Structural insights into instability of the nucleosome driven by histone variant H3T.

The testis-specific histone variant H3T plays a crucial role in chromatin reorganization during spermatogenesis by destabilizing nucleosomes. However, the structure basis for the nucleosome instability driven by H3T is not fully understand. In this study, we determinate the crystal structure of H3T-H4 in complex with histone chaperone ASF1a at 2.8 Å resolution. Our findings reveal that H3T-H4 binds ASF1a similarly to the conventional H3.1-H4 complex. However, significant structural differences are observed in the H3 α1 helix, the N- and C-terminal region of α2, and N-terminal region of L2. These differences are driven by H3T-specific residues, particularly Val111. Unlike the smaller Ala111 in H3.1, we find that bulkier residue Val111 fits well within the ASF1-H3T-H4 complex, but is difficult to arrange in nucleosome structure. Given that H3.1-Ala111/H3T-Val111 is located at the DNA binding and tetramerization interface of H3-H4, it is likely that Ala111Val substitution will lead to the instability of the corresponding area in nucleosome, contributing to instability of H3T-containing nucleosome. These structural findings may elucidate the role of H3T in chromatin reorganization during spermatogenesis.