In tachyzoites, required for the cellular trafficking of guanylate cyclase GC and UGO to the cell membrane (PubMed:30742070). May play a role in the folding of the GC P-type ATPase-like domain to sense vacuolar changes in phosphatidic acid and pH levels which trigger parasite egress (PubMed:30742070). Interacts with GC; the interaction regulates guanylate cyclase GC trafficking and sensing environmental changes. In intracellular tachyzoites, localizes to the apical cap, to the cytoplasm in disperse foci and to the residual body. Conditional knockout in tachyzoites severely impairs lytic parasite growth in host cells without affecting intracellular replication (PubMed:30742070). Loss of egress from host cells; however, tachyzoite motility and host cell invasion are normal (PubMed:30742070). Guanylate cyclase GC is unstable and, together with UGO, remains trapped along the secretory pathway (PubMed:30742070). Loss of microneme secretion in response to phosphatidic acid or a decrease in vacuolar pH (PubMed:30742070). Belongs to the CDC50/LEM3 family.