Mapping the evolution of mitochondrial complex I through structural variation

Respiratory complex I (CI) is a multi-subunit membrane protein complex important for the production of ATP via the oxidative phosphorylation pathway. The structure of CI is roughly conserved across species and is composed of subunits that are either embedded in the membrane or are exposed to the aqueous environment that together form an overall L-shaped 'boot'. The conserved core of CI is generally composed of 14 subunits. Across species, various less conserved 'supernumerary' or 'accessory' subunits have been added. Accessory subunits vary in number across species and can include proteins that are unique to specific lineages. Additionally, there are structural variations in the core subunits between clades. In this Review, we compare seven representative CI structures from divergent eukaryotic lineages to identify what aspects of the CI core subunits are susceptible to variation and classify eukaryotic accessory subunits into those conserved from the last eukaryotic common ancestor (LECA) or those that are lineage specific. Impact statement Understanding the biodiversity and evolution of mitochondrial complex I will reveal patterns that may reflect metabolic niche and can be used to constrain quantitative models of molecular evolution.