Microneme proteins: structural and functional requirements to promote adhesion and invasion by the apicomplexan parasite Toxoplasma gondii

Host-cell invasion by apicomplexan parasites is extremely rapid and relies on a sequence of events that are tightly controlled in time and space. In most Apicomplexa, the gliding motility and host-cell invasion are tightly coupled to the release of microneme proteins at the apical tip of the parasites and their redistribution toward the posterior pole. This movement is dependent on an intact parasite actomyosin system. Micronemes are involved in the trafficking and storage of ligands (MICs) for host-cell receptors that are not only structurally related but also functionally conserved among the Apicomplexa. In Toxoplasma gondii, the repertoire of membrane-spanning microneme proteins includes adhesins such as TgMIC2 and escorters such as TgMIC6. The latter forms a complex with the soluble adhesins, TgMIC1 and TgMIC4 and assures their proper sorting to the mironemes. Escorters are also anticipated to bridge host-cell receptors to the parasite membrane during invasion. Most TgMICs are proteolytically cleaved either during their transport along the secretory pathway and/or after exocytosis. The biological significance of these processing events is largely unknown. One of these processing events targets a conserved motif close to the membrane-spanning domain causing the release of the processed form of the micronemes from the parasite surface. The cleavages occurring after release might contribute to the disassembly of the complexes and thus to fission between the parasitophorous vacuole and the host plasma membrane at the end of the invasion process. Gliding motility and host-cell penetration involve the redistribution of the micronemes toward the posterior pole of the parasites. This capping process involves actin polymerisation, myosin adenosine triphosphatase activation and the establishment of a connection between the MICs-receptor complexes and the actomyosin system of the parasite. The most carboxy-terminal end of the MICs cytoplasmic tails is implicated in this process, but the precise nature of the connection with the actomyosin system remains to be elucidated.