Guanylyl cyclases with the topology of mammalian adenylyl cyclases and an N-terminal P-type ATPase-like domain in Paramecium, Tetrahymena and Plasmodium

We cloned a guanglyl cyclase of 280 kDa from the ciliate Paramecium which h as an N-terminus similar to that of a P-type ATPase and a C-terminus with a topology identical to mammalian adenylyl cyclases, Respective signature se quence motifs are conserved in both domains. The cytosolic catalytic Cia an d C2a segments of the cyclase are inverted. Genes coding for topologically identical proteins with substantial sequence similarities have been cloned from Tetrahymena and were detected in sequences from Plasmodium deposited b y the Malaria Genome Project. After 99 point mutations to convert the Param ecium TAA/TAG-Gln triplets to CAA/CAG, together with partial gene synthesis , the gene from Paramecium was heterologously expressed. In Sf9 cells, the holoenzyme is proteolytically processed into the two domains. Immunocytoche mistry demonstrates expression of the protein in Paramecium and localizes i t to cell surface membranes, The data provide a novel structural link betwe en class III adenylyl and guanylyl cyclases and imply that the protozoan gu anylyl cyclases evolved from an ancestral adenylyl cyclase independently of the mammalian guanylyl cyclase isoforms, Further, signal transmission in C iliophora (Paramecium, Tetrahymena) and in the most important endoparasitic phylum Apicomplexa (Plasmodium) is, quite unexpectedly, closely related.