The Structure and Function of Xenopus NO38-Core, a Histone Chaperone in the Nucleolus

Abstract

Xenopus NO38 is an abundant nucleolar chaperone and a member of the nucleoplasmin (Np) family. Here, we report high-resolution crystal structures of the N-terminal domain of NO38, as a pentamer and a decamer. As expected, NO38 shares the Np family fold. In addition, NO38- and Np-core pentamers each use highly conserved residues and numerous waters to form their respective decamers. Further studies show that NO38 and Np each bind equal amounts of the four core histones. However, NO38 prefers the (H3-H4)(2) tetramer, while Np probably prefers H2A-H2B dimers. We also show that NO38 and Np will each bind noncognate histones when the preferred partner is absent. We suggest that these chaperones must form decamers in order to bind histones and differentiate between histone tetramers and dimers. When taken together, these data imply that NO38 may function as a histone chaperone in the nucleolus.

NSC348884, a nucleophosmin inhibitor disrupts oligomer formation and induces apoptosis in human cancer cells

“…We hypothesized that an SMI, targeting protein-protein interactions of the dimer interface would inhibit NPM function. Since there is no crystal structure available for human NPM, a homology model of human NPM was built according to the Xenopus NO38-core chaperone structure (residues, 1-107) (pdb:1XE0) (Namboodiri et al, 2004), which has a sequence identity of 77% and similarity of >90%. The homology model of NPM demonstrated a highly conserved topology and sidechain interactions of the dimer interface in comparison to Xenopus NO38-core structure.…”

“…Since there is no crystal structure available for human NPM, a homology model of the human NPM was built utilizing 'Modeller' on a Linux workstation (http://salilab.org/modeller) utilizing the highly homologous Xenopus NO38-core chaperone structure (pdb:1XE0) (Namboodiri et al, 2004). The model was energy minimized (Sybyl V7.3, Tripos, MO, USA) and analysed by Procheck V3.5 (http://www.biochem.ucl.ac.uk/Broman/ procheck) and Ramachandran plot for correctness and outliers.…”

“…This is due to the discovery of several specific small molecular agents targeting anti-apoptotic targets such as Bcl-2-Bax (Oltersdorf et al, 2005), MDM2-p53 (Vassilev et al, 2004) and XIAP-SMAC (Oost et al, 2004). Under native condition, NPM exists as oligomers via its Nterminal molecular chaperone domain (Herrera et al, 1996;Hingorani et al, 2000;Namboodiri et al, 2004). We hypothesized that a specific SMI that disrupts the formation of oligomers would inhibit NPM function(s) in cancer cells.…”

“…NPM shuttles between the nucleolus and the cytoplasm, and it also translocates from the nucleolus to the nucleoplasm during the stationary phase of growth or during treatment with certain antitumor drugs (Yung et al, 1990;Chou and Yung, 1995). Under native condition, NPM exists as an oligomer (Herrera et al, 1996;Namboodiri et al, 2004). The structural features of NPM consist of an oligomerization domain, a metal-binding motif, a bipartite nuclear localization signal, phosphorylation sites and a nucleolar localization signal (Wang et al, 1993;Wang et al, 2005).…”

Histone chaperones in nucleosome eviction and histone exchange

“…The structures of several variants of nucleoplasmin ( [42] and references therein), NAP1 and the related SET/TAF-1 [43] [44] (all putative H2A/H2B chaperones) are now known, as is the structure of the H3-H4 chaperone Asf1 [45]. The structures of HIRA and the histone binding subunit of CAF-1 have been predicted through homology modeling [46,47].…”