Nucleophosmin in Its Interaction with Ligands

Abstract

Nucleophosmin (NPM1) is a mainly nucleolar protein that shuttles between nucleoli, nucleoplasm and cytoplasm to fulfill its many functions. It is a chaperone of both nucleic acids and proteins and plays a role in cell cycle control, centrosome duplication, ribosome maturation and export, as well as the cellular response to a variety of stress stimuli. NPM1 is a hub protein in nucleoli where it contributes to nucleolar organization through heterotypic and homotypic interactions. Furthermore, several alterations, including overexpression, chromosomal translocations and mutations are present in solid and hematological cancers. Recently, novel germline mutations that cause dyskeratosis congenita have also been described. This review focuses on NPM1 interactions and inhibition. Indeed, the list of NPM1 binding partners is ever-growing and, in recent years, many studies contributed to clarifying the structural basis for NPM1 recognition of both nucleic acids and several proteins. Intriguingly, a number of natural and synthetic ligands that interfere with NPM1 interactions have also been reported. The possible role of NPM1 inhibitors in the treatment of multiple cancers and other pathologies is emerging as a new therapeutic strategy.

Keywords: AML; B23; acute myeloid leukemia; cancer; nucleophosmin; protein–nucleic acids interactions; protein–protein interactions.

Figure 1

Nucleophosmin (NPM1) functions, structural organization and acute myeloid leukemia (AML) mutations. (A) The main functions played by NPM1 through its interactions with proteins and nucleic acids are represented. (B) NPM1 is composed of a N-terminal domain (NTD) preceded by a methionine-rich short sequence. NTD contains two nuclear export signals (NES) and a short acidic tract (A1). NTD is followed by an intrinsically disordered region (IDR) which contains two acidic tracts (A2 and A3), a bipartite nuclear localization signal (NLS) and a basic region (BR). Finally, a positively charged C-terminal domain (CTD), which contains the nucleolar localization signal (NoLS), is present. (C) Heterozygous AML-associated mutations cause the expression of a protein that is longer by four residues and has a different sequence in the last seven. A novel NES appears in all mutants (underlined residues). Here, the C-terminal sequences of wild-type and the most common mutants are shown.

Figure 2

NPM1 C-terminal domain and interaction with nucleic acids. (A) The structure of the NPM1 C-terminal domain, consisting of three helices forming a bundle, is represented in ribbon. In sticks are highlighted residues forming the hydrophobic core of the three-helix bundle, including W288 and W290, that are affected by AML-related mutations. (B) Structure of the complex between NPM1 CTD (white ribbon) and the G-quadruplex region from the c-MYC promoter (gold ribbon). The phosphate backbone of the G-quadruplex is engaged by a positively charged groove between helices 1 and 2.

Figure 3

NPM1 N-terminal domain and interaction with arginine-rich motifs. NPM1 N-terminal domain (NTD) consists of eight antiparallel beta-strands forming a jellyroll barrel. Monomers associate to form a homopentamer, here represented from above (A) and in front view (B). (C) Arginine-rich motifs, often predicted to be the NoLS in protein partners, interact with residues of two adjacent monomers. Residues identified from the NMR analysis of the NPM1-p14ARF complex are highlighted in this surface representation. (D) Docking analysis of the interaction between NPM1 NTD and the NoLS peptide from Fbw7γ. The peptide adopts an extended conformation and engages two adjacent monomers.