Complexome profile of Toxoplasma gondii mitochondria identifies divergent subunits of respiratory chain complexes including new subunits of cytochrome bc1 complex

Author summary Apicomplexan parasites, such as Toxoplasma and Plasmodium, cause diseases of global importance, such as toxoplasmosis and malaria. The mitochondrial electron transport chain (mETC) and F1Fo-ATP synthase, which provide the parasite with energy and important metabolites, are essential for parasite function. Here, using a proteomic technique called complexome profiling, we report the composition of the Toxoplasma mETC and F1Fo-ATP synthase. In particular, we reveal the compositions of complexes II and III for the first time. Complex III is an important drug target, yet its full protein composition was unknown. We identify new parasite-specific complex III subunits and demonstrate that they are essential for parasite survival and for proper functioning of the mETC. Our study highlights the divergent nature of the apicomplexan mETC and F1Fo-ATP synthase. The mitochondrial electron transport chain (mETC) and F1Fo-ATP synthase are of central importance for energy and metabolism in eukaryotic cells. The Apicomplexa, important pathogens of humans causing diseases such as toxoplasmosis and malaria, depend on their mETC in every known stage of their complicated life cycles. Here, using a complexome profiling proteomic approach, we have characterised the Toxoplasma mETC complexes and F1Fo-ATP synthase. We identified and assigned 60 proteins to complexes II, IV and F1Fo-ATP synthase of Toxoplasma, of which 16 have not been identified previously. Notably, our complexome profile elucidates the composition of the Toxoplasma complex III, the target of clinically used drugs such as atovaquone. We identified two new homologous subunits and two new parasite-specific subunits, one of which is broadly conserved in myzozoans. We demonstrate all four proteins are essential for complex III stability and parasite growth, and show their depletion leads to decreased mitochondrial potential, supporting their assignment as complex III subunits. Our study highlights the divergent subunit composition of the apicomplexan mETC and F1Fo-ATP synthase complexes and sets the stage for future structural and drug discovery studies.

Automated summary

The study reveals the composition of the Toxoplasma mitochondrial electron transport chain (mETC) and F1Fo-ATP synthase using complexome profiling proteomic approach, with a particular focus on complexes II and III. The identification of new parasite-specific complex III subunits sheds light on the essential role of these proteins in parasite survival and proper functioning of the mETC. This highlights the diverse subunit composition of the apicomplexan mETC and F1Fo-ATP synthase complexes, setting the stage for future structural and drug discovery studies.