(A) Subunits ATP6AP1/Ac45, ATP6AP2/PRR, and c″possess transmembrane α-helices in the centre of the c-ring. (B) ATP6AP2/PRR (top) and ATP6AP1/Ac45 (bottom) possess luminal domains that are absent from the structure. SP, signal peptide; Cyt, cytosolic. (C and D) Subunit d1 interacts with the C termini of ATP6AP1/Ac45 (C) and ATP6AP2/PRR (D). (E) Subunits c″, c(8), and ATP6AP2/PRR interact with the second luminal domain of ATP6AP1/Ac45. Scale bar, 10 Å. (F) Subunits c″, ATP6AP1/Ac45, ATP6AP2/PRR, and d1 create a network of interactions that connect the vesicle lumen and the cytoplasm. Scale bar, 25 Å.