Histone and Histone Chaperones


Histone chaperones (HCs), such as Asf1, Rtt106, CAF-1, HIRA, and FACT, play important roles in nucleosome assembly during DNA replication and DNA repair. Newly synthesized H3:H4 dimer is captured by Asf1 and acetylated by histone acetyltransferases (histone H3 Lys56). Following acetylation, H3-H4 dimers are handed off to the next set of chaperones, CAF-1 and Rtt106, which proceed to deposit the histones onto nascent DNA. HIRA is responsible for DNA replication independent histone assembly and FACT is involved in nucleosome disassembly and reassembly. Understanding nucleosome assembly mechanisms by its chaperones is a crucial not only to understand how newly synthesized histones are recruited during DNA replication and repair, but also important to reveal transferring mechanism of epigenetic information to newly incorporated nucleosomes. As a long-term goal, We like to pursue the structural and biochemical studies of HCs and address questions of: What are structural features of those HCs as it is and as a complex with its substrates, histones? How HCs recognize and stabilize histones in solution? How HCs distinguish histone variants, e.g. histone H3.1 by CAF-1 and histone H3.3 by HIRA? What is the effect of H3 acetylation in transferring histones from Asf1 to either Rtt105 or CAF-1 and eventually nucleosome incorporation?


Kap123, a major karyopherin protein of budding yeast, recognizes the nuclear localization signals (NLSs) of cytoplasmic histones H3 and H4 and translocates them into the nucleus during DNA replication. Mechanistic questions include H3- and H4-NLS redundancy toward Kap123 and the role of the conserved diacetylation of cytoplasmic H4 (K5ac and K12ac) in Kap123-mediated histone nuclear translocation. Recently, we reported crystal structures of full-length Kluyveromyces lactis Kap123 alone and in complex with H3- and H4-NLSs. Structures reveal the unique feature of Kap123 that possesses two discrete lysine-binding pockets for NLS recognition. Structural comparison illustrates that H3- and H4-NLSs share at least one of two lysine-binding pockets, suggesting that H3- and H4-NLSs are mutually exclusive. Additionally, acetylation of key lysine residues at NLS, particularly H4-NLS diacetylation, weakens the interaction with Kap123. These data support that cytoplasmic histone H4 diacetylation weakens the Kap123-H4-NLS interaction thereby facilitating histone Kap123-H3-dependent H3:H4/Asf1 complex nuclear translocation. (An et. al. 2017, eLife)