TONSL/TONSOKU binding to histone H3.1 is required for DNA repair.

In plants, TONSOKU (TSK) regulates heterochromatic DNA replication and genome stability by preferentially binding histone H3.1 over its close homolog, histone H3.3. The structure of this interaction showed that a shallow pocket in TSK's poly-tetratricopeptide repeat (TPRTSK) domain discriminates between the variants but did not report the importance of additional interactions underlying its binding to histone H3.1. In this study, we identify additional binding pockets that are important for histone H3.1 binding and TSK-like (TONSL)-dependent DNA repair. Consistently, sponastrime dysplasia-linked mutations in the TPR domain of TONSL impair histone H3.1 binding and fail to rescue TONSL's DNA repair function in human cells. TPRTSK binding is highly regulated by posttranslational modifications (PTMs) on histone H3.1. Unexpectedly, TPRTSK contains two additional pockets that cooperate to recognize mono-methylated K4 and acetylated K27 on histone H3.1, two modifications associated with transcriptional regulatory elements like enhancers and promoters. Overall, our findings reveal that interactions between histone H3.1 and the TPR domains of TSK/TONSL are regulated by multiple PTMs and are functionally relevant for their DNA repair functions.