Determinants of centromeric nucleosome conformation
Centromeric chromatin is defined by the presence of the histone H3 variant CENP-A, which forms a specialized nucleosome required for kinetochore assembly. Compared to canonical H3 nucleosomes, CENP-A nucleosomes exhibit an open DNA conformation that leaves an additional 13 base pairs of DNA accessible at the entry and exit sites. While the CENP-A N-helix has previously been implicated in promoting this enhanced DNA breathing, the contributions of the intrinsically disordered N-terminal tail and adjacent latch regions of CENP-A to nucleosome conformation remain unknown. The intrinsically disordered N-terminal regions of histone H3 are known to facilitate interactions with DNA to stabilize overall nucleosome conformation. Here, we utilized a combination of MNase digestion assays and coarse-grained molecular dynamics simulations of nucleosomes containing targeted swaps of the H3 and CENP-A tail, latch, and N-helix regions to systematically test the contribution of each N-terminal histone region to nucleosomal DNA conformation. Substitution of individual H3 N-terminal regions with those of CENP-A opened the DNA at the entry/exit sites and increased DNA accessibility. While any single CENP-A N-terminal region was sufficient to open the canonical nucleosomal DNA conformation, replacement of any single CENP-A N-terminal region with its H3 counterpart was insufficient to restore the closed DNA conformation characteristic of canonical H3 nucleosomes. Instead, progressive incorporation of multiple H3-derived regions produced increasingly closed DNA conformations, demonstrating that the H3 tail, latch, and N-helices act cooperatively to stabilize wrapped nucleosomal DNA. Taken together, these findings demonstrate that the more restricted DNA breathing of canonical nucleosomes arises from coordinated contributions across multiple N-terminal regions and suggest that the multi-region redundancy in the conformational flexibility of the centromeric nucleosome could emphasize the importance of retaining flexibility in the centromeric nucleosome, even upon post-translational modification and binding to structural proteins.