Abstract

Apicomplexan parasites possess secretory organelles called rhoptries that undergo regulated exocytosis upon contact with the host. This process is essential for the parasitic lifestyle of these pathogens and relies on an exocytic machinery sharing structural features and molecular components with free-living ciliates. Here, we performed a Tetrahymena-based transcriptomic screen to uncover novel exocytic factors in Ciliata and Apicomplexa. We identified membrane-bound proteins, named CRMPs, forming part of a large complex essential for rhoptry secretion and invasion in Toxoplasma. In contrast to previously described rhoptry exocytic factors, TgCRMPs are not required for the assembly of the rhoptry secretion machinery and only transiently associated with the exocytic site - prior to invasion. CRMPs and their partners contain putative host cell-binding domains, and CRMPa shares similarity to GPCR proteins. We propose that the CRMP complex acts as host-molecular sensor to ensure that rhoptry exocytosis occurs when the parasite contacts the host cell.