Common signal transduction features in exocytosis in the alveolates Toxoplasma and Paramecium

Trichocyst discharge in Paramecium is associated with a calcium-dependent dephosphoglucosylation of the protein parafusin (PFUS), which is believed to be a critical step in the exocytosis cascade. After trichocyst release PFUS dissociates from the empty trichocyst capsule and remains in the cytosol until a new trichocyst develops. The minute tachyzoite of the apicomplexan parasite Toxoplasma contains three types of secretory organelle, the exocytoses of two of which, the micronemes and rhoptries, are essential for invasion of host cells. The discharge of micronemes is a calcium-dependent event, and an ortholog of PFUS, named parafusin-related protein (PRP1), has been found to co-localize with antibodies to microneme contents. The distribution of PRP1 changes upon discharge of micronemes, and localized staining of PRP1 disappeared only to reappear as new micronemes were formed. These results suggest that PFUS, PRP1 and probably other PFUS orthologs have a conserved role in calcium-dependent exocytosis in alveolates.