Replication-Independent Histone Deposition by the HIR Complex and Asf1

SummaryThe orderly deposition of histones onto DNA is mediated by conserved assembly complexes, including chromatin assembly factor-1 (CAF-1) and the Hir proteins 1, 2, 3 and 4. CAF-1 and the Hir proteins operate in distinct but functionally overlapping histone deposition pathways in vivo 5 and 6. The Hir proteins and CAF-1 share a common partner, the highly conserved histone H3/H4 binding protein Asf1, which binds the middle subunit of CAF-1 as well as to Hir proteins 7, 8, 9, 10 and 11. Asf1 binds to newly synthesized histones H3/H4 [12], and this complex stimulates histone deposition by CAF-1 7, 11 and 12. In yeast, Asf1 is required for the contribution of the Hir proteins to gene silencing 7 and 13. Here, we demonstrate that Hir1, Hir2, Hir3, and Hpc2 comprise the HIR complex, which copurifies with the histone deposition protein Asf1. Together, the HIR complex and Asf1 deposit histones onto DNA in a replication-independent manner. Histone deposition by the HIR complex and Asf1 is impaired by a mutation in Asf1 that inhibits HIR binding. These data indicate that the HIR complex and Asf1 proteins function together as a conserved eukaryotic pathway for histone replacement throughout the cell cycle.