The protein, called CENP-A or, in flies, CID, makes sense as an initial foundation stone for kinetochores. It substitutes for histone H3 and thus, of all kinetochore proteins, gets closest to the DNA. Human tissue culture cells overexpressing CENP-A did not make extra kinetochores, possibly because of a shortage of other factors. But in fly cells the extra CENP-A incorporated into ectopic chromosomal sites and, at some of those sites, recruited inner and outer kinetochore proteins, and motor proteins. Microtubule connections at these sites appeared to be exerting force on the chromosomes and caused chromosome breaks and aneuploidy.
Additional factors may be needed to help define kinetochore identity, but “this tells us we are on the right track,” says Karpen. He says evolution may have tolerated the occasional creation of new kinetochores because acentric products of chromosome rearrangments need a way to be rescued.
Karpen hopes to find sequences or histone modification patterns that favor either ectopic CENP-A incorporation or recruitment of other kinetochore proteins to these sites. The second challenge will be to find a loading factor specific for CENP-A, in the hope of localizing this factor and thus inducing kinetochore formation at a specific site.