Nucleoplasmin

Nucleoplasmin (NPM2), the first identified molecular chaperone, is a thermostable acidic protein with a pentameric structure. The protein was first isolated from Xenopus species, and is now recognized as a highly conserved histone chaperone found across animals and other eukaryotes.

Family The nucleoplasmin/nucleophosmin (NPM) protein family comprises Nucleophosmin (NPM1), Nucleoplasmin 2 (NPM2), Nucleoplasmin 3 (NPM3), and nucleoplasmin-like proteins (NLP). These proteins typically share a pentameric N-terminal β-sandwich core. While NPM1 functions broadly in nuclear organization and cellular homeostasis, NPM2 is primarily associated with histone chaperoning during early development and is most frequently described as the canonical nucleoplasmin. The human NPM1 gene is also of clinical interest, as mutations in it are linked to acute myeloid leukemia (AML). In mammalian oocytes, NPM1, NPM2, and NPM3 are co-expressed and appear to function cooperatively during sperm chromatin remodeling, with NPM2 serving as the dominant oocyte form and NPM1 and NPM3 providing complementary and partially compensatory activities. This functional overlap is supported by evidence that the three proteins share similar histone chaperone properties and contribute collectively to paternal chromatin reorganization during early development. Human NPM paralogs also differ in their oligomerization behavior. NPM1 and NPM2 assemble into highly stable pentamers that can further associate into decamers, whereas NPM3 primarily forms dimers and becomes pentamer-competent only when incorporated into mixed complexes with NPM1. These hetero-oligomeric assemblies typically contain a 4:1 NPM1:NPM3 stoichiometry and have been demonstrated through crosslinking and blue-native polyacrylamide gel electrophoresis (BN-PAGE) analyses. Incorporation into such mixed pentamers enables NPM3 to acquire full histone chaperone activity, indicating a cooperative mechanism among NPM family members in vivo. In addition, protein–protein interaction network analysis shows that NPM1, NPM2, and NPM3 each participate in distinct interaction clusters, with NPM2 most strongly associated with transcriptional regulation, chromatin structure organization, and cell cycle–related protein networks.

Human proteins Humans express three members of the nucleoplasmin family:

Nucleophosmin (NPM1) Nucleoplasmin 2 (NPM2) Nucleoplasmin 3 (NPM3)

Protozoan nucleoplasmin homologs A nucleoplasmin-like histone chaperone has also been characterized in the malaria parasite Plasmodium falciparum (PfNPM), whose N-terminal domain adopts the same β-sandwich fold seen in vertebrate nucleoplasmins and assembles into a stable pentamer. Phylogenetic analysis places PfNPM with FKBP-type and HD-tuin nucleoplasmin clades rather than with the vertebrate NPM1–3 group, indicating its divergent evolutionary origin. Structural studies of the P. falciparum nucleoplasmin homolog show that both the N- and C-termini extend from the distal face of the pentamer, reflecting an overall topology similar to NPM2 and other metazoan family members. Although this protozoan protein lacks the continuous A1 acidic tract found in vertebrate nucleoplasmins, it contains multiple acidic patches, including residues in loops L2 and L6, that generate a negatively charged surface suited for histone binding. Biophysical analyses further demonstrate that the P. falciparum pentamer is highly resistant to thermal and chemical denaturation, remaining intact at elevated temperatures and in high-salt environments, a property common across nucleoplasmins.

Structure Nucleoplasmin 2 (NPM2) is encoded by a gene on chromosome 8p21.3 and includes 10 exons producing a 214-amino-acid protein. NPM2 has a two-domain architecture consisting of an N-terminal core domain (NTD) and a C-terminal tail domain. The NTD forms a compact β-sandwich of eight antiparallel β-strands that assembles into a homopentamer, generating the characteristic doughnut-shaped oligomer of the nucleoplasmin family. Crystal structures of the Xenopus nucleoplasmin core reveal that each monomer adopts a jellyroll-type β-barrel, and the resulting wedge-shaped subunits pack tightly to form a highly stable pentameric ring. The core domain also contains several conserved sequence motifs that stabilize the hydrophobic β-barrel and mediate contacts between neighboring pentamers, supporting higher-order oligomerization. Under certain conditions, two nucleoplasmin pentamers can associate into a decamer, providing an expanded platform for histone binding and storage. Structural analyses indicate that specific residues—such as Glu57 in the conserved AKEE loop and Gln84 in the adjacent Q-loop—mediate inter-pentamer hydrogen bonding, contributing to decamer stabilization. Computational docking studies further support this interface, identifying Glu57 and Gln84 as key contact residues driving pentamer–pentamer recognition. The C-terminal tail is an intrinsically disordered region containing the acidic A1, A2 and A3 tracts, a bipartite nuclear localization signal, and KR-rich basic segment. Unlike other nucleoplasmins, NPM2 contains a shortened A1 acidic loop consisting of a single glutamic acid residue (Glu37), which may contribute to the inability of its core domain to bind to histones directly. The A2 acidic stretch serves as the principal histone-interaction and regulatory site. Together, this pentameric and highly acidic structure provides a scaffold capable of simultaneously engaging multiple histones. Structural analyses, including crystal and cryo-EM studies of nucleoplasmin-like proteins such as AtFKBP53, confirm that this overall fold and assembly mode are conserved across diverse eukaryotic species.

… excerpt ends here. Continue reading the full article.