Monoclonal antibody against cytoplasmic lectins of Dictyostelium discoideum: cross-reactivity with a membrane glycoprotein, contact site A, and with E. coli beta-galactosidase and lac repressor

Monoclonal antibodies were raised against two soluble, galactose-binding lectins from cells of Dictyostelium discoideum, discoidin I and II. These antibodies reacted not only with both discoidins, but also with a plasma membrane glycoprotein of aggregation competent cells, called contact site A, and with two carbohydrate-binding proteins of E. coli, beta-galactosidase and lac repressor. The possibility that the antibody recognizes a structure common to different carbohydrate-binding proteins is discussed. The two carbohydrate-binding proteins of E. coli share with discoidin I the sequence -Ser-X-X-Ile-His(Pro)-Pro(His)-Leu-Thr- which might be responsible for the cross-reactivity.

Responses of Dictyostelium discoideum amoebae to local stimulation by light

Single amoebae of Dictyostelium discoideum were locally stimulated with microbeams of white and monochromatic light. Low illuminance stimulation favored formation of pseudopodia at the irradiated parts of the cells, high illuminance stimulation locally suppressed the extension of pseudopodia. When the high illuminance light spot was placed on any portion of the cell other than the moving front, no response could be observed. The results are compatible with the assumption that, during their phototactic response, single amoebae detect the direction of light by a shadowing effect caused by pigments like cytochromes, and/or by light scattering of particles in the cytoplasm.

In vivo sulfation of the contact site A glycoprotein of Dictyostelium discoideum

During the development of Dictyostelium discoideum from the growth phase to the aggregation stage, a glycoprotein with an apparent mol. wt. of 80 kd is known to be expressed on the cell surface. This glycoprotein, referred to as contact site A, has been implicated in the formation of species-specific, EDTA-stable contacts of aggregating cells. When developing cells were labeled in vivo with [S]sulfate, the 80-kd glycoprotein was found to be the most prominently sulfated protein. Another strongly sulfated protein had an apparent mol. wt. of 130 kd and was, like the 80-kd glycoprotein, developmentally regulated and associated with the particulate fraction of the cells. The [S]sulfate incorporated into the 80-kd and 130-kd proteins was not present as tyrosine-O-sulfate, a modified amino acid found in many proteins of mammalian cells. D. discoideum cells incubated with [S]sulfate in the presence of tunicamycin, an inhibitor of N-glycosylation, produced a 66-kd protein that reacted with monoclonal antibodies raised against the 80-kd glycoprotein, but no longer contained [S]sulfate. These results suggest that sulfation of the 80-kd glycoprotein occurred on carbohydrate residues. The possible importance of sulfation for a role of the 80-kd glycoprotein in cell adhesion is discussed.

Myosin heavy chain kinase inactivated by Ca2+/calmodulin from aggregating cells of Dictyostelium discoideum

Soluble myosin heavy chain kinases (MHC kinases) were partially purified from growth phase and aggregation-competent cells of Dictyostelium discoideum. In the aggregation-competent cells, two MHC kinases were distinguishable. One of these enzymes, called MHC kinase II, was inactivated by Ca2+ and calmodulin in a highly temperature-dependent reaction. A MHC kinase found in growth phase cells did not have these regulatory properties. Substrate specificities were analysed for MHC kinase II and for the MHC kinase from growth phase cells. Both enzymes phosphorylated threonine residues of the myosin heavy chains of D. discoideum and Physarum polycephalum. Phosphopeptide mapping of D. discoideum myosin and determination of the stoichiometry of its phosphorylation suggested the presence of two phosphorylation sites per heavy chain. Both sites were contained within a 38-kd chymotryptic fragment. The inactivation of MHC kinase II by Ca2+ plus calmodulin suggests this enzyme has a role in the regulation of myosin functions during the chemotactic response of a cell. The phosphorylated myosin had about one third the actin-activated Mg2+-ATPase activity of the non-phosphorylated myosin. Previous findings indicated that stimulation of D. discoideum cells with the chemo-attractant cAMP increases the cytoplasmic Ca2+ concentration. Under these conditions MHC kinase II might be inhibited and the dephosphorylated, more active form of myosin would accumulate.

Monoclonal antibodies against contact sites A of Dictyostelium discoideum: detection of modifications of the glycoprotein in tunicamycin-treated cells

Tunicamycin acts on cell aggregation in Dictyostelium discoideum by changing cell movement and by inhibiting the EDTA-stable type of intercellular adhesion. Tunicamycin-treated cells show unco-ordinated pseudopodial activity such that pseudopods are simultaneously extended from all parts of the cell surface, and the cells are unable to move in straight paths. Concurrent with the inhibition of formation of EDTA-stable contacts, N-glycosylation of a glycoprotein specific for aggregation-competent cells is inhibited. This glycoprotein, previously called contact site A, has an apparent mol. wt. of 80 kilodaltons (kd). In membranes of tunicamycin-treated cells, two components are detected that react with certain monoclonal antibodies against contact sites A: one component of 66 kd, the other of 53 kd apparent mol. wt. Another group of monoclonal antibodies reacts only with the 80-kd glycoprotein and the 66-kd component. These results are in accord with the assumption that the glycoprotein carries two carbohydrate chains, and that the antibodies differ in their requirement for glycosylation of the antigen. Despite the coincidence between blockage of EDTA-stable cell adhesion and inhibited glycosylation of contact sites A, direct involvement of the carbohydrate moieties of this glycoprotein in intercellular adhesion seems questionable. EDTA-stable cell adhesion has not been blocked by Fab fragments from antibodies that specifically react with the glycosylated protein.

Improved purification and N-terminal amino acid sequence determination of the contact site A glycoprotein of Dictyostelium discoideum

On aggregating cells of Dictyostelium discoideum a specific glycoprotein is expressed which is absent from growth phase cells of this organism. The glycoprotein has been related to cell surface antigens, called contact sites A, that are defined by their reaction with aggregation blocking antibody fragments. In the present paper an improved purification scheme for the glycoprotein is described and its N-terminal sequence is given. The purification has been monitored by use of a monoclonal antibody. The contact site A glycoprotein has the properties of an integral membrane protein.

Stage-specific antigens reacting with monoclonal antibodies against contact site A, a cell-surface glycoprotein of Dictyostelium discoideum

Monoclonal antibodies against a glycoprotein presumably involved in adhesion of aggregating Dictyostelium discoideum cells have been used for labeling of the antigen at the cell surface. The antigen is distributed over the whole surface of the cells, apparently in form of small clusters. The antigen appears concomitantly with the acquisition of EDTA-stable adhesiveness typical of aggregation competent cells. In contrast, discoidin I, a lectin whose accumulation during development parallels EDTA-stable adhesiveness in another strain (NC-4), is present in nearly the same amounts of growth phase and aggregating cells of AX2-214, the strain used by use. Thus, no correlation exists in this strain between the expression of discoidin I and the development of cell adhesiveness. The 80 kilodalton glycoprotein typical of aggregation competent cells has been purified by affinity chromatography on a monoclonal antibody column. The purified antigen absorbs adhesion-blocking Fab from rabbits. Another antigen strongly reacting with the same monoclonal antibodies has an apparent molecular weight of 106 000 and is not detectable before slugs are formed.

Electron microscopic mapping of monoclonal antibodies on the tail region of Dictyostelium myosin

The binding sites of five monoclonal antibodies against myosin of Dictyostelium discoideum have been mapped. These antibodies bind to the tail region of the myosin molecule. By rotary shadowing, images of myosin-antibody complexes were obtained in which the mean distance of the midpoint of an antibody molecule from the myosin heads was localized with a precision better than 2 nm (90% confidence limit). Other quantitative data extracted from electron micrographs provided information on the stoichiometry of antibody-myosin interaction. Certain antibodies interacted with myosin molecules only at a ratio of 1:1. Other antibodies formed complexes of two molecules bound to homologous sites on a double-stranded myosin tail. Affinities were estimated and the abilities of different antibodies to cross-connect two myosin molecules were evaluated.

Chemotactic reorientation of granulocytes stimulated with micropipettes containing fMet-Leu-Phe

Human granulocytes were stimulated by means of a micropipette, with an orifice of about 0.2 micrometer in diameter, which contained fMet-Leu-Phe at a concentration of 10(-5) M. The cells were reorientated by extending lamellipodia towards the source of the attractant, often within less than 10 s. Any part of the granulocyte, from the front to the tip of the tail, could be stimulated to produce new lamellipodia. Usually, but not always, this response occurred at the side of the cell nearest to the micropipette. Cells stimulated from behind responded in one of the following ways: (1) Cells that maintained their polarity extended new lamellipodia at one side of the leading front and reorientated by moving in a U-turn towards the micropipette. Occasionally, the leading front was split because one part of the front tried to make a left-hand and the other a right-hand turn. (2) Formation of lamellipodia at the leading front was arrested and new lamellipodia were formed at the tail instead, indicating reversal of polarity. The result was an immediate change in the direction of locomotion by about 180 degrees. (3) A combination of the first 2 forms of behaviour was observed occasionally. Transiently, lamellipodia were extended from cell surface areas both close to and distant from the micropipette. These observations show that parts of a cell can respond independently to chemotactic gradients by extending lamellipodia towards the source of the attractant. The phenomenon can easily be explained by assuming that a temporal change of attractant concentration is recognized.

Folate deaminase and cyclic AMP phosphodiesterase in Dictyostelium discoideum: their regulation by extracellular cyclic AMP and folic acid

Cyclic AMP and folic acid act as chemotactic factors in Dictyostelium discoideum. Both agents, when applied extracellularly, also control cell development from the growth stage to the acquisition of aggregation competence. Cyclic AMP phosphodiesterase and folate deaminase are extracellular enzymes whose activity is regulated during early differentiation of D. discoideum cells. The two enzymes help control the extracellular levels of cyclic AMP and folic acid. The substrates cyclic AMP and folic acid each increase the extracellular activity of folate deaminase as well as phosphodiesterase. The specificity of extracellular phosphodiesterase regulation by cyclic AMP indicates that the effect is mediated by specific cyclic AMP receptors rather than the catalytic site of cell surface phosphodiesterase. To some extent cyclic AMP and folic acid are interchangeable with respect to regulating differentiation and enhancing enzymatic inactivation of intercellular signals. Thus the two extracellular signals may share a common cellular pathway of signal transduction. The regulation of folate deaminase and phosphodiesterase by folic acid does not always parallel the folic acid effects on development. Pulses of folic acid stimulate development of aggregation competence, whereas a continuous flux inhibits. In contrast, either continuous flux or pulses of folic acid increase the deaminase and phosphodiesterase activities.

Protein-bound mono(ADP-ribose) residues in differentiating cells of Dictyostelium discoideum

Changes in protein-bound mono(ADP-ribose) residues during development of Dictyostelium discoideum were determined. NAD(H) levels and the amounts of the NH2OH resistant and sensitive subfractions of mono(ADPR) were found not to be different between exponentially growing and aggregation-competent cells in which mitosis had ceased. Divergent changes occurred at the differentiation stages following aggregation as indicated by an increase in the ratio of these subfractions from approx. 1 during the growth plase and aggregation competent stage to 2 in the grex, a stage which follows cell aggregation. The fraction of NH2OH sensitive conjugates closely followed the changes in total cellular protein, while the NH2OH resistant ADPR protein conjugates, when based on protein, increased during the stages following aggregation. NAD(H) and NADP(H) levels per unit DNA decreased significantly during this period. The mono(ADPR) to DNA ratio in D. discoideum is comparable to that in proliferating Physarum polycephalum and to non-proliferating adult rat liver. The total amount of mono(ADPR) residues per nucleus is, however, approximately 70-fold higher in the liver, indicating that the quantity of mono(ADPR) residues is more closely related to the size of the eukaryotic genome than to cell proliferation.

A transient rise in cyclic AMP levels following chemotactic stimulation of neutrophil granulocytes

A short transient rise of cyclic AMP is observed within 1 minute after primary stimulation of neutrophils with chemotactic serum peptides containing classical anaphylatoxin (CAT). A second administration of these peptides after two minutes failed to produce a second peak of cAMP. Human serum albumin (HSA) which has chemokinetic but no chemotactic activities did not change cAMP levels. There was no significant change in cGMP levels within 1 minute following stimulation of rabbit neutrophils with chemotactic peptides or HSA.

A membrane glycoprotein of aggregating Dictyostelium cells with the properties of contact sites

Aggregating cells of Dictyostelium discoideum form EDTA-stable contacts which are blocked by a Fab (antigen-binding fragment) preparation from antisera raised against membranes. The target site of the blocking Fab fragments has been identified as a specific glycoprotein. In this paper its purification, carbohydrate and amino acid composition are described. Purification was 800-fold, starting with cells lysed by digitonin. The plasma membranes, preserved as ghosts by this treatment, were purified in a two-phase system and extracted with butan-1-ol. The water phase contained predominantly concanavalin-A-binding glycoproteins and was particularly rich in contact sites A. These were further purified on DE-cellulose and sucrose gradients. Sodium dodecylsulphate/polyacrylamide gel electrophoresis of the purified material revealed one major glycoprotein band in the molecular weight region of 80 000 to 90 000, depending on the acrylamide concentration. The sugars found in contact sites A were mannose, N-acetylglucosamine, fucose, and possibly glucose. The protein moeity contained 8% proline and was particularly rich in hydroxy amino acids.

Oscillations of cyclic nucleotide concentrations in relation to the excitability of Dictyostelium cells

Aggregating cells of Dictyostelium discoideum are able to release cyclic AMP periodically. The oscillations of cAMP generation are associated with changes in adenylate cyclase activity. Cyclic AMP receptors on the cell surface are functionally coupled to the oscillating system as evidenced by phase shifts that are induced by small pulses of extracellular cAMP. An important element of the oscillating system is the signal processing from surface receptors to the adenylate cyclase. This pathway exhibits adaptation resulting in the suppression of responses to constant, elevated concentrations of cAMP. The signal input for adenylate cyclase activation is, therefore, a change in the extracellular cAMP concentration with time. Oscillations in the absence of detectable changes of intra- or extracellular cAMP concentrations suggest the possibility that there is a metabolic network in D. discoideum cells that undergoes oscillations without coupling to adenylate cyclase. Cyclic GMP concentrations oscillate with a slight phase difference in advance of that of cAMP, suggesting that the two nucleotide cyclases might not be activated by the same mechanism. Elevation of extracellular calcium exerts an inhibitory effect on the accumulation of cAMP and on the second of the two cGMP peaks.

Cell surface protein kinases in Dictyostelium: are they artifacts?

Evidence for cell surface protein kinases as possible regulatory factors of cell interaction in Dictyostelium discoideum was examined by incubating intact cells with gamma 32P-ATP in the presence and absence of histone. No significant incorporation of 32P was detected in the absence of histone. In its presence strong phosphorylation not only of the histone but also of endogenous proteins was obtained. This was due to the fact that histone made the cell membranes permeable for substrates and proteinkinases. Histone also preserved protein kinase activities which were otherwise lost during homogenization. The total protein kinase activity in histone treated cells was 5 fold higher than in sonicated cells.

Action of a slowly hydrolysable cyclic AMP analogue on developing cells of Dictyostelium discoideum

Adenosine 3′,5′-cyclic phosphorothioate (cAMP-S) is a cyclic AMP (cAMP) analogue which is only slowly hydrolysed by phosphodiesterases of Dictyostelium discoideum. The affinity of cAMP-S to cAMP receptors at the cell surface is only one order of magnitude lower than that of cAMP. cAMP-S can replace cAMP as a stimulant with respect to all receptor-mediated responses tested, including chemotaxis and the induction of cAMP pulses. cAMP-S does not affect growth of D. discoideum but it blocks cell aggregation at a uniform concentration of 5 × 10(−7) M in agar plate cultures of strain NC-4 as well as its axenically growing derivative, Ax-2. Another wild-type strain of D. discoideum, v-12, is able to aggregate on agar plates supplemented with 1 mM cAMP-S. The development of Polysphondylium pallidum and P. violaceum is also highly cAMP-S resistant. In Ax-2 both differentiation from the growth phase to the aggregation-competent stage and chemotaxis are cAMP-S sensitive, whereas in v-12 only chemotaxis is inhibited. v-12 can still form streams of cohering cells and fruiting bodies when chemotaxis is inhibited by cAMP-S. Whereas cAMP induces differentiation into stalk cells at concentrations of 10(−3) or 10(−4) M, cAMP-S has the same effect in strain v-12 at the much lower concentration of 10(−6) M.

Specific binding proteins for cyclic AMP and cyclic GMP in Dictyostelium discoideum

In Dictyostelium discoideum both cyclic AMP and cyclic GMP are regulated by chemotactic stimuli. Binding proteins specific for cAMP and cGMP have been found in aggregation competent cells as well as in cells harvested during growth. The activity of binding proteins was, on the average, lower in the growth phase cells. cAMP binding proteins were separated into 3 fractions, whereas the cGMP binding activity appeared in 1 major peak both on DEAE-cellulose and Sephadex G-200. Protein kinase activity was present in most but not all cyclic necleotide binding fractions; evidence for a relationship is however missing.

A specific glycoprotein as the target site of adhesion blocking Fab in aggregating Dictyostelium cells

During the acquisition of aggregation competence a new antigen appears on the surface of Dictyostelium cells. Univalent antibody fragments (Fab) against this antigen render the cells unable to form the specific type of cell adhesion which is characteristic of aggregating cells. This membrane constituent has been purified and identified as a concanavalin A-binding glycoprotein present at about 2 X 10(5) copies per cell.

PH oscillations in cell suspensions of Dictyostelium discoideum: their relation to cyclic-amp signals

Cells of Dictyostelium discoideum known to release cyclic AMP (cAMP) rhythmically in the form of pulses, change with the same period of about 8 min the pH of their medium. The pH is used here as an indicator to investigate the effect of externally added cAMP pulses on the oscillations. Both a temporary increase in amplitude and a permanent phase shift can be induced. The phase-response curve indicates that the period can be increased and decreased by rhythmic stimulation with cAMP pulses.

Cyclic-AMP stimulated calcium influx into aggregating cells of Dictyostelium discoideum

Within about 10 seconds after stimulation of Dictyostelium discoideum cells with cyclic AMP an increased rate of 45Ca influx was observed. Part of the cellular calcium reappeared in the extra-cellular medium between 1 and 3 minutes after stimulation. No effect of 5'AMP on calcium distribution was found. The transient calcium influx is discussed in connection with chemotaxis and other cyclic-AMP induced responses.

Cyclic GMP regulation and responses of Polysphondylium violaceum to chemoattractants

In cells of the cellular slime mold Polysphondylium violaceum an attractant, which is released during the aggregation stage, causes a transient rise of the cyclic GMP concentration. Cells of this organism develop in shaken suspensions after they have finished growth. Cell development is not accompanied by an increase in the EDTA stability of cell adhesion. Both the developmental regulation and the specificity of chemotactic responses is reflected in the light scattering patterns recorded in cell suspensions: Folic acid causes a strong response in early preaggregation cells and the Polysphondylium attractant does the same in aggregation competent cells, whereas cyclic AMP is inactive in both stages.

Implication of developmentally regulated Concanavalin A binding proteins of Dictyostelium in cell adhesion and cyclic AMP regulation

Contact sites A and cyclic AMP phosphodiesterase are Concanavalin A binding membrane proteins. Both are characteristic for the aggregation phase of Dictyostelium discoideum. Extracellular cyclic AMP phosphodiesterase and an inhibitor of this enzyme can be recovered from the extracellular medium by binding to Concanavalin A-Sepharose.

Adenylyl cyclase and the control of cell differentiation in Dictyostelium dicoideum

Adenylyl cyclase is part of a biochemical network that controls cell differentiation in Dictyostelium discoideum. At a certain stage of development the enzyme is rhythmically activated, with periods of about 8 min. These oscillations are superimposed upon an increase of the basal activity extending over a period of hours. The basal activity remains low in a mutant blocked at an early stage of development. In strain Ax-2 two periods of strongly increasing basal activity have been found: the first from 2 to 4 h after the end of the growth phase, the other beginning at about 8 h. Based on the periodic regulation of adenylyl cyclase, cyclic AMP is released into the extracellular space in the form of pulses. Application of cyclic-AMP pulses, but not its continuous influx, stimulates the increase of basal adenylyl cyclase activity. Two other constituents of the cyclic-AMP signal system cyclic-AMP receptors and cell-surface phosphodiesterase, are similarly controlled. The principal importance of positive feedback loops in a network controlling cell differentiation is discussed.

Cell aggregation and sexual differentiation in pairs of aggregation-deficient mutants of Dictyostelium discoideum

A diffusible aggregation-stimulating factor (ASF) is released from a series of aggregationdeficient mutants. Biochemical markers indicate that these ASF-donor mutants are blocked at a later step of cell differentiation than an ASF-requiring mutant. ASF is able to bridge the initial block of differentiation in the latter mutant, such that development proceeds up to aggregation and even further. ASF is most probably neither identical with cyclic-AMP phosphodiesterase nor with an inhibitor of this enzyme which both are released from donor strains.

In certain combinations of aggregation-deficient mutants, macrocysts, the sexual stages of Dictyostelium, are formed. Also, motile giant cells believed to be zygotes are observed in these mutant combinations. The gamone known to be released from one mating type and to induce macrocysts in the other, is probably not identical with ASF since this factor is produced by mutants derived from either one of both mating types.

Cell aggregation and sexual differentiation in pairs of aggregation-deficient mutants of Dictyostelium discoideum

A diffusible aggregation-stimulating factor (ASF) is released from a series of aggregation-deficient mutants. Biochemical markers indicate that these ASF-donor mutants are blocked at a later step of cell differentiation than an ASF-requiring mutant. ASF is able to bridge the initial block of differentiation in the latter mutant, such that development proceeds up to aggregation and even further. ASF is most probably neither identical with cyclic-AMP phosphodiesterase nor with an inhibitor of this enzyme which both are released from donor strains. In certain combinations of aggregation-deficient mutants, macrocysts, the sexual stages of Dictyostelium, are formed. Also, motile giant cells believed to be zygotes are observed in these mutant combinations. The gamone known to be released from one mating type and to induce macrocysts in the other, is probably not identical with ASF since this factor is produced by mutants derived from either one of both mating types.

Extracellular cyclic-amp phosphodiesterase regulation in agar plate cultures of Dictyostelium discoideum

Extracellular cyclic-AMP phosphodiesterase and the inhibitor of this enzyme is tested in agar plate cultures of two Dictyostelium discoideum wild-type strains and in a mutant which lacks the inhibitor. Under the conditions used, the phosphodiesterase inhibitor is formed in both wild-type strains either before or in an early stage of cell aggregation. During aggregation of one strain the phosphodiesterase activity is extremely low, excluding a necessary function of the enzyme in the aggregation process.

Cell communication by periodic cyclic-AMP pulses

At the surface of aggregating cells of the slime mould, Dictyostelium discoideum, two different sites interacting with extracellular cAMP are detectable: binding sites and cycl-nucleotide phosphodiesterase. Both sites are developmentally regulated. An adequate stimulus for the chemoreceptor system in D. discoideum is the change of cAMP concentration in time, rather than concentration per se: long-term binding of cAMP causes only short-term response. The system is, consequently, adapted to the recognition of pulses rather than to steady-state concentrations of cAMP. The ce,lls are, nevertheless, able to sense stationary spatial gradients and to respond to them by chemotactic orientation. The possibility is discussed that they do so by transforming spatial concentration changes into temporal ones, using extending pseudopods as sensors. The cAMP recognition system is part of a molecular network involved in the generation of spatio-temporal patterns of cellular activities. This system controls the periodic formation of chemotactic signals and their propagation from cell to cell. The phosphodiesterase limits the duration of the cAMP pulses and thus sharply separates the periods of signalling; the binding sites at the cell surface are supposed to be the chemoreceptors. The control of cellular activities via cAMP receptors can be studied with biochemical techniques with cell suspensions in which spatial inhomogeneities are suppressed by intense stirring, whereas the temporal aspect of the spatiotemporal pattern is preserved. Under these conditions it can be shown that the extracellular cAMP concentration changes periodically, and that the phase of the cellular oscillator can be shifted by external pulses of cAMP. It can also be shown that small cAMP pulses induce a high output of cAMP, which demonstrates signal amplification, a function necessary for a cellular relay system.

Short-term binding and hydrolysis of cyclic 3':5'-adenosine monophosphate by aggregating Dictyostelium cells

Transient binding of cyclic AMP to aggregating cells of Dictyostelium discoideum was measured under cyclic GMP excess in order to inhibit cyclic AMP hydrolysis by cell-bound phosphodiesterase. Cyclic GMP extended the period of half-maximal cyclic AMP binding from less than 5 sec to 1-2 min. With the same time course as bound cyclic AMP was released from the cells, labeled 5'-AMP appeared in the medium. Specificity, kinetics, and developmental regulation suggest that the cyclic AMP-binding sites exposed in living cells are identical with receptor sites for the chemotactic response. The functioning of the cyclic AMP-receptor/phosphodiesterase system and its formal similarity with the synaptic acetylcholine-receptor/esterase system are discussed.

Cyclic-AMP-controlled oscillations in suspended Dictyostelium cells: their relation to morphogenetic cell interactions

Periodic spikes of decreased optical density were recorded in cell suspensions of Dictyostelium discoideum. Spike formation as well as changes in the redox state of cytochrome b are facultatively and independently coupled to an oscillating system which is under developmental control and presumably underlies signal transmission in aggregating cells. Cyclic AMP triggers a double response, the slow component resembling the spikes formed during spontaneous oscillations. The fast component shows characteristics of the chemotactic response to cyclic AMP. The receptor system is suggested to sense changes of cyclic AMP concentration in time. Cyclic AMP pulses interact with the oscillating system, resulting in phase shift or suppression of spike formation, and in the induction of oscillations in an early stage of development before the onset of spontaneous oscillations. Continuous flow application of cyclic AMP does not change frequency up to flow rates which extinguish oscillations.

Quantitation of membrane sites in aggregating Dictyostelium cells by use of tritiated univalent antibody

Cell-to-cell adhesion during aggregation of Dictyostelium discoideum cells is completely blocked by univalent antibody (Fab) directed against two classes of target sites: surface structures characteristic for aggregation-competent cells ("contact sites A") and others present also on growth-phase cells ("contact sites B"). 3 x 10(5) Fab molecules bound per cell are sufficient to block contact sites A completely, although the Fab fragments cover not more than 2% of the total cell surface. Up to 8-fold this value can be bound per cell when Fab fragments of another specificity are used, without affecting activity of contact sites A. Blockage of cell-to-cell adhesion therefore depends on the binding of Fab fragments to specific target sites, rather than on the total number of Fab molecules bound per cell. This conclusion is also valid for cell adhesion attributed to contact sites B. Contact sites therefore represent a special class of cell-surface sites which, in cell homogenates as well as in vivo, can be traced by Fab, and which are not identical with the bulk of cell-surface antigens present on aggregating cells.

Dynamics of antigenic membrane sites relating to cell aggregation in Dictyostelium discoideum

Membrane interaction in aggregating cells of Dictyostelium discoideum can be blocked by univalent antibodies directed against specific membrane sites. Using a quantitative technique for measuring cell association, two classes of target sites for blocking antibodies were distinguished and their developmental dynamics studied. One class of these sites is specific for aggregation-competent cells, their quantity rising from virtually 0-level during growth, with a steep increase shortly before cell aggregation. The serological activity of these structures is species specific; they are not detectable in a nonaggregating mutant, but present in a revertant undergoing normal morphogenesis. Patterns of cell assembly in the presence of antibodies show that selective blockage of these membrane sites abolishes the preference for end-to-end association which is typical for aggregating cells. A second class of target sites is present in comparable quantities in particle fractions from both growth-phase and aggregation-competent cells. Blockage of these sites leads to aggregation patterns in which the side-by-side contacts of aggregating cells are abolished. The target sites of aggregation-inhibiting antibodies are suggested to be identical or associated with the molecular units of the cell membrane that mediate cell-to-cell contacts during aggregation. The results indicate that in one cell, two independent classes of contact sites can be simultaneously active.