Part of the CF(0) (base unit) of the ATP synthase. The base unit is thought to translocate protons through membrane (inner membrane in mitochondria, thylakoid membrane in plants, cytoplasmic membrane in bacteria). The B subunits are thought to interact with the stalk of the CF(1) subunits. This domain should not be confused with the ab CF(1) proteins (in the head of the ATP synthase) which are found in Pfam:PF00006