a, Schematic diagram showing experimentally determined domain boundaries of mBcs1. In the apo structure, the model for the transmembrane (TM) domain begins at residue L29 and ends at I49. The Bcs1-specific domain spans the range of residues 49 to 165. The AAA domain is composed of the RecA-like (residues 165–354) and α-helical domains (residues 355–418). The Walker A residue K236 and Walker B residue E282 are indicated. b,c, EM density maps and atomic models for apo mBcs1 (b) and for the ATPγS-bound mBcs1 (c) in two orthogonal orientations: side view (left) and view from the matrix side (right). Each subunit is colored differently. The TM, Bcs1-specific and AAA regions of the protein are indicated. Most of the protein resides in the mitochondrial matrix. The ‘Interstitial gap’ is defined for the less-dense part of the Bcs1-specific region connecting to the AAA regions.