Tortoise, a novel mitochondrial protein, is required for directional responses of Dictyostelium in chemotactic gradients.

We have identified a novel gene, Tortoise (TorA), that is required for the efficient chemotaxis of Dictyostelium discoideum cells. Cells lacking TorA sense chemoattractant gradients as indicated by the presence of periodic waves of cell shape changes and the localized translocation of cytosolic PH domains to the membrane. However, they are unable to migrate directionally up spatial gradients of cAMP. Cells lacking Mek1 display a similar phenotype. Overexpression of Mek1 in torA- partially restores chemotaxis, whereas overexpression of TorA in mek1- does not rescue the chemotactic phenotype. Regardless of the genetic background, TorA overexpressing cells stop growing when separated from a substrate. Surprisingly, TorA-green fluorescent protein (GFP) is clustered near one end of mitochondria. Deletion analysis of the TorA protein reveals distinct regions for chemotactic function, mitochondrial localization, and the formation of clusters. TorA is associated with a round structure within the mitochondrion that shows enhanced staining with the mitochondrial dye Mitotracker. Cells overexpressing TorA contain many more of these structures than do wild-type cells. These TorA-containing structures resist extraction with Triton X-100, which dissolves the mitochondria. The characterization of TorA demonstrates an unexpected link between mitochondrial function, the chemotactic response, and the capacity to grow in suspension.

G protein beta subunit-null mutants are impaired in phagocytosis and chemotaxis due to inappropriate regulation of the actin cytoskeleton.

Chemotaxis and phagocytosis are basically similar in cells of the immune system and in Dictyostelium amebae. Deletion of the unique G protein beta subunit in D. discoideum impaired phagocytosis but had little effect on fluid-phase endocytosis, cytokinesis, or random motility. Constitutive expression of wild-type beta subunit restored phagocytosis and normal development. Chemoattractants released by cells or bacteria trigger typical transient actin polymerization responses in wild-type cells. In beta subunit-null cells, and in a series of beta subunit point mutants, these responses were impaired to a degree that correlated with the defect in phagocytosis. Image analysis of green fluorescent protein-actin transfected cells showed that beta subunit- null cells were defective in reshaping the actin network into a phagocytic cup, and eventually a phagosome, in response to particle attachment. Our results indicate that signaling through heterotrimeric G proteins is required for regulating the actin cytoskeleton during phagocytic uptake, as previously shown for chemotaxis. Inhibitors of phospholipase C and intracellular Ca2+ mobilization inhibited phagocytosis, suggesting the possible involvement of these effectors in the process.

Switching of chemoattractant receptors programs development and morphogenesis in Dictyostelium: receptor subtypes activate common responses at different agonist concentrations.

One of the common functional features among G-protein coupled receptors is the occurrence of multiple subtypes involved in similar signal transduction events. The cAMP chemoattractant receptor family of Dictyostelium discoideum is composed of four receptors (cAR1-cAR4), which are expressed sequentially throughout the developmental transition from a unicellular to a multicellular organism. The receptors differ in affinity for cAMP and in the sequences of their C-terminal domains. In this study, we constitutively expressed cAR1, cAR2, and cAR3 as well as a series of chimeric and mutant receptors and assessed the capacity of each to mediate chemotaxis, activation of adenylyl cyclase and actin polymerization, and rescue the developmental defect of car1-/car3- cells. We found that various receptors and mutants sense different concentration ranges of cAMP but all can mediate identical responses during the aggregation stage of development. The responses displayed very similar kinetics, suggesting no major differences in regulatory properties attributable to the C-terminal domains. We speculate that switching of receptor subtypes during development enables the organism to respond to the changing concentrations of the chemoattractant and thereby program morphogenesis appropriately.

Phosphorylation of chemoattractant receptors is not essential for chemotaxis or termination of G-protein-mediated responses.

In several G-protein-coupled signaling systems, ligand-induced receptor phosphorylation by specific kinases is suggested to lead to desensitization via mechanisms including receptor/G-protein uncoupling, receptor internalization, and receptor down-regulation. We report here that elimination of phosphorylation of a chemoattractant receptor of Dictyostelium, either by site-directed substitution of the serines or by truncation of the C-terminal cytoplasmic domain, completely prevented agonist-induced loss of ligand binding but did not impair the adaptation of several receptor-mediated responses including the activation of adenylyl and guanylyl cyclases and actin polymerization. In addition, the phosphorylation-deficient receptors were capable of mediating chemotaxis, aggregation, and differentiation. We propose that for chemoattractant receptors agonist-induced phosphorylation regulates surface binding activity but other phosphorylation-independent mechanisms mediate response adaptation.

Regulation of actin polymerization in cell-free systems by GTPgammaS and Cdc42.

We have established a cell-free system to investigate pathways that regulate actin polymerization. Addition of GTPgammaS to lysates of polymorphonuclear leukocytes (PMNs) or Dictyostelium discoideum amoeba induced formation of filamentous actin. The GTPgammaS appeared to act via a small G-protein, since it was active in lysates ofD. discoideum mutants missing either the alpha2- or beta-subunit of the heterotrimeric G-protein required for chemoattractant-induced actin polymerization in living cells. Furthermore, recombinant Cdc42, but not Rho or Rac, induced polymerization in the cell-free system. The Cdc42-induced increase in filamentous actin required GTPgammaS binding and was inhibited by a fragment of the enzyme PAK1 that binds Cdc42. In a high speed supernatant, GTPgammaS alone was ineffective, but GTPgammaS-loaded Cdc42 induced actin polymerization, suggesting that the response was limited by guanine nucleotide exchange. Stimulating exchange by chelating magnesium, by adding acidic phospholipids, or by adding the exchange factors Cdc24 or Dbl restored the ability of GTPgammaS to induce polymerization. The stimulation of actin polymerization did not correlate with PIP2 synthesis.

Random mutagenesis of the cAMP chemoattractant receptor, cAR1, of Dictyostelium. Mutant classes that cause discrete shifts in agonist affinity and lock the receptor in a novel activational intermediate.

The cAMP chemoattractant receptor, cAR1, of Dictyostelium transduces extracellular cAMP signals via G protein-dependent and G protein-independent mechanisms. While site-directed mutagenesis studies of G protein-coupled receptors have provided a host of information regarding the domains essential for various functions, many mechanistic and structural questions remain to be resolved. We therefore carried out polymerase chain reaction-mediated random mutagenesis over a large part of the cAR1 sequence (from TMIII through the proximal part of the cytoplasmic tail). We devised a rapid screen for loss-of-function mutations based on the essential role of cAR1 in the developmental program of Dictyostelium. Although there were an average of two amino acid substitutions per receptor, approximately 90% of the mutants were able to substitute for wild-type cAR1 when expressed in receptor null cells. About 2% were loss-of-function mutants that expressed wild-type levels of receptor protein. We used biochemical screens to select about 100 of these mutants and chose eight representative mutants for extensive characterization. These fell into distinct classes. One class had a conditional defect in cAMP binding that was reversed by high salt. Another large class had decreased affinity under all conditions. Curiously, the decreases were clustered into three discrete intervals. One of the most interesting class of mutants lost all capacity for signal transduction but was phosphorylated in response to agonist binding. This latter finding suggests that there are at least two activated states of cAR1 that can be recognized by different downstream effectors.

The aimless RasGEF is required for processing of chemotactic signals through G-protein-coupled receptors in Dictyostelium.

BACKGROUND: Ras proteins are small GTP-binding proteins that play an essential role in a wide range of processes, particularly in mammalian growth control. They act as molecular switches, being inactive when GDP is bound, and active when associated with GTP. Activation is accomplished by guanine nucleotide exchange factors (RasGEFs); when RasGEFs interact with Ras proteins, GDP is allowed to escape, and is replaced by GTP. Dictyostelium responds to chemoattractants through typical seven transmembrane domain receptors and heterotrimeric G proteins. There are at least five different Dictyostelium Ras genes, whose functions are not yet known. RESULTS: We have isolated the aimless gene, which encodes the Dictyostelium homologue of RasGEFs, during a screen for insertional mutants that fail to aggregate. We found that aimless null mutants grew at a normal rate, but were severely impaired in both chemotaxis and activation of adenylyl cyclase, both of which are critical for the early stages of development. Although coupling between receptors and their G proteins is unaffected, and several cyclic AMP (cAMP)-mediated responses appear normal, activation of adenylyl cyclase by receptors and GTP gamma S (a non-hydrolyzable GTP analogue) is reduced by up to 95%. The motility of mutant cells appears normal, suggesting a true defect in gradient sensing. CONCLUSIONS: The discovery of the aimless gene adds an interesting new member to the family of RasGEFs. Our data suggest an unforeseen role for a RasGEF, and therefore presumably a complete Ras pathway, in the processing of chemotactic signals through G-protein-coupled receptors.

Agonist-induced loss of ligand binding is correlated with phosphorylation of cAR1, a G protein-coupled chemoattractant receptor from Dictyostelium.

The parallel agonist-induced phosphorylation, alteration in electrophoretic mobility, and loss of ligand binding of a guanine nucleotide-binding regulatory protein (G protein)-coupled chemoattractant receptor from Dictyostelium (cAR1) depend upon a cluster of five C-terminal domain serine residues (Caterina, M. J., Hereld, D., and Devreotes, P.N. (1995) J. Biol. Chem. 270, 4418-4423). Analysis of mutants lacking combinations of these serines revealed that either Ser303 or Ser304 is required; mutants lacking both serines are defective in all of these responses. Interestingly, several mutants, including those substituted at only Ser299, Ser302, or Ser303 or at non-serine positions within the third cytoplasmic loop, displayed an unstable mobility shift; the alteration was rapidly reversed upon cAMP removal. These mutants also exhibited subnormal extents of loss of ligand binding, which is assessed after removal of the ligand. For the wild-type receptor, we found that the stability of phosphorylation depends upon the concentration and duration of agonist pretreatment. This suggests that, following phosphorylation of Ser303 or Ser304, cAR1 undergoes a further transition (EC50 approximately 140 nM, t 1/2 approximately 4 min) to a relatively phosphatase-resistant state. We used this insight to show that, under all conditions tested, the extent of loss of binding is correlated with the fraction of cAR1 in the altered mobility form. We discuss possible relationships between cAR1 phosphorylation and loss of ligand binding.

Localization of ligand-induced phosphorylation sites to serine clusters in the C-terminal domain of the Dictyostelium cAMP receptor, cAR1.

When Dictyostelium cells are stimulated with cyclic adenosine 3',5'-monophosphate (cAMP), the major surface cAMP receptor expressed in early development, cAR1, undergoes a rapid phosphorylation and parallel decrease in electrophoretic mobility which may serve to regulate the activity of this G protein-coupled receptor. Biochemical analyses indicate the electrophoretic mobility shift is caused by phosphorylation of serine residues within the C-terminal cytoplasmic domain. The 18 serines of this domain are grouped in four clusters, designated 1 to 4 (in N- to C-terminal order). Two approaches were taken to determine the distribution of phosphorylation sites among the serine clusters. First, a proteolytic analysis of the C-terminal domain was performed. Second, mutants lacking various combinations of the serine clusters were created by site-directed mutagenesis and their abilities to undergo ligand-induced modification were determined. Both approaches yielded corroborative results consistent with the following model: the stimulus induces the addition of approximately two phosphates to cluster 1 and one to cluster 2; basal phosphorylation occurs predominantly in cluster 3 and to a lesser extent in cluster 2; and cluster 4 is not phosphorylated. The phosphorylation-deficient receptor mutants should be useful for establishing the role of ligand-induced phosphorylation of cAR1 in chemotaxis, cell-cell signaling, and gene expression.

Structurally distinct and stage-specific adenylyl cyclase genes play different roles in Dictyostelium development.

We have isolated two adenylyl cyclase genes, designated ACA and ACG, from Dictyostelium. The proposed structure for ACA resembles that proposed for mammalian adenylyl cyclases: two large hydrophilic domains and two sets of six transmembrane spans. ACG has a novel structure, reminiscent of the membrane-bound guanylyl cyclases. An aca- mutant, created by gene disruption, has little detectable adenylyl cyclase activity and fails to aggregate, demonstrating that cAMP is required for cell-cell communication. cAMP is not required for motility, chemotaxis, growth, and cell division, which are unaffected. Constitutive expression in aca- cells of either ACA or ACG, which is normally expressed only during germination, restores aggregation and the ability to complete the developmental program. ACA expression restores receptor and guanine nucleotide-regulated adenylyl cyclase activity, while activity in cells expressing ACG is insensitive to these regulators. Although they lack ACA, which has a transporter-like structure, the cells expressing ACG secrete cAMP constitutively.

Purification of the surface cAMP receptor in Dictyostelium.

We have previously identified and demonstrated reversible ligand-induced modification of the major cell surface cAMP receptor in Dictyostelium discoideum. The receptor, or a subunit of it, has been purified to homogeneity by hydroxylapatite chromatography followed by two-dimensional preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purification was monitored by following 32Pi incorporated by photoaffinity labeling with 8-azido-[32P]cAMP or by in vivo labeling with 32Pi. Two interconvertible forms of the receptor, designated R (Mr 40,000) and D (Mr 43,000), co-purified. Two-dimensional peptide maps of independently purified and 125I-iodinated R and D forms of the receptor were nearly identical but did have several distinct peptides. The estimated 6000-fold purification required is consistent with the number of cell surface binding sites assuming there are not multiple binding sites/polypeptide. In the accompanying article we report the generation of a monospecific polyclonal antiserum which has helped to further elucidate the physical properties and developmental regulation of the cAMP receptor.

The surface cyclic AMP receptor in Dictyostelium. Levels of ligand-induced phosphorylation, solubilization, identification of primary transcript, and developmental regulation of expression.

A monospecific polyclonal antiserum to the surface cAMP receptor of Dictyostelium has been developed by immunization with purified receptor immobilized on particles of polyacrylamide and on nitrocellulose paper. In Western blots, the antiserum displays high affinity and specificity for both the R (Mr 40,000) and D (Mr 43,000) forms of the receptor previously identified by photoaffinity labeling with 8-azido-[32P] cAMP. These bands, labeled with the photoaffinity label or with 32 Pi, were quantitatively and specifically immunoprecipitated, supporting co-purification data that all represent the same polypeptide. The R form, found in unstimulated cells, contained at least 0.2 mol of phosphate/mol of receptor. The D form, generated by cAMP stimulation of intact cells, contained at least 4 mol of phosphate/mol of receptor. In the absence of detergents, the receptor was exclusively located on membranes. The receptor was solubilized effectively in Triton X-100 and sedimented as a broad peak of 5-7 S on sucrose velocity gradients. Western blots of membranes isolated at different times after starvation indicate that the appearance of cell surface cAMP binding sites during the aggregation stage of development (5-6 h) is due to de novo synthesis of receptor protein. Pulse labeling with [35S]methionine indicated that the receptor is most rapidly synthesized during the preaggregation stage of development (1-3 h), prior to its maximal accumulation in membranes. The serum specifically immunoprecipitates a polypeptide of Mr 37,000 from an in vitro translation reaction using RNA isolated from preaggregation stage cells. The time course of expression of the mRNA coding for the Mr 37,000 polypeptide parallels the rate of receptor synthesis in vivo.