4EI2:
reference: Crystal Structure of a Ba(2+)-Bound Gating Ring Reveals Elementary Steps in RCK Domain Activation., Smith FJ, Pau VP, Cingolani G, Rothberg BS, Structure. 2012 Dec 5;20(12):2038-47. doi: 10.1016/j.str.2012.09.014. Epub 2012, Oct 18. PMID: 23085076

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