Hydrophobic and acidic moments of a nucleoplasmin NP-core chaperone.

A recent crystal structure, at atomic resolution, of the NO38-core chaperone has revealed a decamer comprised of a dimer of pentamers, with each pentamer consisting of closely coupled eight-stranded beta-barrel monomers. This N-terminal core domain of the chaperone shares the Nucleoplasmin family fold and is presumed to assist the binding of the core histones in their assembly into nucleosomes during DNA replication and repair. The present work provides a measure of the hydrophobic residue burial about the different interfaces and centers of the NO38-core multimeric structure. While the hydrophobic "pentameric ring," comprised of the hydrophobic cores of the monomers and prevalence of non-polar residues at their interfaces is observed, a hydrophobic bias with respect to the center of the pentamer is also found, and consequently also expected to contribute to the thermostability of the multimer. Structural and chromatographic analysis had shown the NO38-core chaperone to bind (H3-H4)2 histone tetramers as well as H2A-H2B dimers. The acidic dipole, which reflects the spatial disposition of the acidic residues of the core monomer points to the lateral region of the monomers comprising the oligomers, and thereby, shows it to be the region of charge that would optimally complement the basic charge of the histones in their electrostatic binding to the chaperone. It is also pointed out that the prevalence of basic residues on the short helices of the histone cores also provides regions of charge that would complement histone binding to the chaperone.