NASP maintains histone H3-H4 homeostasis through two distinct H3 binding modes

Histone chaperones regulate all aspects of histone metabolism. NASP is a major histone chaperone for H3-H4 dimers critical for preventing histone degradation. Here, we identify two distinct histone binding modes of NASP and reveal how they cooperate to ensure histone H3-H4 supply. We determine the structures of a sNASP dimer, a complex of a sNASP dimer with two H3 alpha 3 peptides, and the sNASP-H3-H4-ASF1b co-chaperone complex. This captures distinct functionalities of NASP and identifies two distinct binding modes involving the H3 alpha 3 helix and the H3 alpha N region, respectively. Functional studies demonstrate the H3 alpha N-interaction represents the major binding mode of NASP in cells and shielding of the H3 alpha N region by NASP is essential in maintaining the H3-H4 histone soluble pool. In conclusion, our studies uncover the molecular basis of NASP as a major H3-H4 chaperone in guarding histone homeostasis.