The structure of nucleosome assembly protein 1.
Nucleosome assembly protein 1 (NAP-1) is an integral component in the establishment, maintenance, and dynamics of eukaryotic chromatin. It shuttles histones into the nucleus, assembles nucleosomes, and promotes chromatin fluidity, thereby affecting the transcription of many genes. The 3.0 A crystal structure of yeast NAP-1 reveals a previously uncharacterized fold with implications for histone binding and shuttling. A long alpha-helix is responsible for homodimerization via a previously uncharacterized antiparallel non-coiled-coil, and an alpha/beta domain is implicated in protein-protein interaction. A nuclear export sequence that is embedded in the dimerization helix is almost completely masked by an accessory domain that contains several target sites for casein kinase II. The four-stranded antiparallel beta-sheet that characterizes the alpha/beta domain is found in all histone chaperones, despite the absence of homology in sequence, structural context, or quaternary structure. To our knowledge, this is the first structure of a member of the large NAP family of proteins and suggests a mechanism by which the shuttling of histones to and from the nucleus is regulated.