Molecular biology of Dictyostelium development. Proceedings of a symposium. Airlie, Virginia, November 7-12, 1987

cAMP-dependent signal pathways in unicellular eukaryotes

The review summarizes current data about mechanisms of signal transduction with participation of cAMP (cyclic adenosine monophosphate) and elements of the complex cAMP-protein kinase A (PKA) signal pathway in unicellular eukaryotes. Conceptions of evolutionary origin of eukaryotic signal transduction systems are developed.

The Dictyostelium MAP kinase kinase DdMEK1 regulates chemotaxis and is essential for chemoattractant-mediated activation of guanylyl cyclase

We have identified a MAP kinase kinase (DdMEK1) that is required for proper aggregation in Dictyostelium. Null mutations produce extremely small aggregate sizes, resulting in the formation of slugs and terminal fruiting bodies that are significantly smaller than those of wild-type cells. Time-lapse video microscopy and in vitro assays indicate that the cells are able to produce cAMP waves that move through the aggregation domains. However, these cells are unable to undergo chemotaxis properly during aggregation in response to the chemoattractant cAMP or activate guanylyl cyclase, a known regulator of chemotaxis in Dictyostelium. The activation of guanylyl cyclase in response to osmotic stress is, however, normal. Expression of putative constitutively active forms of DdMEK1 in a ddmek1 null background is capable, at least partially, of complementing the small aggregate size defect and the ability to activate guanylyl cyclase. However, this does not result in constitutive activation of guanylyl cyclase, suggesting that DdMEK1 activity is necessary, but not sufficient, for cAMP activation of guanylyl cyclase. Analysis of a temperature-sensitive DdMEK1 mutant suggests that DdMEK1 activity is required throughout aggregation at the time of guanylyl cyclase activation, but is not essential for proper morphogenesis during the later multicellular stages. The activation of the MAP kinase ERK2, which is essential for chemoattractant activation of adenylyl cyclase, is not affected in ddmek1 null strains, indicating that DdMEK1 does not regulate ERK2 and suggesting that at least two independent MAP kinase cascades control aggregation in Dictyostelium.

Wave propagation in aggregation fields of the cellular slime mouldDictyostelium discoideum

A mathematical model developed earlier to describe adaptation, relay and oscillation in the cellular slime mouldDictyostelium discoideumis used here to study various aspects of wave propagation in aggregation fields. We first show that travelling waves of cyclic AMP do not result from Turing (diffusive) instabilities. We then display the numerically computed dispersion relation for travelling periodic waves in one space dimension, and compare the results with the experimentally measured relation. Numerical results on phase locking in axisymmetric fields are also presented and the failure of propagation at low cell densities is discussed. Finally, we demonstrate that this model supports spiral waves whose wavelength and speed agree well with the experimental observ­ations.

A density-sensing factor regulates signal transduction in Dictyostelium

Dictyostelium discoideum initiates development when cells overgrow their bacterial food source and starve. To coordinate development, the cells monitor the extracellular level of a protein, conditioned medium factor (CMF), secreted by starved cells. When a majority of the cells in a given area have starved, as signaled by CMF secretion, the extracellular level of CMF rises above a threshold value and permits aggregation of the starved cells. The cells aggregate using relayed pulses of cAMP as the chemoattractant. Cells in which CMF accumulation has been blocked by antisense do not aggregate except in the presence of exogenous CMF. We find that these cells are viable but do not chemotax towards cAMP. Videomicroscopy indicates that the inability of CMF antisense cells to chemotax is not due to a gross defect in motility, although both video and scanning electron microscopy indicate that CMF increases the frequency of pseudopod formation. The activations of Ca2+ influx, adenylyl cyclase, and guanylyl cyclase in response to a pulse of cAMP are strongly inhibited in cells lacking CMF, but are rescued by as little as 10 s exposure of cells to CMF. The activation of phospholipase C by cAMP is not affected by CMF. Northern blots indicate normal levels of the cAMP receptor mRNA in CMF antisense cells during development, while cAMP binding assays and Scatchard plots indicate that CMF antisense cells contain normal levels of the cAMP receptor. In Dictyostelium, both adenylyl and guanylyl cyclases are activated via G proteins. We find that the interaction of the cAMP receptor with G proteins in vitro is not measurably affected by CMF, whereas the activation of adenylyl cyclase by G proteins requires cells to have been exposed to CMF. CMF thus appears to regulate aggregation by regulating an early step of cAMP signal transduction.

Seven helix cAMP receptors stimulate Ca2+ entry in the absence of functional G proteins in Dictyostelium

Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of signals across the plasma membrane. The best characterized cAR, cAR1, couples to the heterotrimeric guanine nucleotide-binding protein (G protein) alpha-subunit G alpha 2 to mediate activation of adenylyl and guanylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dependent responses including receptor phosphorylation, loss of ligand binding (LLB), and Ca2+ influx through a G alpha 2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g beta- strain (Lilly, P., Wu. L., Welker, D. L., and Devreotes, P. N. (1993) Genes & Dev. 7,986-995), which lacks G protein activity. Both cell lines failed to aggregate, a process requiring the G alpha 2 and G beta- subunits. In contrast, cAR1 phosphorylation in cAR1/g beta- cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G beta+ controls. cAMP-induced LLB was also normal in the cAR1/g beta- cells. Finally, cAR1/g beta- cells and cAR3/g beta- cells showed a Ca2+ response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G beta+ controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control strains. Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response, possibly by modulating other intracellular Ca2+ signal transduction pathways.

Cloning and targeted mutations of G alpha 7 and G alpha 8, two developmentally regulated G protein alpha-subunit genes in Dictyostelium

GTP-binding protein (G protein)-mediated signal transduction pathways play essential roles during the aggregation and differentiation process of Dictyostelium. In addition to the five known G protein alpha-subunit genes, we recently identified three novel alpha-subunit genes, G alpha 6, G alpha 7, and G alpha 8, using the polymerase chain reaction technique. We present here a more complete analysis of G alpha 7 and G alpha 8. The cDNAs of these two genes were cloned, and their complete nucleotide sequences were determined. Sequence analyses indicate that G alpha 8 possesses some unusual features. It lacks the "TCATDT" motif, a sequence of amino acids highly conserved among G alpha subunits, and has an additional 50 amino acids at its C-terminus consisting of long stretches of asparagine. Moreover, G alpha 8 is unusually resistant to protease digestion, which may indicate a slow GTP hydrolysis rate. The possible functions of these alpha-subunits were assessed by generating mutants lacking G alpha 7 or G alpha 8 by gene targeting through homologous recombination and by overexpressing G alpha 7 or G alpha 8 protein. Overexpression of G alpha 7 resulted in abnormal morphogenesis starting at the slug stage, whereas analysis of the other strains failed to reveal any obvious growth or developmental defects under either normal or stressful conditions. The implications of these results are discussed.

A G protein-based model of adaptation in Dictyostelium discoideum

A new model is proposed based on signal transduction via G proteins for adaptation of the signal relay process in the cellular slime mold Dictyostelium discoideum. The kinetic constants involved in the model are estimated from Dictyostelium discoideum and other systems. A qualitative analysis of the model shows how adaptation arises, and numerical computations show that the model agrees with observations in both perfusion and suspension experiments. Several experiments that can serve to test the model are suggested.

A G-protein beta-subunit is essential for Dictyostelium development

Recent studies have demonstrated that G-protein-linked signal transduction pathways play a significant role in the developmental program of the simple eukaryotic organism Dictyostelium. We have reported previously the isolation of a G-protein beta-subunit and present here a more complete analysis of this gene. Low-stringency Southern blots and RFLP mapping studies suggest that the beta-subunit is a unique gene found on linkage group II. Its deduced amino acid sequence of 347 residues is approximately 60% identical to those of the human, Drosophila, and Caenorhabditis elegans beta-subunits. The carboxy-terminal 300 residues are about 70% identical; the amino-terminal 50 residues are quite divergent, containing only 10 identities. At all stages of growth and development, a single 1.9-kb beta-subunit mRNA is present at a high level, and a specific antibody detects a single 37-kD protein. We propose that G-protein heterotrimers are formed when this beta-subunit couples with each of the eight distinct G-protein alpha-subunits that are transiently expressed during development. Targeted disruption of the beta-subunit gene had no effect on the viability of haploid cells, but resulted in the inability of cells to aggregate.

The surface cyclic AMP receptors, cAR1, cAR2, and cAR3, promote Ca2+ influx in Dictyostelium discoideum by a G alpha 2-independent mechanism

Activation of surface folate receptors or cyclic AMP (cAMP) receptor (cAR) 1 in Dictyostelium triggers within 5-10 s an influx of extracellular Ca2+ that continues for 20 s. To further characterize the receptor-mediated Ca2+ entry, we analyzed 45Ca2+ uptake in amoebas overexpressing cAR2 or cAR3, cARs present during multicellular development. Both receptors induced a cAMP-dependent Ca2+ uptake that had comparable kinetics, ion selectivity, and inhibitor profiles as folate- and cAR1-mediated Ca2+ uptake. Analysis of mutants indicated that receptor-induced Ca2+ entry does not require G protein alpha subunits G alpha 1, G alpha 2, G alpha 3, G alpha 4, G alpha 7, or G alpha 8. Overexpression of cAR1 or cAR3 in g alpha 2- cells did not restore certain G alpha 2-dependent events, such as aggregation, or cAMP-mediated activation of adenylate and guanylate cyclases, but these strains displayed a cAMP-mediated Ca2+ influx with kinetics comparable to wild-type aggregation-competent cells. These results suggest that a plasma membrane-associated Ca(2+)-influx system may be activated by at least four distinct chemoreceptors during Dictyostelium development and that the response may be independent of G proteins.

Lithium, an inhibitor of cAMP-induced inositol 1,4,5-trisphosphate accumulation in Dictyostelium discoideum, inhibits activation of guanine-nucleotide-binding regulatory proteins, reduces activation of adenylylcyclase, but potentiates activation of guanylyl cyclase by cAMP

Li+ drastically alters pattern formation in Dictyostelium by inhibiting cAMP-induced prespore-gene expression and promoting cAMP-induced prestalk-gene expression. We reported previously that Li+ inhibits inositol monophosphatases in this organism and strongly reduces basal and cAMP-stimulated inositol 1,4,5-trisphosphate levels. We show here that Li+ also reduces cAMP-induced accumulation of cAMP, but promotes cAMP-induced accumulation of cGMP. This effect is not due to inhibition of cGMP hydrolysis or inhibition of adaptation and may therefore reflect stimulation of guanylyl-cyclase activation. Li+ does not affect the binding of cAMP to surface receptors but interferes with the interaction between receptors and guanine-nucleotide-binding regulatory (G) proteins. These effects are complex; in the absence of Mg2+, Li+ increases guanosine 5'-[gamma-thio]triphosphate(GTP[S])-binding activity to similar levels as 1 mM Mg2+. However, while Mg2+ potentiates cAMP-induced stimulation of GTP[S]-binding activity, Li+ effectively inhibits stimulation. Li+ also inhibits cAMP-stimulated, but not basal high-affinity GTP-ase activity, indicating an inhibitory effect on cAMP-induced activation of G-proteins. Our data suggest that in addition to inositolphosphate metabolism, the activation of G-proteins may be a second biochemical target for Li+ effects on pattern formation and signal transduction in Dictyostelium.

Selective induction of gene expression and second-messenger accumulation in Dictyostelium discoideum by the partial chemotactic antagonist 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate

During development of the cellular slime mold Dictyostelium discoideum, cAMP induces chemotaxis and expression of different classes of genes by means of interaction with surface cAMP receptors. We describe a cAMP derivative, 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate (8-CPT-cAMP), which inhibits cAMP-induced chemotaxis at low concentrations but induces chemotaxis at supersaturating concentrations. This compound, moreover, selectively activates expression of aggregative genes but not of postaggregative genes. 8-CPT-cAMP induces normal cGMP and cAMP accumulation but in contrast to cAMP, which increases inositol 1,4,5-trisphosphate levels, 8-CPT-cAMP decreases inositol 1,4,5-trisphosphate levels. The derivative induces reduced activation of guanine nucleotide regulatory proteins, which may cause its defective activation of inositol 1,4,5-trisphosphate production. Our data suggest that disruption of inositolphospholipid signaling impairs chemotaxis and expression of a subclass of cAMP-regulated genes.

cAMP signal transduction pathways regulating development of Dictyostelium discoideum

Dictyostelium discoideum development is regulated through receptor/G protein signal transduction using cAMP as a primary extracellular signal. Signaling pathways will be discussed as well as the regulation and function of individual cAMP receptors and G alpha subunits. Finally potential downstream targets including protein kinases and nuclear events will be explored.

Involvement of cyclic AMP cell surface receptors and G-proteins in signal transduction during slug migration of Dictyostelium discoideum

The presence of G-proteins, interacting with cAMP surface receptors, was investigated in vegetative cells, aggregation-competent cells, and migrating slugs of Dictyostelium discoideum. Our results indicate that G-proteins are present in all stages. In vegetative cells there is a limited number of cAMP receptors but no effect of GTP tau S on cAMP binding could be detected; in addition, no effect of cAMP on GTP tau S binding or GTPase activity was observed. In both aggregation-competent cells and slugs GTP tau S inhibits cAMP binding, while cAMP stimulates GTP tau S binding and high-affinity GTPase. Since the presence of G-proteins coupled to cAMP receptors could be demonstrated in slugs, the involvement of the effector enzymes adenylate cyclase and phospholipase C was investigated. The results show that adenylate cyclase activity is stimulated by GTP tau S in both stages and that in cells from migrating slugs the Ins(1,4,5)P3 production is increased upon stimulation with cAMP. The possible involvement of G-proteins in signal transduction during the slug stage of D. discoideum is discussed.

Dictyostelium discoideum lipids modulate cell-cell cohesion and cyclic AMP signaling

During Dictyostelium discoideum development, cell-cell communication is mediated through cyclic AMP (cAMP)-induced cAMP synthesis and secretion (cAMP signaling) and cell-cell contact. Cell-cell contact elicits cAMP secretion and modulates the magnitude of a subsequent cAMP signaling response (D. R. Fontana and P. L. Price, Differentiation 41:184-192, 1989), demonstrating that cell-cell contact and cAMP signaling are not independent events. To identify components involved in the contact-mediated modulation of cAMP signaling, amoebal membranes were added to aggregation-competent amoebae in suspension. The membranes from aggregation-competent amoebae inhibited cAMP signaling at all concentrations tested, while the membranes from vegetative amoebae exhibited a concentration-dependent enhancement or inhibition of cAMP signaling. Membrane lipids inhibited cAMP signaling at all concentrations tested. The lipids abolished cAMP signaling by blocking cAMP-induced adenylyl cyclase activation. The membrane lipids also inhibited amoeba-amoeba cohesion at concentrations comparable to those which inhibited cAMP signaling. The phospholipids and neutral lipids decreased cohesion and inhibited the cAMP signaling response. The glycolipid/sulfolipid fraction enhanced cohesion and cAMP signaling. Caffeine, a known inhibitor of cAMP-induced adenylyl cyclase activation, inhibited amoeba-amoeba cohesion. These studies demonstrate that endogenous lipids are capable of modulating amoeba-amoeba cohesion and cAMP-induced activation of the adenylyl cyclase. These results suggest that cohesion may modulate cAMP-induced adenylyl cyclase activation. Because the complete elimination of cohesion is accompanied by the complete elimination of cAMP signaling, these results further suggest that cohesion may be necessary for cAMP-induced adenylyl cyclase activation in D. discoideum.

Control of cAMP-induced gene expression by divergent signal transduction pathways

A compilation of literature data and recent experiments led to the following conclusions regarding cyclic adenosine 3':5' monophosphate (cAMP) regulation of gene expression. Several classes of cAMP-induced gene expression can be discriminated by sensitivity to stimulation kinetics. The aggregation-related genes respond only to nanomolar cAMP pulses. The prestalk-related genes respond both to nanomolar pulses and persistent micromolar stimulation. The prespore specific genes respond only to persistent micromolar stimulation. The induction of the aggregation- and prestalk-related genes by nanomolar cAMP pulses may share a common transduction pathway, which does not involve cAMP, while involvement of the inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway is unlikely. Induction of the expression of prespore and prestalk-related genes by micromolar cAMP stimuli utilizes divergent signal processing mechanisms. cAMP-induced prespore gene expression does not involve cAMP and probably also not cyclic guanosine 3'.5' monophosphate (cGMP) as intracellular intermediate. Involvement of cAMP-induced phospholipase C (PLC) activation in this pathway is suggested by the observation that IP3 and 1,2-diacylglycerol (DAG) can induce prespore gene expression, albeit in a somewhat indirect manner and by the observation that Li+ and Ca2+ antagonists inhibit prespore gene expression. Cyclic AMP induction of prestalk-related gene expression is inhibited by IP3 and DAG and promoted by Li+, and is relatively insensitive to Ca2+ antagonists, which indicates that PLC activation does not mediate prestalk-related gene expression. Neither prespore nor prestalk-related gene expression utilizes the sustained cAMP-induced pHi increase as intracellular intermediate.

Multiple genes for cell surface cAMP receptors in Dictyostelium discoideum

We have cloned and characterized three genes (CAR1, CAR2, CAR3) encoding potential cell surface, cyclic adenosine 3':5' monophosphate (AMP) receptors from Dictyostelium discoideum. The three proteins are predicted to be substantially similar in amino acid sequence throughout most of their transmembrane (TM) and loop domains but are distinctly different in their carboxyl terminal segments. In addition, all three genes possess an intron which interrupts an equivalent codon of TM3. CAR1 is expressed early in development when the cAMP relay system is being established. As development proceeds multiple size forms of CAR1 RNA are detected which apparently result from differences in their 5'-untranslated regions. Late in development levels of CAR1 RNA decrease. In contrast, CAR2 encodes a single sized RNA which is expressed only during postaggregative development. CAR3 expression is approximately 10% of CAR1 during early development, is maximal during tight aggregate formation but declines thereafter. Only one size class of CAR3 mRNA is detected throughout development. Because RNA for each of the three genes is present in postaggregative cells, it was of interest to determine the cell type distribution of each RNA. Gene-specific probes were hybridized to RNAs isolated from cells of Percoll gradient-enriched prespore and prestalk fractions and relative levels of hybridization compared. CAR1 and CAR3 show approximately the same pattern of accumulation; a 3-4 fold enrichment in prestalk cells. CAR2, however, is highly enriched in prestalk cells, more than 10 fold relative to prespore cells.

Expression of a cAMP receptor gene of Dictyostelium and evidence for a multigene family

We have previously reported the cloning of cDNAs for a Dictyostelium cell-surface cAMP receptor that is a member of the family of G-protein-linked receptors. Here, we report the organization and the developmental expression of this cAMP receptor gene, designated CAR1. CAR1 is a single copy gene that contains two intervening sequences. CAR1 mRNA levels are low in growing cells, rise to peak expression at 5-10 hr of development when the cAMP signaling system is maximally active, and decrease as development proceeds. At 5 hr the predominant mRNA species is approximately 1.9 kb, by 10 hr the mRNA is heterogeneous with sizes of approximately 1.9-2.1 kb, but during culmination only the 2.1 kb mRNA is detected. The variety of mRNA sizes results from differences in 5'-untranslated regions. Studies using developmental mutants with aberrant cAMP-signaling patterns indicate that pulsatile action of cAMP promotes maximal expression of CAR1 during early development. Low stringency hybridization of CAR1 probes to genomic DNA detects additional, related sequences, suggesting that there are several genes that encode a family of structurally similar receptors. Multiple functions previously attributed to the cAMP receptor instead may be fulfilled by distinct receptor subtypes encoded by specific genes.

Dictyostelium discoideum lipids modulate cell-cell cohesion and cyclic AMP signaling

During Dictyostelium discoideum development, cell-cell communication is mediated through cyclic AMP (cAMP)-induced cAMP synthesis and secretion (cAMP signaling) and cell-cell contact. Cell-cell contact elicits cAMP secretion and modulates the magnitude of a subsequent cAMP signaling response (D. R. Fontana and P. L. Price, Differentiation 41:184-192, 1989), demonstrating that cell-cell contact and cAMP signaling are not independent events. To identify components involved in the contact-mediated modulation of cAMP signaling, amoebal membranes were added to aggregation-competent amoebae in suspension. The membranes from aggregation-competent amoebae inhibited cAMP signaling at all concentrations tested, while the membranes from vegetative amoebae exhibited a concentration-dependent enhancement or inhibition of cAMP signaling. Membrane lipids inhibited cAMP signaling at all concentrations tested. The lipids abolished cAMP signaling by blocking cAMP-induced adenylyl cyclase activation. The membrane lipids also inhibited amoeba-amoeba cohesion at concentrations comparable to those which inhibited cAMP signaling. The phospholipids and neutral lipids decreased cohesion and inhibited the cAMP signaling response. The glycolipid/sulfolipid fraction enhanced cohesion and cAMP signaling. Caffeine, a known inhibitor of cAMP-induced adenylyl cyclase activation, inhibited amoeba-amoeba cohesion. These studies demonstrate that endogenous lipids are capable of modulating amoeba-amoeba cohesion and cAMP-induced activation of the adenylyl cyclase. These results suggest that cohesion may modulate cAMP-induced adenylyl cyclase activation. Because the complete elimination of cohesion is accompanied by the complete elimination of cAMP signaling, these results further suggest that cohesion may be necessary for cAMP-induced adenylyl cyclase activation in D. discoideum.

Intracellular vesicle movement, cAMP and myosin II in Dictyostelium

Dictyostelium amoebae were analyzed before and after rapid addition of 10(-6) M cAMP for cellular motility, dynamic shape changes, and intracellular particle movement. Before cAMP addition, amoebae moved in a persistent anterior fashion and were elongate with F-actin localized predominantly in the anterior pseudopod. Intracellular particles moved rapidly and anteriorly. Within seconds after 10(-6) M cAMP addition, cells stopped translocating, pseudopod formation ceased, intracellular particle movement was depressed, and F-actin was lost from the pseudopod and concomitantly relocalized in the cell cortex. After 10 seconds, expansion zones reappeared but were small and no longer anteriorly localized. Vesicle movement partially rebounded but was no longer anteriorly directed. The myosin II null mutant HS2215 exhibited both depressed cellular translocation and vesicle movement. The addition of cAMP to HS2215 cells did not result in any detectable change in the random, depressed movement of particles. The results with HS2215 suggest that myosin II is essential for (1) rapid cellular translocation, (2) cellular polarity, (3) rapid particle movement, (4) anteriorly directed particle movement, and (5) the cAMP response. Electron micrographs suggest that at least half of the particles examined in this study contain in turn smaller membrane bound vesicles or multilamellar membrane bodies. The possible role of these vesicles is discussed.

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.

cAMP-mediated inhibition of intracellular particle movement and actin reorganization in Dictyostelium

Before addition of cAMP, Dictyostelum amoebae rapidly translocating in buffer are elongate, exhibit expansion zones primarily at the anterior end and filamentous actin (F-actin) localization primarily in the anterior pseudopodia. Intracellular particle movement is primarily in the anterior direction, and the average rate of particle movement is roughly five times the rate of cellular translocation. Within seconds after the addition of 10(-6)M cAMP, there is a dramatic suppression of cellular translocation, an inhibition of pseudopod formation, a freeze in cellular morphology, a dramatic depression in intracellular particle movement, loss of F-actin localization in pseudopodia concomitant with relocalization of F-actin in the general cytoplasmic cortex under the plasma membrane, and a doubling of F-actin content. After 10 s, expansion zones are again visible at the cell perimeter, but they no longer are localized in the original anterior portion of the cell. There is a slight rebound in particle movement after 10 s, but particles with persistent tracks now show no directionality towards the original anterior portion of the cell, as they did before cAMP addition. Finally, in parallel with the resumption of peripheral expansion and the small rebound in particle movement, there is a decrease in total cellular F-actin to the untreated level. The pattern of microtubule organization is unaffected by the addition of cAMP.