Toxoplasma ferlin1 is a versatile and dynamic mediator of microneme trafficking and secretion

Calcium-dependent exocytosis of the microneme organelles that facilitate host cell invasion is critical for obligate intracellular apicomplexan parasites such as . Ferlins represent a protein family with roles in exocytosis containing multiple Ca-sensing C2 domains. Here we defined the role of ’s ferlin 1 (FER1) in microneme biology. FER1 localized dynamically to several compartments of the parasite’s secretory pathway as well as to an apical spot near the site of microneme secretion. FER1 function was dissected by overexpression of a variety of N-terminally tagged alleles causing dominant negative phenotypes. This demonstrated FER1 traffics microneme organelles at several discrete steps of their natural trajectories: 1. from ELC to the subpellicular microtubules; 2. along the subpellicular microtubules to the apical end; 3. into the conoid; 4. and inferred from observed retrograde transport from the subpellicular microtubules, recycling of micronemes from mother to daughter parasites. Furthermore, full-length FER1 overexpression results in a squirt of microneme release sufficient for host cell egress. This indicates FER1 facilitates fusion of the most apical, radially organized micronemes with the plasma membrane. Moreover, FER1 acts differentially on the Rab5A/C-dependent and - independent microneme sub-populations. Finally, apical FER1 overlaps with the presence of VP1, a pyrophosphatase proton pump. Integrating all new insights, we propose a model of microneme exocytosis wherein the radial micronemes constitute a readily releasable vesicle pool primed by acidification.