Chen et al. reveal how Drosophila cells assemble the specialized histone CENP-A on centromeric DNA.
Eukaryotic centromeres are defined by the histone H3 variant CENP-A, which is specifically incorporated into the nucleosomes that package centromeric chromatin. In mammalian cells, CENP-A is deposited at centromeres by a chaperone protein called HJURP. This protein is conserved in budding yeast but is missing from several eukaryotic lineages, including insects, nematodes, and fish. One candidate to fulfill HJURP’s function in flies is a protein called CAL1, whose depletion results in the loss of CENP-A from Drosophila centromeres.
Chen et al. found that mis-targeting CAL1 to noncentromeric DNA induced the incorporation of CENP-A and the assembly of ectopic centromeres and kinetochores. CAL1’s N terminus—which binds to CENP-A—was sufficient to assemble CENP-A into chromatin, whereas CAL1’s C terminus localized the protein to centromeres by binding to a centromeric protein called CENP-C.
CAL1 could also incorporate CENP-A, but not histone H3, into nucleosomes in vitro. Nucleosomes containing Drosophila CENP-A have been reported to be tetrameric, but CAL1 assembled CENP-A into octameric nucleosomes that wrapped DNA into a negative supercoil, similar to regular, H3-containing nucleosomes and the centromeric, CENP-A–containing nucleosomes of other organisms.
Though apparently unrelated to HJURP, CAL1 is therefore the Drosophila CENP-A assembly factor. Senior author Barbara Mellone now wants to investigate why flies evolved to use a different CENP-A chaperone than the one used by yeast and mammals.