Nanoscale Structure, Interactions, and Dynamics of Centromere Nucleosomes

Centromeres are specific segments of chromosomes responsible for the accurate chromosome segregation process. Centromeres are comprised of two types of nucleosomes: canonical nucleosomes containing an octamer of H2A, H2B, H3, and H4 histones, and CENP-A nucleosomes in which H3 is replaced with its analog CENP-A histone. This modification leads to the difference in the nuclear of DNA turns around the histone core, wrapping efficiency. This value is 121 bp of DNA, considerably less than 147 bp found in canonical nucleosomes. We used Atomic Force Microscopy (AFM) to characterize nanoscale features for both types of nucleosomes assembled on the same template, enabling us to evaluate the effect of internucleosomal interaction. We found that CENP-A mononucleosomes have a lower internucleosomal affinity than canonical H3 nucleosomes. We applied time-lapse, high-speed AFM (HS-AFM) to characterize the dynamics of nucleosomes. For both nucleosomes, spontaneous unwrapping of DNA was observed, and this process occurs via a transient state with ∼100 bp DNA wrapped around the core, followed by a rapid dissociation of DNA. The unwrapping process is asymmetric, so when the dissociation starts on one arm, it enlarges the size of the dissociated arm. Additionally, HS-AFM revealed higher stability of CENP-nucleosomes compared with H3 ones, in which dissociation of the histone core occurs prior to the nucleosome dissociation. The histone core of CENP-A nucleosomes remains intact even after the dissociation of DNA.