Models for the architecture of the human inner kinetochore CCAN complex on centromeric α-satellite CENP-A nucleosome arrays

Human kinetochores assemble onto centromeric DNA defined by tandem copies of thousands of the 171 bp α-satellite repeat sequence. The centromere-specific CENP-A nucleosome (CENP-A ^Nuc ) recruits the inner kinetochore constitutive centromere associated network (CCAN) complex. A previous cryo-EM structure of CCAN bound to a CENP-A ^Nuc reconstituted with a 171 bp α-satellite repeat sequence showed how extranucleosomal linker DNA threads through an internal tunnel in the CCAN complex. To understand the higher-order architecture of the inner kinetochore assembled onto α-satellite repeat arrays we have determined cryo-EM structures of CCAN with longer DNA sequences. These include free DNA and single and dimeric α-satellite repeats with CENP-A nucleosomes. We show from the structures of CCAN bound to both free DNA and monomeric CENP-A ^Nuc that CCAN engages 65-70 bp of DNA comprising 30-35 bp of an upstream α-satellite repeat. This upstream DNA interacts with the histone fold domain subunits of the CENP-TWSX module in a manner resembling how DNA is wrapped in nucleosomes. A complex of CCAN assembled onto a dimeric α-satellite repeat with two CENP-A nucleosomes showed that CCAN can only be accommodated on the linker DNA by unwrapping DNA from the CENP-TWSX module together with 25 bp of DNA from the upstream nucleosome. We discuss the implications of these results for models of CCAN assembly on arrays of α-satellite chromatin containing CENP-A ^Nuc .