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Structures of Tetrahymena’s respiratory chain reveal the diversity of eukaryotic core metabolism

Respiration is a core biological energy–converting process whose last steps are carried out by a chain of multisubunit complexes in the inner mitochondrial membrane. To probe the functional and structural diversity of eukaryotic respiration, we examined the respiratory chain of the ciliate Tetrahymena thermophila (Tt). Using cryo–electron microscopy on a mixed sample, we solved structures of a supercomplex between Tt complex I (Tt-CI) and Tt-CIII2 (Tt-SC I+III2) and a structure of Tt-CIV2. Tt-SC I+III2 (~2.3 megadaltons) is a curved assembly with structural and functional symmetry breaking. Tt-CIV2 is a ~2.7-megadalton dimer with more than 50 subunits per protomer, including mitochondrial carriers and a TIM83-TIM133–like domain. Our structural and functional study of the T. thermophila respiratory chain reveals divergence in key components of eukaryotic respiration, thereby expanding our understanding of core metabolism. Description Big complexes from single-celled eukaryote Mitochondria use a series of membrane-embedded enzymes to transform chemical energy first into an electrochemical gradient and then into ATP. The series of chemical steps that take place are conserved in aerobic organisms, but there is considerable structural divergence in different lineages. Working with extracted mitochondrial membranes, Zhou et al. determined structures of mitochondrial electron transfer chain complexes from a model ciliate (see the Perspective by Huynen and Elurbe). Complex I and a dimer of complex III form a supercomplex as in many other eukaryotes, but both have important fundamental differences in architecture and functional apparatus. Complex IV is massive relative to previously known structures, with more than half of the mass composed of components not seen in homologous structures, including mitochondrial carrier proteins and a translocase that may be involved in assembly or may be a relic of such components. —MAF Structural analysis of Tetrahymena mitochondrial respiratory complexes illuminates the diversity of eukaryotic cellular respiration.