Fig. 4: Models for the rhoptry secretion system in C. parvum and T. gondii.
From: In situ ultrastructures of two evolutionarily distant apicomplexan rhoptry secretion systems
Salient findings of this study include conserved ultrastructures in C. parvum and T. gondii for—shaping the rhoptries (via helical filaments in the neck); priming rhoptries for secretion (via the apical vesicle (AV) and the rhoptry secretory apparatus (RSA) that dock rhoptries to the plasma membrane); and for regulating rhoptry secretion (via RSAs that likely perform controlled fusion of the AV with the plasma membrane in response to rhoptry secretion stimuli). In their resting state, the RSAs hold the AV away from the plasma membrane thus preventing spontaneous fusion (top insets). Likewise, the ‘collar’ proteins in C. parvum and the rhoptry tip densities in T. gondii likely regulate the fusion of rhoptries with the AV. The two fusion events together would complete the conduit for rhoptry proteins’ secretion (denoted by * in bottom insets). T. gondii, in order to secrete multiple rhoptries (including the docked pair at the AV), likely requires multiple AVs. The microtubule-associated vesicles (MVs) are possibly trafficked to the cell apical tip (denoted by red arrows and arrow heads) to facilitate this function.
