Binding of inhibitor alters kinetic and physical properties of extracellular cyclic AMP phosphodiesterase from Dictyostelium discoideum

Extracellular Processing of Molecular Gradients by Eukaryotic Cells Can Improve Gradient Detection Accuracy

Eukaryotic cells sense molecular gradients by measuring spatial concentration variation through the difference in the number of occupied receptors to which molecules can bind. They also secrete enzymes that degrade these molecules, and it is presently not well understood how this affects the local gradient perceived by cells. Numerical and analytical results show that these enzymes can substantially increase the signal-to-noise ratio of the receptor difference and allow cells to respond to a much broader range of molecular concentrations and gradients than they would without these enzymes.

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.

A cAMP signaling model explains the benefit of maintaining two forms of phosphodiesterase in Dictyostelium

Starving Dictyostelium cells respond chemotactically to cell-generated waves of cyclic adenosine -3',5'- monophosphate (cAMP) that guide cell aggregation toward a signaling center. In this process, a large number of cells are recruited, resulting in the formation of aggregation territories that are essential for fruiting body formation. The enzyme PdsA phosphodiesterase (PDE), a crucial component of the signaling system, breaks down the external cAMP and can be either membrane-bound or secreted. The existence of two such forms is unusual in cell biology, and it remains to be determined why they have both been maintained through evolution. Here, using a model of the cAMP signaling system, I show that colonies can successfully organize into aggregates over a wider range of initial cell densities when both forms of PDE are present in an appropriately tuned ratio than when only a single form is present. The model indicates that membrane-bound PDE maintains aggregation-territory integrity in colonies with high initial cell density, whereas the secreted form is important for wave propagation at low cell densities. Thus, the ultimate retention of both forms can increase territory size. These findings have implications for other excitable media, including Ca(2+) propagation in cardiac cells and propagation of electrical excitation in nerve axons, since these systems have similar features of spatial nonuniform "release" and "degradation" of the relevant signals.

cAMP signaling in Dictyostelium. Complexity of cAMP synthesis, degradation and detection

cAMP plays a pivotal role in control of cell movement, differentiation and response to stress in all phases of the Dictyostelium life cycle. The multitudinous functions of cAMP require precise spatial and temporal control of its production, degradation and detection. Many novel proteins have recently been identified that critically modulate the cAMP signal. We focus in this review on the properties and functions of the three adenylyl cyclases and the three cAMP-phosphodiesterases that are present in Dictyostelium, and the network of proteins that regulate the activity of these enzymes. We also briefly discuss the two modes of detection of cAMP.

Characterization of a cAMP-stimulated cAMP phosphodiesterase in Dictyostelium discoideum

A cyclic nucleotide phosphodiesterase, PdeE, that harbors two cyclic nucleotide binding motifs and a binuclear Zn(2+)-binding domain was characterized in Dictyostelium. In other eukaryotes, the Dictyostelium domain shows greatest homology to the 73-kDa subunit of the pre-mRNA cleavage and polyadenylation specificity factor. The Dictyostelium PdeE gene is expressed at its highest levels during aggregation, and its disruption causes the loss of a cAMP-phosphodiesterase activity. The pdeE null mutants show a normal cAMP-induced cGMP response and a 1.5-fold increase of cAMP-induced cAMP relay. Overexpression of a PdeE-yellow fluorescent protein (YFP) fusion construct causes inhibition of aggregation and loss of the cAMP relay response, but the cells can aggregate in synergy with wild-type cells. The PdeE-YFP fusion protein was partially purified by immunoprecipitation and biochemically characterized. PdeE and its Dictyostelium ortholog, PdeD, are both maximally active at pH 7.0. Both enzymes require bivalent cations for activity. The common cofactors Zn(2+) and Mg(2+) activated PdeE and PdeD maximally at 10 mm, whereas Mn(2+) activated the enzymes to 4-fold higher levels, with half-maximal activation between 10 and 100 microm. PdeE is an allosteric enzyme, which is approximately 4-fold activated by cAMP, with half-maximal activation occurring at about 10 microm and an apparent K(m) of approximately 1 mm. cGMP is degraded at a 6-fold lower rate than cAMP. Neither cGMP nor 8-Br-cAMP are efficient activators of PdeE activity.

A cell-counting factor regulating structure size in Dictyostelium

Developing Dictyostelium cells form large aggregation streams that break up into groups of 0.2 x 10(5) to 1 x 10(5) cells. Each group then becomes a fruiting body. smlA cells oversecrete an unknown factor that causes aggregation streams to break up into groups of approximately 5 x 10(3) cells and thus form very small fruiting bodies. We have purified the counting factor and find that it behaves as a complex of polypeptides with an effective molecular mass of 450 kD. One of the polypeptides is a 40-kD hydrophilic protein we have named counting. In transformants with a disrupted counting gene, there is no detectable secretion of counting factor, and the aggregation streams do not break up, resulting in huge (up to 2 x 10(5) cell) fruiting bodies.

A Dictystelium mutant with defective aggregate size determination

Starved Dictyostelium cells aggregate into groups of roughly 10(5) cells. We have identified a gene which, when repressed by antisense transformation or homologous recombination, causes starved cells to form large numbers of small aggregates. We call the gene smlA for small aggregates. A roughly 1.0 kb smlA mRNA is expressed in vegetative and early developing cells, and the mRNA level then decreases at about 10 hours of development. The sequence of the cDNA and the derived amino acid sequence of the SmlA protein show no significant similarity to any known sequence. There are no obvious motifs in the protein or large regions of hydrophobicity or charge. Immunofluorescence and staining of Western blots of cell fractions indicates that SmlA is a 35x10(3) Mr cytosolic protein present in all vegetative and developing cells and is absent from smlA cells. The absence of SmlA does not affect the growth rate, cell cycle, motility, differentiation, or developmental speed of cells. Synergy experiments indicate that mixing 5% smlA cells with wild-type cells will cause the wild-type cells to form smaller fruiting bodies and aggregates. Although there is no detectable SmlA protein secreted from cells, starvation medium conditioned by smlA cells will cause wild-type cells to form large numbers of small aggregates. The component in the smlA-conditioned media that affects aggregate size is a molecule with a molecular mass greater than 100x10(3) Mr that is not conditioned media factor, phosphodiesterase or the phosphodiesterase inhibitor. The data thus suggest that the cytosolic protein SmlA regulates the secretion or processing of a secreted factor that regulates aggregate size.

Origin and evolution of circular waves and spirals in Dictyostelium discoideum territories

Randomly distributed Dictyostelium discoideum cells form cooperative territories by signaling to each other with cAMP. Cells initiate the process by sending out pulsatile signals, which propagate as waves. With time, circular and spiral patterns form. We show that by adding spatial and temporal noise to the levels of an important regulator of external cAMP levels, the cAMP phosphodiesterase inhibitor, we can explain the natural progression of the system from randomly firing cells to circular waves whose symmetries break to form double- and single- or multi-armed spirals. When phosphodiesterase inhibitor is increased with time, mimicking experimental data, the wavelength of the spirals shortens, and a proportion of them evolve into pairs of connected spirals. We compare these results to recent experiments, finding that the temporal and spatial correspondence between experiment and model is very close.

Purification and properties of periplasmic 3':5'-cyclic nucleotide phosphodiesterase. A novel zinc-containing enzyme from the marine symbiotic bacterium Vibrio fischeri

The 3':5'-cyclic nucleotide phosphodiesterase (CNP) of Vibrio fischeri, due to its unusual location in the periplasm, allows this symbiotic bacterium to utilize extracellular 3':5'-cyclic nucleotides (e.g. cAMP) as sole sources of carbon and energy, nitrogen, and phosphorus for growth. The enzyme was purified to apparent homogeneity by a four-step procedure: chloroform shock, ammonium sulfate precipitation, and chromotography on DEAE-Sephacel and Cibacron Blue 3GA-agarose. The active enzyme consists of a single polypeptide with a mass of 34 kDa. At 25 degrees C, it has a pH optimum of 8.25, a Km for cAMP of 73 microns, and a Vmax of 3700 mumol of cAMP hydrolyzed/min/mg protein (turnover number of 1.24 x 10(5)/min). The specific activity of the V. fischeri enzyme is approximately 20-fold greater than that of any previously characterized CNP when comparisons of activity are made at the same assay temperature. Activity increases with temperature up to 60 degrees C. The CNP contains 2 atoms of zinc/monomer, and zinc, copper, magnesium, and calcium can restore activity of the apoenzyme to varying degrees. The exceptional specific activity of the enzyme and its unusual location in the periplasm support proposals that the enzyme enables the bacterium to scavenge 3':5'-cyclic nucleotides in seawater and that the enzyme plays a role in cAMP-mediated host-symbiont interactions.

Involvement of intracellular calcium in protein secretion in Dictyostelium discoideum

We reported previously that Ca2+ depletion of Dictyostelium discoideum cells severely inhibits extracellular cyclic nucleotide phosphodiesterase (PD) synthesis at a post-transcriptional step. In this study, further experiments were performed to learn more about the nature of this phenomenon. Examination of the polysomal distribution of PD transcripts in control cells and in cells depleted of Ca2+ by incubation with EGTA and A23187 (EA) suggested that inhibition of PD production does not involve translational control. Kinetic analysis of this inhibitory process revealed that soluble, intracellular PD activity, synthesized from either the 2.4 or 1.9 kb PD mRNA, decreased very rapidly upon addition of EA. Furthermore, this decrease in activity was accompanied by the preferential loss of PD-related polypeptides, indicating a proteolytic event. EA-induced PD degradation required cellular energy and concomitant protein synthesis but was unaffected by most of the lysosomotropic agents tested. Therefore, PD proteolysis might not occur in the lysosome. In cell fractionation experiments, the EA-sensitive, intracellular PD activity comigrated with a rough ER marker in Percoll/KCl gradients. In addition to its effect on the PD, EA were also observed to inhibit production and rapidly lower the intracellular levels of another secreted glycoprotein, the PD inhibitor. Together, these results suggest that depletion of some intracellular Ca2+ store(s) in Dictyostelium, possibly the ER, disrupts the normal function of the secretory pathway, resulting in selective degradation of certain proteins.

Chemotactic sorting to cAMP in the multicellular stages of Dictyostelium development

Dictyostelium transformants that overproduce the extracellular form of cyclic nucleotide phosphodiesterase and so accumulate a reduced amount of cAMP are blocked in development after aggregation in the form of a tight mound, prior to formation of the apical tip. In such mounds, prespore cell differentiation is repressed, and the apical accumulation of prestalk cells is greatly retarded. When a source of cAMP is placed below the arrested mounds, prestalk cells that would normally migrate in an apical direction instead sort downwards to the substratum. Thus, by acting as the chemoattractant that draws prestalk cells to the apex, cAMP signaling directs the formation of a patterned structure.

Cyclic nucleotide phosphodiesterase of Dictyostelium discoideum and its glycoprotein inhibitor: structure and expression of their genes

The genes coding for the cyclic nucleotide phosphodiesterase (PD) and the PD inhibitory glycoprotein (PDI) have been cloned and characterized. The PDI gene was isolated as a 1.6 kb genomic fragment, which included the coding sequence containing two small introns and 510 nucleotides of non-translated 5' sequence. From the deduced amino acid sequence we predict a protein with a molecular weight (MW) of 26,000 that, in agreement with previous data, contains 15% cysteine residues. Genomic Southern blot analysis indicates that only one gene encodes the inhibitor. Northern blot analysis shows a single transcript of 0.95 kb. The PDI gene is expressed early in development with little transcript remaining following aggregation. The appearance of PDI mRNA is prevented by the presence of cAMP, but when cAMP is removed the transcript appears within 30 minutes. When cAMP is applied to cells expressing PDI the transcript disappears with a half-life of less than 30 minutes. The PD gene of D. discoideum is transcribed into three mRNAs: a 1.9 kb mRNA specific for growth, a 2.4 kb mRNA specific for aggregation, and a 2.2 kb mRNA specific for late development. The 2.2 kb mRNA is also specific for prestalk cells, and is induced by differentiation-inducing factor. All three mRNAs contain the same coding sequence, and differ only in their 5' non-coding sequences. Each mRNA is transcribed from a different promoter, and by using the chloramphenicol acyltransferase gene as a reporter, we have shown that each promoter displays the same regulation as its cognate mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of DdrasG gene expression during Dictyostelium development

DdrasG gene expression during the early development of Dictyostelium discoideum has been examined in detail. The amount of DdrasG-specific mRNA increased approximately twofold during the first 2 to 3 h of development and then declined rapidly, reaching negligible levels by the aggregation stage. The increase in mRNA levels that occurred during the first 2 to 3 h of development also occurred during differentiation in cell suspensions and was enhanced when cells were shaken rapidly. This initial increase was unaffected by cell density. When cells were set up to differentiate on filters, the addition of a glucose-amino acid mixture slightly delayed differentiation and had a similar effect on the expression of the gene. The decline in DdrasG expression during development did not occur when cells were treated with cycloheximide, suggesting that the expression of a developmentally regulated gene product is essential for the reduction of DdrasG gene mRNA. There was no decrease in DdrasG mRNA level during differentiation in shake suspension, but the decrease did occur upon application of pulses of cyclic AMP to shaking cultures. The application of a continuously high level of cyclic AMP delayed the increase in expression of the gene and did not result in the subsequent decline. These results suggest that the induction of a functional cyclic AMP relay system is important in reducing DdrasG gene mRNA levels.

Regulation of DdrasG gene expression during Dictyostelium development

DdrasG gene expression during the early development of Dictyostelium discoideum has been examined in detail. The amount of DdrasG-specific mRNA increased approximately twofold during the first 2 to 3 h of development and then declined rapidly, reaching negligible levels by the aggregation stage. The increase in mRNA levels that occurred during the first 2 to 3 h of development also occurred during differentiation in cell suspensions and was enhanced when cells were shaken rapidly. This initial increase was unaffected by cell density. When cells were set up to differentiate on filters, the addition of a glucose-amino acid mixture slightly delayed differentiation and had a similar effect on the expression of the gene. The decline in DdrasG expression during development did not occur when cells were treated with cycloheximide, suggesting that the expression of a developmentally regulated gene product is essential for the reduction of DdrasG gene mRNA. There was no decrease in DdrasG mRNA level during differentiation in shake suspension, but the decrease did occur upon application of pulses of cyclic AMP to shaking cultures. The application of a continuously high level of cyclic AMP delayed the increase in expression of the gene and did not result in the subsequent decline. These results suggest that the induction of a functional cyclic AMP relay system is important in reducing DdrasG gene mRNA levels.

Cyclic AMP oscillations in suspensions of Dictyostelium discoideum

A model developed previously for signal relay and adaptation in the cellular slime mould Dictyostelium discoideum is shown to account for the observed oscillations of calcium and cyclic AMP in cellular suspensions. A qualitative argument is given which explains how the oscillations arise, and numerical computations show how characteristics such as the period and amplitude of the periodic solutions depend on parameters in the model. Several extensions of the basic model are investigated, including the effect of cell aggregation and the effect of time delays in the activation and adaptation processes. The dynamics of mixed cell populations in which only a small fraction of the cells are capable of autonomous oscillation are also studied.

Rescue of a Dictyostelium discoideum mutant defective in cyclic nucleotide phosphodiesterase

One of the developmentally induced gene products that is essential for chemotaxis of Dictyostelium amoebae is a cyclic nucleotide phosphodiesterase. The enzyme can be secreted or exist in a membrane bound form. This enzyme is missing in the mutant HPX235 which, as a consequence, does not aggregate unless exogenous cAMP phosphodiesterase is supplied. We have introduced multiple copies of the cloned phosphodiesterase gene into mutant amoebae and restored aggregation. The formation of anatomically correct fruiting bodies, which does not occur when exogenous enzyme is added, is also restored by transformation with the gene. The construct that we have used gives rise only to secreted phosphodiesterase and therefore the membrane bound form of the enzyme is not absolutely required for normal aggregation and morphogenesis.

Genetics of early Dictyostelium discoideum development

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The cyclic nucleotide phosphodiesterase of Dictyostelium discoideum: the structure of the gene and its regulation and role in development

The cyclic nucleotide phosphodiesterase (phosphodiesterase) of Dictyostelium discoideum plays an essential role in development by hydrolyzing the cAMP used as a chemoattractant by aggregating cells. We have studied the biochemistry of the phosphodiesterase and a functionally related protein, the phosphodiesterase inhibitor protein, and have cloned the cognate genes. A 1.8-kb and a 2.2-kb mRNA are transcribed from the single-phosphodiesterase gene. The 2.2-kb mRNA comprises the majority of the phosphodiesterase mRNA found in differentiating cells and is transcribed only during development from a promoter at least 2.5 kb upstream of the translational start site. The 1.8-kb phosphodiesterase mRNA is detected at all stages of growth and development, is present at lower levels than the developmentally induced mRNA, and is transcribed from a site proximal to the protein-coding region. The phosphodiesterase gene contains a minimum of three exons, and a 2.3-kb intron, the longest yet reported for this organism. We have shown that the pdsA gene and four fgd genes affect the accumulation of the phosphodiesterase mRNAs, and we believe that these loci represent a significant portion of the genes regulating expression of the phosphodiesterase. The phosphodiesterase gene was introduced into cells by transformation and used as a tool to explore the effects of cAMP on the terminal stages of development. In cells expressing high levels of phosphodiesterase activity, final morphogenesis cannot be completed, and differentiated spore and stalk cells do not form. We interpret these results to support the hypothesis that cAMP plays an essential role in organizing cell movements in late development as well as in controlling the aggregation of cells in the initial phase of the developmental program.

Chemotaxis and cell motility in the cellular slime molds

Chemotaxis and cell motility have essential roles to play throughout the developmental cycle of the cellular slime molds. The particular emphasis of this review, however, will be on the amoeboid stages of the life cycle. The nature of the chemoattractants and their detection will be discussed as will the possible mechanisms that may account for the directed locomotion of amoebae. Intracellular chemoattractant-elicited molecular responses thought to play a role in transduction of extracellular signals into a motility response will also be examined. Furthermore, relationships of these transduction pathway components with changes in assembly states of the cytoskeletal proteins contributing to shape change and cell movement will be assessed. Theories of amoeboid movement involving these cytoskeletal proteins will be compared and discussed in terms of their relevance to cellular slime mold motility.

Behavior of a fluorescent analogue of calmodulin in living 3T3 cells

We have prepared and partially characterized a lissamine-rhodamine B fluorescent analogue of calmodulin, LRB-CM. The analogue had a dye/protein ratio of approximately 1.0 and contained no free dye or contaminating labeled proteins. LRB-CM was indistinguishable from native calmodulin upon SDS PAGE and in assays of phosphodiesterase and myosin light chain kinase. The emission spectrum of LRB-CM was insensitive to changes in pH, ionic strength, and temperature over the physiological range, but the apparent quantum yield was influenced somewhat by divalent cation concentration. LRB-CM injected into living Swiss 3T3 fibroblasts became associated with nitrobenzoxadiazole-phallacidin staining stress fibers in some interphase cells. LRB-CM and acetamidofluorescein-labeled actin co-injected into the same cell both became associated with fibers in some cells, but in most cases association of the two analogues with fibers was mutually exclusive. This suggests that calmodulin may differ from actin in the timing of incorporation into stress fibers or that we have distinguished distinct populations of stress fibers. We were able to detect no direct interaction of LRB-CM with actin by fluorescence photobleaching recovery (FRAP) of aqueous solutions. Interaction of LRB-CM with myosin light chain kinase also was not detected by FRAP. This suggests that the mean lifetime of the calmodulin-myosin light chain kinase complex is too short to affect the diffusion coefficient of calmodulin. We examined various fluorescent derivatives of proteins and dextrans as suitable control molecules for quantitative fluorescent analogue cytochemistry in living cells. Fluorescein isothiocyanate-dextrans were found to be preferable to all the proteins tested, since their mobilities in cytoplasm were inversely dependent on molecular size and there was no evidence of binding to intracellular components. In contrast, FRAP of LRB-CM in the cytoplasm of living 3T3 cells suggested that the analogue interacts with intracellular components with a range of affinities. The mobility of LRB-CM in the cytoplasm was sensitive to treatment of the cells with trifluoperazine, which suggests that at least some of the intracellular binding sites are specific for calmodulin in the calcium-bound form. FRAP of LRB-CM in the nuclei of living 3T3 cells indicated that the analogue was highly mobile within the nucleus but entered the nucleus from the cytoplasm much more slowly than fluorescein isothiocyanate-dextran of comparable molecular size and much more slowly than predicted from its mobility in cytoplasm.

Extracellular cGMP phosphodiesterase related to the rod outer segment phosphodiesterase isolated from bovine and monkey retinas

A phosphodiesterase (PDE) has been characterized in the interphotoreceptor matrix (IPM) of light-adapted fresh bovine retinas. It is obtained through a gentle rinsing of the retinal surface under conditions where the light-activated rod outer segment (ROS) enzyme remains attached. The enzyme has an apparent native molecular weight of 350 000 by gel filtration and appears as a doublet at Mr 47 000 and 45 000 on sodium dodecyl sulfate-polyacrylamide gels. It has an apparent Km value for cGMP of 33 microM and an apparent Km value for cAMP of 2200 microM. It is activated 3-6-fold by protamine and over 40-fold by trypsin. Protamine has no effect on the Km for cGMP while trypsin decreases the Km for cGMP by a factor of 2. The enzyme occurs in at least two forms as evidenced by two distinct peaks of activity after gel electrophoresis under nondenaturing conditions. A heat-stable inhibitor is tightly bound to the enzyme. The inhibitor obtained from the IPM PDE inhibits 98% of the activity of the trypsin-activated ROS PDE: conversely, the inhibitor obtained by boiling the ROS PDE completely inhibits the trypsin-activated IPM enzyme. A high-affinity monoclonal antibody to the active site of the ROS PDE, ROS 1 [Hurwitz, R., Bunt-Milan, A.H., & Beavo, J. (1984) J. Biol. Chem. 259, 8612-8618], quantitatively absorbs the IPM PDE. These observations indicate a clear relationship between these two PDEs even though their location, sizes, and specific functions in the retina appear to be distinct.

Developmental regulation of the cyclic-nucleotide-phosphodiesterase mRNA of Dictyostelium discoideum. Analysis by cell-free translation and immunoprecipitation

Extracellular cyclic-nucleotide phosphodiesterase of Dictyostelium discoideum has previously been purified and characterized [Orlow et al. (1981) J. Biol. Chem. 256, 7620-7627]. Antisera have been raised against the purified enzyme. Following cell-free translation of RNA extracted from cells at various stages of development and immunoprecipitation with anti-phosphodiesterase serum, cAMP phosphodiesterase synthesized in vitro and labeled with L-[35S]methionine can be detected by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and fluorography. The cell-free translation product is an Mr-48 000 polypeptide and can be immunoprecipitated with antiserum raised against active Mr-50 000 cAMP phosphodiesterase or antiserum raised against heat-denatured cAMP phosphodiesterase. Purified native cAMP phosphodiesterase blocks immunoprecipitation of the cAMP-phosphodiesterase polypeptide synthesized in vitro. A detectable level of cAMP-phosphodiesterase mRNA is present in axenically grown cells. After starvation of the cells in phosphate buffer for 1 h an increase of translatable cAMP-phosphodiesterase mRNA occurs, followed by a decrease and another increase. When cells are starved in the presence of the slowly hydrolyzed cAMP analogue, adenosine 3',5'-thiophosphate, the level of translatable cAMP-phosphodiesterase mRNA increases about tenfold and does not show a temporary decline. A maximum of 0.015% of the total acid-insoluble radioactivity is incorporated into the Mr-48 000 cAMP-phosphodiesterase polypeptide.

Detection and regulation of the mRNA for the inhibitor of extracellular cAMP phosphodiesterase of Dictyostelium discoideum

The inhibitor of the cAMP phosphodiesterase of Dictyostelium discoideum is a cysteine-rich glycoprotein, which binds to the enzyme and inactivates it. When the inhibitor is removed, enzymatic activity is restored. Following translation in vitro of RNA from developing cells and immunoprecipitation with anti-inhibitor serum, newly synthesized inhibitor can be detected by sodium dodecylsulfate/polyacrylamide gel electrophoresis and fluorography. The inhibitor can be labeled using [35S]cysteine but not [35S]methionine, in agreement with the previously determined amino acid composition, and can be detected after cell-free translation only if it has been previously acetylated. Purified native inhibitor blocks immunoprecipitation of the inhibitor polypeptide synthesized in vitro. No inhibitor mRNA was detected in growing cells. Translatable mRNA was present 2 h after the beginning of starvation, reached a maximal level after 3 h, and decreased thereafter. Addition of 1 mM cAMP at the beginning of starvation delayed the appearance of translatable inhibitor mRNA. In the presence of 5 microM adenosine cyclic-3',5'-phosphorothioate, a slowly hydrolyzed cAMP analogue, no translatable mRNA could be detected. Following removal of the analogue, the mRNA appeared within one hour and inhibitor was secreted after another hour.

Reversible heat activation of alkaline phosphatase of dictyostelium discoideum and its developmental implication

The activities of some enzymes increase during the development of the cellular slime mould, Dictyostelium discoideum. Because optimal specific activity is attained by eachenzyme at a specific developmental stage, development can be followed as a function of enzyme activity. The activity of one of these enzymes, alkaline phosphatase, increases markedly during the late stages of development, making it a potentially useful marker for terminal differentiation. It has been suggested that this increase in activity is due to de novo enzyme synthesis because the increase in activity does not occur in the presence of inhibitors of RNA and protein synthesis. However, we present evidence here of reversible heat activation of membrane-bound alkaline phosphatase which suggests that the increase in alkaline phosphatase activity may be due to an unmasking of pre-existing enzyme by a novel membrane regulatory mechanism.

Isolation and properties of calmodulin from Dictyostelium discoideum

A calcium-dependent regulatory protein (calmodulin) was purified from vegetative amoebae of Dictyostelium discoideum. The properties of Dictyostelium calmodulin are similar but not identical to those of bovine brain calmodulin. Calmodulin activity was not detected in extracts of Saccharomyces cerevisiae or Escherichia coli.