detect immunocytochemically reactive endogenous lectin on the cell surface of Dictyostelium discoideum

The endogenous lectins of Dictyostelium discoideum, called discoidins I and II, have been implicated in cell cohesion during the associative phase of this organism. In an effort to repeat and extend the studies of these putative cell-surface proteins, we attempted a variety of immunocytochemical techniques. Antibodies to a mixture of the purified discoidins were raised in rabbit. Both living and fixed cells were examined by indirect immunoferritin labeling using whole antiserum and by direct immunolabeling using purified specific IgG adsorbed to colloidal gold. Cells, at the appropriate stage, of strains A3, NC-4, and WS-582 were tested. In no instance were cell surface antigens detected despite meticulous efforts to duplicate the published techniques and to extend and refine them. Specific localization was found only in the cytosol and on the cytoplasmic face of certain endomembrane vesicles, and much less so on outer nuclear and mitochondrial membranes, in inadvertently disrupted cells. In no case was specific label found on either side of the plasma membrane or on food vacuoles. Exogenously supplied discoidins, bound to cells, were successfully localized by our technique. We conclude that the discoidins are not present on the cell surface, or are there in undetectable quantities, during the associative phase. We suggest that previous demonstrations of these proteins at the cell surface were artifacts resulting from the way in which the cells were handled, which caused the binding of externalized discoidins, possibly those released from lysed cells. We believe that the current notion that the discoidins play a direct role in cell cohesion by virtue of their carbohydrate-binding capacity should be reexamined. We suggest that the true role of the discoidins is solely intracellular.

Ion dependence of the discoidin I lectin from Dictyostelium discoideum

The lectin discoidin I from Dictyostelium discoideum requires divalent cations for binding activity. The data indicate that calcium is the preferred ion in vitro. In contrast, the lectin activity of discoidin II is independent of divalent ions.

Monoclonal antibodies: use to detect developmentally regulated antigens on D. discoideum amebae

We have used monoclonal antibodies to detect developmentally regulated cell surface antigens on D. discoideum amebae. A study of an antigen detected using an antibody produced by a hybridoma line implicates a previously undescribed component in the process of cell aggregation. This antigen (consisting of a doublet of 69,000 and 73,000 molecular weight) is first detected during the early hours of cell starvation and is present until cells begin slug formation. The developmental appearance of the antigen is not controlled by cAMP pulses and is distinct from that of Contact A sites. Fab fragments directed against the antigen are potent inhibitors of aggregation but do not inhibit the differentiation of cells to aggregation competence.

Cell-surface discoidin in aggregating cells of Dictyostelium discoideum

Both discoidin I and discoidin II have been detected on the surface of aggregating (10 h developmental stage) cells of Dictyostelium discoideum NC4 by radioiodination of the cell-surface followed by immunoprecipitation and sodium dodecyl sulphate/polyacrylamide-gel-electrophoretic analysis. Approx. 92% of cell-surface discoidin I and 72% of cell-surface discoidin II can be eluted with 0.5 M-galactose, showing that most of each endogenous lectin is not present as integral membrane protein but rather is bound to cell-surface discoidin receptors. Two-dimensional polyacrylamide-gel-electrophoretic analysis of discoidin I suggests that the native tetramer may be a hetero-multimer composed of both Ia and Ib subunits. Cell-surface discoidin I also contains both types of subunit, but it is not clear whether both subunits have corresponding cell-surface receptors.

Discoidins I and II: common and unique regions on two lectins implicated in cell--cell cohesion in Dictyostelium discoideum

As Dictyostelium discoideum amoebae differentiate from the noncohesive to the mutually cohesive state, they synthesize two galactose-binding lectins--discoidins I and II--which have been implicated as obligatory components of the morphogenetic cell-cell recognition and cohesion system. These proteins have been shown to have similar amino acid compositions and subunit Mr and overlapping but distinct carbohydrate recognition specificities. We have performed extensive immunochemical and biochemical analyses to study the structural relationships between these two molecules and to eventually identify structural and functional domains. Antisera raised against highly purified preparations of discoidin I and discoidin II were tested for their reactivities against each protein by both immunoprecipitation and double diffusion analyses. The patterns of crossreactivity indicated the presence of shared as well as unique antigenic determinants. This interpretation was supported by two-dimensional thin-layer peptide map analysis and by studies with purified peptides. Of approximately 10-12 peptides observed after exhaustive tryptic digestion of each radioiodinated lectin, 3 appeared to be common to both. These putative common peptides were purified, and the corresponding peptides from discoidins I and II were found to behave identically by two-dimensional thin-layer analysis, gel filtration, and susceptibility to chymotrypsin. The finding of common and unique regions in discoidins I and II suggests analogies with other families of recognition proteins and may have important functional implications for these cell-cell recognition molecules.

Chemical carcinogenesis -- the price for DNA - repair?

This essay examines the possibility of merging the mutation theory of cancer with the hypothesis that cancer is a change in the state of the differentiation of cells. It is suggested that during normal development DNA rearrangements occur, concerning genes which code for differentiation specific cell communication proteins. These proteins are responsible for the proper functioning of growth control in a multicellular organism. DNA-damaging agents - mutagens - induce DNA repair enzymes, some of which may catalyse illegitimate genome rearrangements, thus leading to a change of the balance between growth and differentiation. A cell with a selective advantage may arise and become the origin of a tumor.

Characterization of sequence elements at the 5' end of a discoidin I gene isolated from Dictyostelium discoideum

The discoidin I genes of Dictyostelium discoideum encode a family of three closely related developmentally regulated lectins which may play a role in cell cohesion. We have isolated a genomic clone containing the 5' half of a discoidin I gene with 12kb of flanking sequence and we have identified the discoidin I polypeptide encoded by this gene. We have determined part of the nucleotide sequence of the cloned segment and have hence determined the N-terminal amino acid sequence of the encoded polypeptide. The nucleotide sequence upstream of the AUG initiation codon was unusually AT rich, with 94% AT base pairs in the 171 nucleotides for which sequence was determined. However, having determined the start point of transcription ('cap' site) two striking features of similarity with other eukaryotic structural genes became apparent. The sequence TATAAA was present 33 nucleotides upstream from the 'cap' site and the sequence CCAAT was present a further 20 nucleotides upstream.

Immunochemical analysis of discoidins I and II at the cell surface in wild type and aggregation-defective mutants of Dictyostelium discoideum

The endogenous lectins discoidins I and II are believed to be primary components of the morphogenetic cell cohesion system of D discoideum. We have developed two immunochemical methods to analyze the association of the discoidins with the cell surface. One method is a two-state specific antibody binding assay in which intact cells are incubated on ice with rabbit serum (either control serum or antidiscoidin I and II), washed, then incubated with 125I-Protein A. Specific antibody binding is defined as the difference between percent radioactivity bound with antidiscoidin versus control serum during the first stage. Substantial specific binding was observed with developed A3 cells but not with vegetative cells, and nearly all of the activity could be removed by preadsorption of the antiserum with discoidin-Sepharose. As a complementary method, quantitative immunoadsorption analysis was performed in which we tested the ability of intact cells to remove antibodies reactive with purified 125I-discoidin I or II. Developed cells, but not vegetative cells, were capable of adsorbing antibodies reactive with discoidin I as well as those reactive with discoidin II. This represents the first demonstration that both lectins are present on the surface of cohesive cells. These procedures, coupled with other methods to analyze soluble discoidin in cell extracts, were used to study discoidin expression in wild type cells and in two newly isolated aggregation-defective mutants. Strain EB-32 fails to aggregate and displays little or no discoidin in cell extracts or at the cell surface. On the other hand, strain EB-18 forms loose amorphous mounds, and expresses substantial quantities of the discoidins, both in cell extracts and at the cell surface. These mutants should prove valuable in studying the organization and regulation of discoidins I and II at the surface of aggregating cells.

An analysis of discoidin I binding sites in Dictyostelium discoideum (NC4)

Vegetative wild-type (strain NC4) D. discoideum cells and cells at the 10h stage of development (aggregation) were harvested in the presence of 0.5 M-galactose to remove any endogenous discoidin I already bound to the cell surface, and fixed with glutaraldehyde. Affinity-purified 125I-labelled discoidin I bound to these fixed cells in a specific manner, greater than or equal to 95% of binding being inhibited by 0.5 M-galactose. Binding of 125I-labelled discoidin I was essentially complete in 90 min at 22 degrees C. Based on specific radioactivity measurements, vegetative (0h) D. discoideum (NC4) cells bind approx. 8.4 x 10(5) discoidin I tetramers/cell and aggregated (10h) cells bind 5.1 x 10(5) discoidin I tetramers/cell, each exhibiting apparent positive co-operativity of binding with highest limiting affinity constants (Ka) of approx. 1 x 10(7) and 2 x 10(7) M-1, respectively. Klebsiella aerogenes, the food source used for growth of D. discoideum NC4 amoebae, also binds 125I-labelled discoidin I and this is greater than 99% inhibited by 0.5 M-galactose. However, at the levels of bacterial contamination present, greater than 97% of 125I-labelled discoidin I binding to D. discoideum cell preparations was to the cells themselves. Confirmation of the number of discoidin I tetramers bound per D. discoideum cell was obtained by elution of bound 125I-labelled discoidin I followed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and then quantification by scanning of stained discoidin I bands.

Drastic changes in accumulation and synthesis of plasma-membrane proteins during aggregation of Dictyostelium discoideum

Protein compositions of plasma membranes of Dictyostelium discoideum and their developmental changes during aggregation were analyzed by two-dimensional polyacrylamide gel electrophoresis. More than 150 polypeptide species could be detected with the growth-phase cells by staining. Most of them (about 120 spots) were conserved during aggregation, although their relative intensities of staining changes. On the initiation of cell differentiation, new polypeptides were accumulated on the plasma membrane and at least 110 new polypeptides were detected at the late-aggregation stage. This accumulation of new components was completely inhibited when the growth-phase cells were incubated in a liquid medium containing 5 mM EDTA. Studies on protein synthesis suggested that the accumulation and appearance of the new protein components was due to de novo synthesis. Pulse-label experiments with [35S]methionine showed that out of 200 proteins detected during aggregation, about 90 proteins showed various stage-specific patterns of synthesis and the rest were synthesized consistently. Actin and discoidin present in the plasma membrane were identified by reference to purified actin and discoidin. Actin synthesized de novo accumulated progressively on the plasma membrane during aggregation. Discoidin also accumulated on the plasma membrane, but a large amount was also recovered in a cytoplasmic soluble fraction. Discoidin I was separated into two spots with different pI values on a two-dimensional gel.

Identification of endogenous binding proteins for the lectin discoidin-I in Dictyostelium discoideum

Recent biochemical and genetic evidence has shown that the endogenous lectin discoidin-I is involved in intercellular adhesion during development of the cellular slime mold Dictyostelium discoideum. We have prepared discoidin-I affinity columns and used them to isolate the lectin receptors. By using the cell surface radioiodination method, 11 discoidin-I binding proteins were identified in wild-type NC4 cells by gel electrophoresis, with apparent molecular weights of 95,000, 85,000, 78,000, 72,000, 60,000, 33,000, 31,000, 28,000, 25,000, 21,000, and 15,000. Only three (gp33, gp31, and gp28) were under developmental regulation. The amount of gp31 increased 8- to 10-fold during aggregation, and it was the predominant discoidin-I binding protein synthesized at the aggregation stage. Discoidin-I binding proteins derived from aggregation stage cells were potent inhibitors of discoidin-I in a hemagglutination assay in vitro. The same preparation was found to promote aggregation of cells bearing discoidin-I on the surface, suggesting a multivalent interaction.

Developmental regulation of glycoprotein biosynthesis in Dictyostelium

We have examined the glycoprotein-linked oligosaccharides assembled during the life cycle of Dictyostelium discoideum, and found their expression to be dramatically dependent upon the stage of development. During early development mature glycans have a high mannose character, and a substantial proportion acquire a fucose residue that correlates with endo-H resistance. One-third of the glycans also acquire sulfate residues. These glycans diminish in importance during aggregation. The mature glycans expressed during aggregation. The mature glycans expressed during late development contain fewer mannose residues, from five to ten mannose residues, and are characterized by the absence of sulfate residues and by the presence of fucose residues on endo-H-sensitive glycans. These glycans make their appearance coincident with the construction of tips on tight cell mounds. At this stage glycans characteristic of both early and late stages occur simultaneously. Developmental regulation of the wide array of protein-linked glycans expressed during the life cycle of Dictyostelium discoideum may be as simple as the controlled transition from a group of structures that are assembled by the vegetative cells to a group of structures that are assembled by the terminally differentiating cells. The potential biological significance of this transition is discussed.