4APS:
reference: Alternating access mechanism in the POT family of oligopeptide transporters., Solcan N, Kwok J, Fowler PW, Cameron AD, Drew D, Iwata S, Newstead S, EMBO J. 2012 Jun 1. doi: 10.1038/emboj.2012.157. PMID: 22659829

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