Structural Basis for Mis18 Complex Assembly: Implications for Centromere Maintenance

The centromere, defined by the enrichment of CENP-A (a Histone H3 variant)-containing nucleosomes, is a specialised chromosomal locus that acts as a microtubule attachment site. During each round of the cell cycle, CENP-A levels undergo DNA replication-mediated dilution. To maintain centromere identity, CENP-A levels must be restored. A central player mediating this process is the Mis18 complex (composed of Mis18α, Mis18β and Mis18BP1), which recruits the CENP-A specific chaperone HJURP to centromeres for CENP-A deposition. Here, using a multipronged approach we provide the structural basis for the assembly of the Mis18 complex. We show that the Mis18α/β hetero-trimer (2 Mis18α:1 Mis18β) is assembled by the formation of a triple helical bundle with a Mis18α/βYippee hetero-dimer and Mis18αYippee monomer on opposite ends. Two such Mis18α/β hetero-trimers, each bound to a Mis18BP1, assemble as a hetero-octamer via Mis18αYippee homo-dimerisation. Evaluation of structure-guided separation of function mutants in cells reveal structural determinants essential for Mis18 complex assembly and function.