Distinct histone H3-H4 binding modes of sNASP reveal the basis for cooperation and competition of histone chaperones.

Chromosomal duplication requires de novo assembly of nucleosomes from newly synthesized histones, and the process involves a dynamic network of interactions between histones and histone chaperones. sNASP and ASF1 are two major histone H3-H4 chaperones found in distinct and common complexes, yet how sNASP binds H3-H4 in the presence and absence of ASF1 remains unclear. Here we show that, in the presence of ASF1, sNASP principally recognizes a partially unfolded Nα region of histone H3, and in the absence of ASF1, an additional sNASP binding site becomes available in the core domain of the H3-H4 complex. Our study also implicates a critical role of the C-terminal tail of H4 in the transfer of H3-H4 between sNASP and ASF1 and the coiled-coil domain of sNASP in nucleosome assembly. These findings provide mechanistic insights into coordinated histone binding and transfer by histone chaperones.