Dynamic protein interaction networks such as DNA double-strand break (DSB) signaling are modulated by post-translational modifications. The DNA repair factor 53BP1 is a rare example of a protein whose post-translational modification-binding function can be switched on and off. 53BP1 is recruited to DSBs by recognizing histone lysine methylation within chromatin, an activity directly inhibited by the 53BP1-binding protein TIRR. X-ray crystal structures of TIRR and a designer protein bound to 53BP1 now reveal a unique regulatory mechanism in which an intricate binding area centered on an essential TIRR arginine residue blocks the methylated-chromatin-binding surface of 53BP1. A 53BP1 separation-of-function mutation that abolishes TIRR-mediated regulation in cells renders 53BP1 hyperactive in response to DSBs, highlighting the key inhibitory function of TIRR. This 53BP1 inhibition is relieved by TIRR-interacting RNA molecules, providing proof-of-principle of RNA-triggered 53BP1 recruitment to DSBs.
Bibliographical noteFunding Information:
We are very grateful to R. Alkire, N. Duke, and J. Lazarz at Argonne National Laboratory for their outstanding assistance. X-ray diffraction data were collected at Argonne National Laboratory, Structural Biology Center (SBC) at the Advanced Photon Source. SBC is operated by UChicago Argonne, LLC, for the US Department of Energy, Office of Biological and Environmental Research, under contract DE-AC02-06CH11357. This research was supported by NIH grants R01 CA132878, R01 GM116829, and P50 CA136393 (Mayo Clinic Ovarian Cancer SPORE developmental project) to G.M.; and by NIH grants R01 CA208244 and R01CA142698, DoD grant W81XWH-15-0564/ OC140632, a Leukemia and Lymphoma Society Scholar grant, and the Claudia Adams Barr Program in Innovative Basic Cancer Research to D.C. M.V.B. was supported by DoD grant W81XWH-16-1-0391 and a Liz Tilberis award from the Ovarian Cancer Research Fund Alliance. G.C. received a Fellowship Award from the Mayo Clinic Cancer Center Fraternal Order of Eagles Funds. J.R.C. and C.O. were supported by a Cancer Research UK Career Development Fellowship Grant (C52690/A19270).