RON12, a novel Plasmodium-specific rhoptry neck protein important for parasite proliferation.

Apicomplexan parasites invade host cells by a conserved mechanism: parasite proteins are secreted from apical organelles, anchored in the host cell plasma membrane, and then interact with integral membrane proteins on the zoite surface to form the moving junction (MJ). The junction moves from the anterior to the posterior of the parasite resulting in parasite internalization into the host cell within a parasitophorous vacuole (PV). Conserved as well as coccidia-unique rhoptry neck proteins (RONs) have been described, some of which associate with the MJ. Here we report a novel RON, which we call RON12. RON12 is found only in Plasmodium and is highly conserved across the genus. RON12 lacks a membrane anchor and is a major soluble component of the nascent PV. The bulk of RON12 secretion happens late during invasion (after parasite internalization) allowing accumulation in the fully formed PV with a small proportion of RON12 also apparent occasionally in structures resembling the MJ. RON12, unlike most other RONs is not essential, but deletion of the gene does affect parasite proliferation. The data suggest that although the overall mechanism of invasion by Apicomplexan parasites is conserved, additional components depending on the parasite-host cell combination are required.

A novel Sushi domain-containing protein of Plasmodium falciparum.

Using bioinformatics analyses of the completed malaria genome sequence, we have identified a novel protein with a potential role in erythrocyte invasion. The protein (PFD0295c, ) has a predicted signal sequence and transmembrane domain and a sequence near the C-terminus of the protein shows significant similarity with Sushi domains. These domains, which exist in a wide variety of complement and adhesion proteins, have previously been shown to be involved in protein-protein and protein-ligand interactions. Orthologous genes have also been identified in the genomes of several other Plasmodium species, suggesting a conserved function for this protein in Plasmodium. Our results show that this protein is located in apical organelles and we have therefore designated the protein apical Sushi protein (ASP). We show that the expression of ASP is tightly regulated in the intraerythrocytic stages of the parasite and that it undergoes post-translational proteolytic processing. Based on our observations of timing of expression, location and proteolytic processing, we propose a role for ASP in erythrocyte invasion.