title: Born to be exported: COOH-terminal nuclear export signals of different strength ensure cytoplasmic accumulation of nucleophosmin leukemic mutants.
abstract: Creation of a nuclear export signal (NES) motif and loss of tryptophans (W) 288 and 290 (or 290 only) at the COOH terminus of nucleophosmin (NPM) are both crucial for NPM aberrant cytoplasmic accumulation in acute myelogenous leukemia (AML) carrying NPM1 mutations. Hereby, we clarify how these COOH-terminal alterations functionally cooperate to delocalize NPM to the cytoplasm. Using a Rev(1.4)-based shuttling assay, we measured the nuclear export efficiency of six different COOH-terminal NES motifs identified in NPM mutants and found significant strength variability, the strongest NES motifs being associated with NPM mutants retaining W288. When artificially coupled with a weak NES, W288-retaining NPM mutants are not exported efficiently into cytoplasm because the force (W288) driving the mutants toward the nucleolus overwhelms the force (NES) exporting the mutants into cytoplasm. We then used this functional assay to study the physiologic NH(2)-terminal NES motifs of wild-type NPM and found that they are weak, which explains the prominent nucleolar localization of wild-type NPM. Thus, the opposing balance of forces (tryptophans and NES) seems to determine the subcellular localization of NPM. The fact that W288-retaining mutants always combine with the strongest NES reveals mutational selective pressure toward efficient export into cytoplasm, pointing to this event as critical for leukemogenesis.
authors.affiliation: Section of Hematology and Immunology, University of Perugia, Perugia, Italy.
authors.forename: Niccolò
authors.initials: N
authors.lastname: Bolli
authors.forename: Ildo
authors.initials: I
authors.lastname: Nicoletti
authors.forename: M Felicetta
authors.initials: MF
authors.lastname: De Marco
authors.forename: Barbara
authors.initials: B
authors.lastname: Bigerna
authors.forename: Alessandra
authors.initials: A
authors.lastname: Pucciarini
authors.forename: Roberta
authors.initials: R
authors.lastname: Mannucci
authors.forename: Maria Paola
authors.initials: MP
authors.lastname: Martelli
authors.forename: Arcangelo
authors.initials: A
authors.lastname: Liso
authors.forename: Cristina
authors.initials: C
authors.lastname: Mecucci
authors.forename: Francesco
authors.initials: F
authors.lastname: Fabbiano
authors.forename: Massimo F
authors.initials: MF
authors.lastname: Martelli
authors.forename: Beric R
authors.initials: BR
authors.lastname: Henderson
authors.forename: Brunangelo
authors.initials: B
authors.lastname: Falini
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D002471:Cell Transformation, Neoplastic; D003593:Cytoplasm / Q000378:metabolism; D004252:DNA Mutational Analysis; D005347:Fibroblasts / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D015470:Leukemia, Myeloid, Acute / Q000378:metabolism; D051379:Mice; D008856:Microscopy, Fluorescence; D009154:Mutation; D041681:NIH 3T3 Cells; D009687:Nuclear Proteins / Q000737:chemistry* / Q000235:genetics*; D000090243:Nucleophosmin; D010957:Plasmids / Q000378:metabolism; D017434:Protein Structure, Tertiary
chemical_list: C000715570:Npm1 protein, mouse; D009687:Nuclear Proteins; D000090243:Nucleophosmin