4GCZ:
reference: Full-length structure of a sensor histidine kinase pinpoints coaxial coiled coils as signal transducers and modulators., Diensthuber RP, Bommer M, Gleichmann T, Moglich A, Structure. 2013 Jul 2;21(7):1127-36. doi: 10.1016/j.str.2013.04.024. Epub 2013, Jun 6. PMID: 23746806

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