Identification of four candidate cGMP targets in Dictyostelium

In Dictyostelium, a transient increase in intracellular cGMP is important for cytoskeletal rearrangements during chemotaxis. There must be cGMP-binding proteins in Dictyostelium that regulate key cytoskeletal components after treatment with chemoattractants, but to date, no such proteins have been identified. Using a bioinformatics approach, we have found four candidate cGMP-binding proteins (GbpA-D). GbpA and -B have two tandem cGMP-binding sites downstream of a metallo beta-lactamase domain, a superfamily that includes cAMP phosphodiesterases. GbpC contains the following nine domains (in order): leucine-rich repeats, Ras, MEK kinase, Ras guanine nucleotide exchange factor N-terminal (RasGEF-N), DEP, RasGEF, cGMP-binding, GRAM, and a second cGMP-binding domain. GbpD is related to GbpC, but is much shorter; it begins with the RasGEF-N domain, and lacks the DEP domain. Disruption of the gbpC gene results in loss of all high-affinity cGMP-binding activity present in the soluble cellular fraction. GbpC mRNA levels increase dramatically 8 h after starvation is initiated. GbpA, -B, and -D mRNA levels show less dramatic changes, with gbpA mRNA levels highest 4 h into starvation, gbpB mRNA levels highest in vegetative cells, and gbpD levels highest at 8 h. The identification of these genes is the first step in a molecular approach to studying downstream effects of cGMP signaling in Dictyostelium.