The family of AAA+ proteins in eukaryotes has many members in various cellular compartments with a broad spectrum of functions in protein unfolding and degradation. The mitochondrial AAA protein Bcs1 plays an unusual role in protein translocation. It is involved in the topogenesis of the Rieske protein, Rip1, and thereby in the biogenesis of the cytochrome bc(1) complex of the mitochondrial respiratory chain. Bcs1 mediates the export of the folded FeS domain of Rip1 across the mitochondrial inner membrane and the insertion of its transmembrane segment into an assembly intermediate of the cytochrome bc(1) complex. We discuss structural elements of the Bcs1 protein compared to other AAA proteins in an attempt to understand the mechanism of its function. In this context, we discuss human diseases caused by mutations in Bcs1 that lead to different properties of the protein and subsequently to different symptoms. (C) 2012 Elsevier Inc. All rights reserved.