Structure of the yeast histone H3-ASFI interaction: implications for chaperone mechanism, species-specific interactions, and epigenetics

Background: The histone H3/H4 chaperone AsfI (anti-silencing function 1) is required for the establishment and maintenance of proper chromatin structure, as well as for genome stability in eukaryotes. AsfI participates in both DNA replication-coupled ( RC) and replication-independent ( RI) histone deposition reactions in vitro and interacts with complexes responsible for both pathways in vivo. AsfI is known to directly bind histone H3, however, high-resolution structural information about the geometry of this interaction was previously unknown. Results: Here we report the structure of a histone/histone chaperone interaction. We have solved the 2.2 angstrom crystal structure of the conserved N-terminal immunoglobulin fold domain of yeast AsfI ( residues 2-155) bound to the C-terminal helix of yeast histone H3 ( residues 121-134). The structure defines a histone-binding patch on AsfI consisting of both conserved and yeast-specific residues; mutation of these residues abrogates H3/H4 binding affinity. The geometry of the interaction indicates that AsfI binds to histones H3/H4 in a manner that likely blocks sterically the H3/H3 interface of the nucleosomal four-helix bundle. Conclusion: These data clarify how AsfI regulates histone stoichiometry to modulate epigenetic inheritance. The structure further suggests a physical model in which AsfI contributes to interpretation of a "histone H3 barcode" for sorting H3 isoforms into different deposition pathways.