Our goal is to determine the structure of heptameric AAA+ ATPase Bcs1 bound to its native substrate Rip1. Rip1 is a critical component of the Complex III of mitochondrial electron transport chain. To be incorporated into Complex III, Rip1 first needs to fold and incorporate an iron-sulfur center in the matrix of the mitochondria. It maintains this center, presumably as a folded protein, when translocated from the mitochondrial matrix to the inner membrane by Bcs1. While the structures of Bcs1 without substrate have been determined, the mechanism of translocation of folded Rip1 by Bcs1 is still unclear. To elucidate the working mechanism of Bcs1, we are going to determine the structures of Bcs1-Rip1 complex at different stages of translocation by cryo-EM.