Abstract
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-CIII(2) (Tt-SC I+III(2)) and a structure of Tt-CIV(2). Tt-SC I+III(2) (~2.3 megadaltons) is a curved assembly with structural and functional symmetry breaking. Tt-CIV(2) is a ~2.7-megadalton dimer with more than 50 subunits per protomer, including mitochondrial carriers and a TIM8(3)-TIM13(3)-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.