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© The Rockefeller University Press, 0021-9525/2000//239 $5.00
The Journal of Cell Biology, Volume 148, Number 2, , 2000 239-246

Brief Report

Nuclear Eukaryotic Initiation Factor 4e (Eif4e) Colocalizes with Splicing Factors in Speckles



Josée Dostiea, Flavio Lejbkowicza, and Nahum Sonenberga

a Department of Biochemistry and Cancer Centre, McGill University, Montréal, Québec H3G 1Y6, Canada
Department of Biochemistry and Cancer Centre, McGill University, 3655 Drummond Street, Montréal, Québec H3G 1Y6, Canada.(514) 398-1287(514) 398-7274

nsonen{at}med.mcgill.ca

The eukaryotic initiation factor 4E (eIF4E) plays a pivotal role in the control of protein synthesis. eIF4E binds to the mRNA 5' cap structure, m7GpppN (where N is any nucleotide) and promotes ribosome binding to the mRNA. It was previously shown that a fraction of eIF4E localizes to the nucleus (Lejbkowicz, F., C. Goyer, A. Darveau, S. Neron, R. Lemieux, and N. Sonenberg. 1992. Proc. Natl. Acad. Sci. USA. 89:9612–9616). Here, we show that the nuclear eIF4E is present throughout the nucleoplasm, but is concentrated in speckled regions. Double label immunofluorescence confocal microscopy shows that eIF4E colocalizes with Sm and U1snRNP. We also demonstrate that eIF4E is specifically released from the speckles by the cap analogue m7GpppG in a cell permeabilization assay. However, eIF4E is not released from the speckles by RNase A treatment, suggesting that retention of eIF4E in the speckles is not RNA-mediated. 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB) treatment of cells causes the condensation of eIF4E nuclear speckles. In addition, overexpression of the dual specificity kinase, Clk/Sty, but not of the catalytically inactive form, results in the dispersion of eIF4E nuclear speckles.

Key Words: nuclear proteins • peptide initiation factor • RNA caps • RNA splicing • cap-binding protein

© 2000 The Rockefeller University Press

Josée Dostie and Flavio Lejbkowicz contributed equally to this work.

Flavio Lejbkowicz's present address is Department of Microbiology, Rambam Medical Center, Haifa, Israel 31096.

Abbreviations used in this paper: CBC, cap-binding complex; DRB, 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole; eIF4E, eukaryotic initiation factor 4E; RT, room temperature.


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