Two classes of chromatin domains are important for fidelity of human centromeres

The centromere is a vital locus on each chromosome which seeds the kinetochore, allowing for a physical connection between the chromosome and the mitotic spindle. At the heart of the centromere is the centromere-specific histone H3 variant CENP-A/CENH3. Throughout the cell cycle the constitutive centromere associated network is bound to CENP-A chromatin, but how this protein network modifies CENP-A nucleosome dynamics in vivo is unknown. Here, using a combination of biophysical and biochemical analyses we provide evidence for the existence of two populations of CENP-A nucleosomes that co-exist at human centromeres. Disrupting the balance of these two populations by overexpressing CENP-C results in reduced levels of centromeric RNA polymerase 2, impair de novo CENP-A loading, and correlate with significant mitotic defects. Mutants that either serve as a sink for excess CENP-C, or which are unable to bind CENP-C rescue the mitotic defects. These data support a model where a balance between both CENP-A populations is required for mitotic fidelity.