Regulation of guanylate cyclase by a guanine nucleotide binding protein, G alpha 2, in Dictyostelium discoideum

Unusual Guanylyl Cyclases and cGMP Signaling in Dictyostelium discoideum

cGMP is used as a second messenger in many eukaryotes. cGMP signaling requires at least three components: Guanylyl cyclases synthesize cGMP from GTP. Specific cGMP-binding proteins propagate the signal, usually by phosphorylation of their target proteins. Finally, phosphodiesterases terminate the cGMP signal by hydrolyzing cGMP to 5'cGMP. Recently, all guanylyl cyclases and most of the cGMP target proteins and phosphodiesterases of the cellular slime mold Dictyostelium discoideum have been identified. Characterization of these enzymes show them to be structurally and evolutionarily distinct from their bacterial and metazoan counterparts. In this chapter we review the properties of the Dictyostelium guanylyl cyclases and discuss their role in the unusual cGMP pathway of Dictyostelium.

A model for cGMP signal transduction in Dictyostelium in perspective of 25 years of cGMP research

The chemoattactant mediated cGMP response of Dictyostelium cells was discovered about twenty-five years ago. Shortly thereafter, guanylyl cyclases, cGMP-phosphodiesterases and cGMP-binding proteins were detected already in lysates, but the encoding genes were discovered only very recently. The deduced proteins appear to be very different from proteins with the same function in metazoa. In this review we discuss these new findings in perspective of the previously obtained biochemical and functional data on cGMP in Dictyostelium.

GTPgammaS regulation of a 12-transmembrane guanylyl cyclase is retained after mutation to an adenylyl cyclase

DdGCA is a Dictyostelium guanylyl cyclase with a topology typical for mammalian adenylyl cyclases containing 12 transmembrane-spanning regions and two cyclase domain. In Dictyostelium cells heterotrimeric G-proteins are essential for guanylyl cyclase activation by extracellular cAMP. In lysates, guanylyl cyclase activity is strongly stimulated by guanosine 5'-3-O-(thio) triphosphate (GTPgammaS), which is also a substrate of the enzyme. DdGCA was converted to an adenylyl cyclase by introducing three point mutations. Expression of the obtained DdGCA(kqd) in adenylyl cyclase-defective cells restored the phenotype of the mutant. GTPgammaS stimulated the adenylyl cyclase activity of DdGCA(kqd) with properties similar to those of the wild-type enzyme (decrease of K(m) and increase of V(max)), demonstrating that GTPgammaS stimulation is independent of substrate specificity. Furthermore, GTPgammaS activation of DdGCA(kqd) is retained in several null mutants of Galpha and Gbeta proteins, indicating that GTPgammaS activation is not mediated by a heterotrimeric G-protein but possibly by a monomeric G-protein.

Guanylate cyclase in Dictyostelium discoideum with the topology of mammalian adenylate cyclase

The core of adenylate and guanylate cyclases is formed by an intramolecular or intermolecular dimer of two cyclase domains arranged in an antiparallel fashion. Metazoan membrane-bound adenylate cyclases are composed of 12 transmembrane spanning regions, and two cyclase domains which function as a heterodimer and are activated by G-proteins. In contrast, membrane-bound guanylate cyclases have only one transmembrane spanning region and one cyclase domain, and are activated by extracellular ligands to form a homodimer. In the cellular slime mould, Dictyostelium discoideum, membrane-bound guanylate cyclase activity is induced after cAMP stimulation; a G-protein-coupled cAMP receptor and G-proteins are essential for this activation. We have cloned a Dictyostelium gene, DdGCA, encoding a protein with 12 transmembrane spanning regions and two cyclase domains. Sequence alignment demonstrates that the two cyclase domains are transposed, relative to these domains in adenylate cyclases. DdGCA expressed in Dictyostelium exhibits high guanylate cyclase activity and no detectable adenylate cyclase activity. Deletion of the gene indicates that DdGCA is not essential for chemotaxis or osmo-regulation. The knock-out strain still exhibits substantial guanylate cyclase activity, demonstrating that Dictyostelium contains at least one other guanylate cyclase.

Two opposite signal transducing mechanisms regulate a G-protein-coupled guanylyl cyclase

Membrane-bound guanylyl cyclase (GC) is regulated by muscarinic receptors (mAChRs). Carbamylcholine (CC) induces a "dual" biological response on GC activity. Thus, an activation is observed at 0.1 nM and a maximal response at 1 nM CC. However, at higher agonist concentration (> 100 nM), there is an agonist-dependent inhibition of GC. This CC dual response is affected by 4-DAMP and HDD (M3 antagonists), which produce a right-shift of the CC curve; the maximal CC dose response with 4-DAMP is more potent than that with HDD. Moreover, AFDX-DS (an M2 antagonist) increases basal activity and decreases the agonist-dependent inhibition. Neither the CC response nor the CC maximal dose responses are affected by pirenzepine (PZ, M1 antagonist). The agonist-dependent stimulation of GC activity is inhibited by 4-DAMP showing a -log IC50 = 8.4 +/- 0.4, while AFDX116 DS poorly inhibits such activity with a -log IC50 = 5.0 +/- 0.2. The agonist-independent (basal) GC activity also was inhibited by 4-DAMP, in a dose-dependent manner, with an IC50 = 8.5 +/- 0.2. Nonetheless, other muscarinic antagonists (PZ and HDD) were not able to inhibit this basal GC. Pertussis toxin treatment produces a complete blockade of the agonist-dependent inhibition of GC with a full expression of the agonist-dependent activation of membrane-bound GC. These results indicate that membrane-bound GC is regulated by muscarinic agents through two opposite signaling pathways; one involves the activation of GC via an M3 mAchR coupled to a PTX-insensitive G protein, while the GC inhibition is mediated through a PTX-sensitive Gi/o protein possibly coupled to an M2 mAChR.