Nucleophosmin Serves as a Rate-Limiting Nuclear Export Chaperone for the Mammalian Ribosome

Abstract

Nucleophosmin (NPM) (B23) is an essential protein in mouse development and cell growth; however, it has been assigned numerous roles in very diverse cellular processes. Here, we present a unified mechanism for NPM's role in cell growth; NPM directs the nuclear export of both 40S and 60S ribosomal subunits. NPM interacts with rRNA and large and small ribosomal subunit proteins and also colocalizes with large and small ribosomal subunit proteins in the nucleolus, nucleus, and cytoplasm. The transduction of NPM shuttling-defective mutants or the loss of Npm1 inhibited the nuclear export of both the 40S and 60S ribosomal subunits, reduced the available pool of cytoplasmic polysomes, and diminished overall protein synthesis without affecting rRNA processing or ribosome assembly. While the inhibition of NPM shuttling can block cellular proliferation, the dramatic effects on ribosome export occur prior to cell cycle inhibition. Modest increases in NPM expression amplified the export of newly synthesized rRNAs, resulting in increased rates of protein synthesis and indicating that NPM is rate limiting in this pathway. These results support the idea that NPM-regulated ribosome export is a fundamental process in cell growth.

ACKNOWLEDGMENTS

We are grateful to Myla Ashfaq and Mike Benjamin for excellent technical assistance and Alan Diehl, Joe Baldassare, Helen Piwnica-Worms, Sheila Stewart, and other members of the laboratory of J.D.W. for discussion, critical reading of the manuscript, and support.

M.K. is supported by the Cancer Biology Pathway through the Washington University Siteman Cancer Center, and D.Y.A.D. is supported by the BioMed RAP Program. This work was funded by grants from the National Institutes of Health to P.P.P. and J.D.W. and the Susan G. Komen Breast Cancer Race for the Cure and the Pew Scholars Program in Biomedical Sciences to J.D.W. This research was supported by the National Center for Research Resources of the National Institutes of Health grant P41RR000954 and the W. M. Keck Foundation.