A respiratory microdomain links ubiquinone biosynthesis to succinate dehydrogenase in an apicomplexan mitochondrion lacking Complex I

Mitochondrial complex I (CI) is a major entry point for NADH-derived electrons and an important site of redox regulation in most eukaryotes. However, apicomplexan parasites, including Toxoplasma gondii, lack CI, raising a fundamental question of how mitochondrial respiration is organized in these parasites. Here, we identify a parasite-specific mitochondrial microdomain that physically and functionally couples ubiquinone (UQ) biosynthesis to succinate dehydrogenase (SDH). Using proximity labeling, membrane-based yeast two-hybrid analyses, reciprocal co-immunoprecipitations, and blue native PAGE, we show that UQ biosynthesis proteins associate with parasite-specific SDH subunits and co-migrate as a high-molecular-weight assembly. Knockdown of TgCoq4, a component of the UQ biosynthesis pathway, reduced SDH activity, and adding exogenous UQ to the assay did not restore it, indicating that UQ availability is not sufficient for efficient succinate oxidation. FCCP stimulates malate- and TMPD/ascorbate-supported respiration to maximal capacity but fails to do so with succinate as the substrate. This result, combined with reactive oxygen species (ROS) data, suggests that succinate causes over-reduction of the UQ pool, limiting further electron flow. We also find that CIII associates with the UQ synthesis machinery, suggesting a larger protein assembly. Together, these findings reveal that UQ biosynthesis and electron transport are physically and functionally integrated in T. gondii, defining a previously unrecognized organization of mitochondrial respiration in an organism lacking CI.