6YUD:
reference: Tetramerisation of the CRISPR ring nuclease Crn3/Csx3 facilitates cyclic oligoadenylate cleavage., Athukoralage JS, McQuarrie S, Gruschow S, Graham S, Gloster TM, White MF, Elife. 2020 Jun 29;9. pii: 57627. doi: 10.7554/eLife.57627. PMID: 32597755

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