[Guanylyl cyclases of unicellular eukaryotes: structure, function, and regulatory properties]

Guanylyl cyclases (GCs), catalyzing the synthesis of the second messenger cGMP, are key elements of the signaling systems of animals of different phylogenetic levels including unicellular eukaryotes. In the review the literature data concerning unusual GCs observed in unicellular eukaryotes and having the structural-functional organization and topology similar to those of mammalian membrane-bound adenylyl cyclases, are analyzed. Among these GCs there are bifunctional membrane-bound GCs of ciliates and Plasmodium, which have both C-terminal cyclase domain related to mammalian adenylyl cyclases and N-terminal domain with ten membrane-spanning regions homologous to P-type ATPases. The developed by the author comparative analysis of primary structures of GC ATPase domains showed that the domains are high conservative and the motifs, which are closely linked to functional activity of ATPase transporters, are preserved in the domains. It is suggested that ATPase domains carry out either receptor or regulatory functions in GC molecules. Dual substrate specificity of cyclases of unicellular organisms and its possible role in revealing of GC activity in fungi and trypanosomes, lacking GC encoded genes, are discussed. The molecular mechanisms of the functioning of GCs, the regulation of GC activity by different agents, and the participation of these enzymes in control of the processes, such as chemotaxis, aggregation, movement, gametogenesis and photophobis response, are analyzed.