The Fo sector of the ATP synthase is a membrane bound complex which mediates proton transport. It is composed of nine different polypeptide subunits (a, b, c, d, e, f, g F6, A6L). The function of subunit g is currently unknown. The conserved region covers all but the very N-terminus of the member sequences. No prokaryotic members have been identified thus far [1].

1: Fo membrane domain of ATP synthase from bovine heart mitochondria: purification, subunit composition, and reconstitution with F1-ATPase. Collinson IR, Runswick MJ, Buchanan SK, Fearnley IM, Skehel JM, van Raaij MJ, Griffiths DE, Walker JE; Biochemistry 1994;33:7971-7978. PMID:8011660