2RU6:
reference: Close Identity between Alternatively Folded State N2 of Ubiquitin and the Conformation of the Protein Bound to the Ubiquitin-Activating Enzyme., Kitazawa S, Kameda T, Kumo A, Yagi-Utsumi M, Baxter NJ, Kato K, Williamson MP, Kitahara R, Biochemistry. 2014 Jan 28;53(3):447-9. doi: 10.1021/bi401617n. Epub 2014 Jan 10. PMID: 24401037

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