4PXK:
reference: Crystal Structure of Escherichia coli-Expressed Haloarcula marismortui Bacteriorhodopsin I in the Trimeric Form., Shevchenko V, Gushchin I, Polovinkin V, Round E, Borshchevskiy V, Utrobin P, Popov A, Balandin T, Buldt G, Gordeliy V, PLoS One. 2014 Dec 5;9(12):e112873. doi: 10.1371/journal.pone.0112873., eCollection 2014. PMID: 25479443

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