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

The core of adenylate and guanylate cyclases is formed by an intramolecular ol intermolecular dimer of two cyclase domains arranged in an antiparallel fashion. Metazoan membrane-bound adenylate cyclases are composed of 12 tra nsmembrane spanning regions, and two cyclase domains which function as a he terodimer and are activated by G-proteins. In contrast, membrane-bound guan ylate 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 guanylat e cyclase activity is induced after cAMP stimulation; a G-protein-coupled c AMP receptor and G-proteins are essential for this activation. We have clon ed a Dictyostelium gene, DdGCA, encoding a protein with 12 transmembrane sp anning regions and two cyclase domains. Sequence alignment demonstrates tha t the two cyclase domains are transposed, relative to these domains in aden ylate cyclases. DdGCA expressed in Dictyostelium exhibits high guanylate cy clase activity and no detectable adenylate cyclase activity. Deletion of th e gene indicates that DdGCA is not essential for chemotaxis or osmo-regulat ion. The knock-out strain still exhibits substantial guanylate cyclase acti vity, demonstrating that Dictyostelium contains at least one other guanylat e cyclase.