2X2V:
reference: A new type of proton coordination in an F(1)F(o)-ATP synthase rotor ring., Preiss L, Yildiz O, Hicks DB, Krulwich TA, Meier T, PLoS Biol. 2010 Aug 3;8(8):e1000443. PMID: 20689804

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).