Essential microneme protein that plays an important role in host cell invasion. Part of the moving junction (MJ) complex, a ringlike structure formed between the plasma membranes of the apical tip of the parasite and the target host cell. During invasion, the MJ migrates from the anterior to the posterior of the parasite, leading to internalization of the parasite into a parasitophorous vacuole (PV). Component of the moving junction (MJ) complex, composed of AMA1, a transmembrane protein on the parasite surface, and a complex of the rhoptry neck proteins RON2, RON4, RON5 and RON8 localized to the cytoplasmic face of the host plasma membrane. Interacts (via ectodomain) with RON2 (via C-terminus); RON2 serves as the receptor for AMA1 on the host plasma membrane. AMA1 and the RON proteins are initially in distinct compartments within the parasite, namely the micronemes and the rhoptries, and interaction happens only upon initiation of invasion when the micronemes and rhoptries discharge. Initially localizes to micronemes, specialized secretory organelles of apicomplexan parasites important for host cell invasion. Relocalizes to the surface membrane upon host invasion by tachyzoites. Found predominantly on the apical end of the parasite surface. A cleavage product of the large ectodomain is released into the medium by extracellular parasites. Proteolytically cleaved during invasion within its transmembrane domain, releasing a soluble form from the tachyzoite surface. The cytosolic tail generated by ROM4 cleavage during invasion may trigger parasite replication within the parasitophorous vacuole. Severely compromises the parasite in its ability to invade host cells. Inhibits secretion of the rhoptries, organelles whose discharge is coupled to active host cell penetration. Belongs to the apicomplexan parasites AMA1 family. PubMed:21205639 reported that AMA1 may also be involved in a signaling pathway leading to replication. However, PubMed:22523242 refutes this model.