Catalyzes the synthesis of the second messenger cGMP from GTP (PubMed:30992368, PubMed:31235476, PubMed:30742070). During the tachyzoite lytic growth cycle in host cells, detects and transduces environmental changes in potassium, phosphatidic acid and pH levels (PubMed:31235476, PubMed:30992368). By producing cGMP in response to these environmental changes, activates PKG and thereby regulates PKG-dependent microneme secretion which is essential for tachyzoite motility, host cell attachment invasion of and egress from host cells (PubMed:30449726, PubMed:30992368, PubMed:31235476, PubMed:30742070). May play a role in the fission of connected tachyzoites at their basal pole during egress (PubMed:30742070). Does not display flippase activity towards phosphatidylserine, phosphatidic acid or phosphatidylcholine (PubMed:30742070). GTP = 3',5'-cyclic GMP + diphosphate Binds 2 magnesium ions per subunit (By similarity). Is also active with manganese (in vitro) (By similarity). Interacts with chaperone CDC50.1; the interaction regulates guanylate cyclase GC trafficking and sensing environmental changes (PubMed:30742070). Interacts with UGO; the interaction regulates guanylate cyclase GC trafficking and catalytic activity (PubMed:30742070). Localizes to the apical cap in extracellular tachyzoites (PubMed:30449726, PubMed:30992368). During tachyzoite intracellular replication in the host cell, localizes to the apical cap, to the cytoplasm in disperse foci and to the residual body (PubMed:30992368, PubMed:31235476, PubMed:30742070). Expressed in tachyzoites. The P-type ATPase-like domain is required for GC apical localization and microneme secretion in tachyzoites (PubMed:30449726). May act as a sensor for external changes in ionic conditions such as potassium or pH levels (Probable). May act as a sensor for vacuolar changes in phosphatidic acid or pH levels (Probable). Does not display flippase activity towards phosphatidylserine, phosphatidic acid or phosphatidylcholine (PubMed:30742070). Conditional knockout in tachyzoites blocks lytic parasite growth in host cells (PubMed:30449726, PubMed:30992368, PubMed:31235476, PubMed:30742070). Reduces tachyzoite attachment to host cell (PubMed:30449726, PubMed:30992368). Prevents invasion of and egress from host cells without affecting tachyzoite replication inside the host cell (PubMed:30449726, PubMed:30992368, PubMed:31235476, PubMed:30742070). Impairs tachyzoite gliding motility (PubMed:30449726, PubMed:30992368, PubMed:31235476). Impairs stimulated microneme secretion without affecting basal microneme secretion or dense granule protein secretion (PubMed:30449726, PubMed:30992368, PubMed:30742070). cGMP production is reduced (PubMed:30992368, PubMed:31235476). cGMP production and microneme secretion is impaired in response to phosphatidic acid (PubMed:30742070). Increase in cytoplasmic Ca(2+) levels in response to a decrease in extracellular potassium levels or in pH is impaired (PubMed:30992368). Basal phosphatidic acid levels are reduced (PubMed:30992368). Infection of mice with knockout parasites (strain ME49) does not cause lethal toxoplasmosis; mice show severe reduction in parasite burden and have no detectable cyst formation in tissues (PubMed:30449726). In the N-terminal section; belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IV subfamily. In the C-terminal section; belongs to the adenylyl cyclase class-4/guanylyl cyclase family. The guanylate cyclase domains 1 and 2 lack catalytic activity when expressed on their own or in combination.