2HMV:
reference: The RCK Domain of the KtrAB K(+) Transporter: Multiple Conformations of an Octameric Ring., Albright RA, Ibar JL, Kim CU, Gruner SM, Morais-Cabral JH, Cell. 2006 Sep 22;126(6):1147-59. PMID: 16990138

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