5MSJ:
reference: The Mammalian Proteasome Activator PA28 Forms an Asymmetric alpha4beta3 Complex., Huber EM, Groll M, Structure. 2017 Aug 22. pii: S0969-2126(17)30247-2. doi:, 10.1016/j.str.2017.07.013. PMID: 28867616

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