Fig. 4. Subunit-a interactions throughout the F_o-subcomplex.

a Subunit-a canonical six-helical core surrounded by the conserved subunits b, d, f, i/j, k and 8. It forms a non-canonical interaction with subunit-b′. The c-ring is grey. Window corresponds to the region shown in b close-up view. b Subunit-a forms a dimer interface with subunits a′, f′, ATPTT5-6′, with a phosphatidylethanolamine bound close to the interaction site. c Close-up of subunit-a, showing the six-helical core fold (H1-H6_a) with N- and C-terminal extensions a labeled. d Overview of F_o-subcomplex cryo-EM density showing peripheral location of subunit-a N-terminal extension and interacting subunits (subunit-colored, rest transparent gray). Close-up view shows subunit-a N-terminal extension scaffolding of helices from other F_o-subunits.