Identification and characterization of factors that govern apicoplast segregation in Toxoplasma gondii

The apicoplast is an essential plastid organelle of apicomplexan parasites that supports indispensable metabolic pathways, and its successful inheritance during cell division is critical for parasite survival. Here, we identify two previously uncharacterized proteins, APP1 and APP2, as key components of the apicoplast segregation machinery in Toxoplasma gondii. We show that APP1 is an inner membrane complex associated protein that is expressed during daughter bud development and localizes near the centrosomes and apicoplast. Loss of APP1 results in apicoplast loss and missegregation and growth defects without perturbing centrosome duplication or other organelles. Deletion mapping reveals regions of APP1 that are essential for apicoplast inheritance. Proximity labeling of APP1 further identified APP2, which localizes adjacent to APP1. APP2 is also found at the poles and along the length of the elongating apicoplast during organelle partitioning. Disruption of APP2 phenocopies loss of APP1, leading to lethal apicoplast inheritance defects. Interdependence analyses suggest that APP1 anchors the segregation machinery at the daughter IMC while APP2 associates with the apicoplast membrane, together forming a bridge that links the apicoplast to the centrosome during endodyogeny. These findings uncover a previously unknown molecular architecture controlling apicoplast segregation and highlight new parasite-specific targets for therapeutic intervention.