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

A parasite-specific mitochondrial microdomain in Toxoplasma gondii physically and functionally couples ubiquinone biosynthesis to succinate dehydrogenase, forming a high-molecular-weight assembly.

• Why it matters: Apicomplexans lack mitochondrial complex I, leaving unclear how their mitochondria organize electron entry and redox control to support respiration.
• What they did: Using proximity labeling, membrane-based yeast two-hybrid, reciprocal co-immunoprecipitation, and blue native PAGE, the study links UQ biosynthesis proteins to parasite-specific SDH subunits; TgCoq4 knockdown reduced SDH activity, and exogenous UQ did not restore it.
• The result: FCCP drove maximal malate- and TMPD/ascorbate-supported respiration but not succinate-supported respiration, and ROS data suggest succinate over-reduces the UQ pool; CIII also associates with the UQ machinery.

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