Thr¹⁹⁹ phosphorylation targets nucleophosmin to nuclear speckles and represses pre‐mRNA processing

Abstract

Nucleophosmin (NPM) is a multifunctional phosphoprotein, being involved in ribosome assembly, pre-ribosomal RNA processing, DNA duplication, nucleocytoplasmic protein trafficking, and centrosome duplication. NPM is phosphorylated by several kinases, including nuclear kinase II, casein kinase 2, Polo-like kinase 1 and cyclin-dependent kinases (CDK1 and 2), and these phosphorylations modulate the activity and function of NPM. We have previously identified Thr 199 as the major phosphorylation site of NPM mediated by CDK2/cyclin E (and A), and this phosphorylation is involved in the regulation of centrosome duplication. In this study, we further examined the effect of CDK2-mediated phosphorylation of NPM by using the antibody that specifically recognizes NPM phosphorylated on Thr 199 . We found that the phospho-Thr 199 NPM localized to dynamic sub-nuclear structures known as nuclear speckles, which are believed to be the sites of storage and/or assembly of premRNA splicing factors. Phosphorylation on Thr 199 by CDK2/cyclin E (and A) targets NPM to nuclear speckles, and enhances the RNA-binding activity of NPM. Moreover, phospho-Thr 199 NPM, but not unphosphorylated NPM, effectively represses pre-mRNA splicing. These findings indicate the involvement of NPM in the regulation of pre-mRNA processing, and its activity is controlled by CDK2-mediated phosphorylation on Thr 199 .

A dose-dependent tug of war involving the NPM1 leukaemic mutant, nucleophosmin, and ARF

“…3 Shuttling and subcellular localization of NPM1 is, in turn, regulated by specific functional domains of the protein and post-translational modifications, such as sumoylation and phosphorylation. [4][5][6][7] Moreover, NPM1 partners, such as p14ARF, can influence NPM1 activity, modulating its nucleocytoplasmic shuttling. 8,9 Although functionally important, the shuttling pool of the protein is minimal and at immunocytochemistry, the localization of NPM1 is characteristically restricted to the nucleolus.…”

Nucleolin and nucleophosmin: nucleolar proteins with multiple functions in DNA repair

“…NPM1 binds to and regulates the activity of a range of numerous transcription factors including p53, c-Myc, androgen receptor, Miz1, NFκB, AP2α and CRCF, as well as regulating global RNA Pol II transcription through an interaction with HEXIM1 (Colombo et al 2011;Gurumurthy et al 2008). In conjunction with this transcriptional regulation of expression, NPM1 may act post-transcriptionally through loading onto mRNAs during polyadenylation and, possibly, promotion of their processing within nuclear speckles (Palaniswamy et al 2006;Tarapore et al 2006). In B cell lineages, NPM1 has been found in a complex with NCL, PARP1 and SWAP70 (termed the SWAP complex), which promotes the recombination of IgH gene switch regions (Borggrefe et al 1998).…”

Altered nucleophosmin transport in acute myeloid leukaemia with mutated NPM1: molecular basis and clinical implications

“…Finally, post-translational NPM1wt modifications, 32 including phosphorylation, [33][34][35] ubiquitination 36 and SUMOylation, 37 may also contribute to regulating cellular traffic of wild-type NPM1.…”