Nucleophosmin Is Essential for Ribosomal Protein L5 Nuclear Export

Abstract

Nucleophosmin (NPM/B23) is a key regulator in the regulation of a number of processes including centrosome duplication, maintenance of genomic integrity, and ribosome biogenesis. While the mechanisms underlying NPM function are largely uncharacterized, NPM loss results in severe dysregulation of developmental and growth-related events. We show that NPM utilizes a conserved CRM1-dependent nuclear export sequence in its amino terminus to enable its shuttling between the nucleolus/nucleus and cytoplasm. In search of NPM trafficking targets, we biochemically purified NPM-bound protein complexes from HeLa cell lysates. Consistent with NPM's proposed role in ribosome biogenesis, we isolated ribosomal protein L5 (rpL5), a known chaperone for the 5S rRNA. Direct interaction of NPM with rpL5 mediated the colocalization of NPM with maturing nuclear 60S ribosomal subunits, as well as newly exported and assembled 80S ribosomes and polysomes. Inhibition of NPM shuttling or loss of NPM blocked the nuclear export of rpL5 and 5S rRNA, resulting in cell cycle arrest and demonstrating that NPM and its nuclear export provide a unique and necessary chaperoning activity to rpL5/5S.

We thank J. Alan Diehl and Joachim Hauber for plasmid constructs as well as Loryn Rikimaru, Yijun Yi, and Myla Ashfaq for their excellent technical assistance. We are extremely grateful to Alan Diehl, Chuck Sherr, Greg Longmore, Helen Piwnica-Worms, David Gutmann, and Joseph Baldassare for insightful discussions.

Y.Y. and A.J.A. are recipients of a grant-in-aid from the Department of Defense Breast Cancer Research Program (BC030793). L.B.M. is supported by the Department of Defense Prostate Cancer Research Program under award number PC040009. H.L. is supported by a grant from the National Cancer Institute (CA-095441). J.D.W. thanks the Pew Charitable Trusts and is a recipient of grant-in-aid from the National Institutes of Health (GM-066032).

Views and opinions of, and endorsements by, the author(s) do not reflect those of the U.S. Army or the Department of Defense.