Structural Insights into ceNAP1 Chaperoning Activity toward ceH2A-H2B

In eukaryotes, nucleosome assembly is crucial for genome integrity. The histone chaperone NAP1 plays an important role in histone folding, storage, and transport, as well as histone exchange and nucleosome assembly. At present, the molecular basis of these activities is not fully understood. We have solved high-resolution crystal structures of Caenorhabditis elegans NAP1 (ceNAP1) in complex with its cognate substrates: the C. elegans H2A-H2B dimer (ceH2A-H2B) and the H2A.Z-H2B dimer (ceH2A.Z-H2B). Our structural and biochemical data reveals the acidic concave surface is relevant to tetramerization, and uncovers how a ceNAP1 homodimer uses its concave surface to asymmetrically recognize a ceH2A-H2B or ceH2A.Z-H2B heterodimer. Intriguingly, an "acidic strip" within the concave surface of ceNAP1 is crucial for binding histones, including H2A-H2B, H3-H4, and histone variants. Thus, our results provide insight into the molecular mechanisms of NAP1 histone chaperone activity.