2QI9:
reference: Asymmetry in the structure of the ABC transporter-binding protein complex BtuCD-BtuF., Hvorup RN, Goetz BA, Niederer M, Hollenstein K, Perozo E, Locher KP, Science. 2007 Sep 7;317(5843):1387-90. Epub 2007 Aug 2. PMID: 17673622

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