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A more recent version of this article appeared on January 1, 2006
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Submitted on September 11, 2005
Revised on October 7, 2005
Accepted on October 18, 2005
*Laboratory of Nucleopore Biology, Institute of Molecular and Cell Biology, National University of Singapore, Singapore 117609, Singapore; Laboratory of Molecular Virology, Centre for DNA Fingerprinting and Diagnostics, Hyderabad 500076, India
Monitoring Editor: Joseph Gall
Grn1p from fission yeast and GNL3L from human cells, two putative GTPases from the novel HSR1_MMR1 GTP-binding protein subfamily with circularly permuted G-motifs play a critical role in maintaining normal cell growth. Deletion of Grn1 resulted in a severe growth defect, a marked reduction in mature rRNA species with a concomitant accumulation of the 35S pre-rRNA transcript and failure to export the ribosomal protein Rpl25a from the nucleolus. Deleting any of the Grn1p G-domain motifs resulted in a null phenotype and nuclear/nucleolar localization consistent with the lack of nucleolar export of pre-ribosomes accompanied by a distortion of nucleolar structure. Heterologous expression of GNL3L in a grn1
mutant restored processing of 35S pre-rRNA, nuclear export of Rpl25a and cell growth to wild type levels. Genetic complementation in yeast and siRNA knockdown in HeLa cells confirmed the homologous proteins Grn1p and GNL3L are required for growth. Failure of two similar HSR1_MMR1 putative nucleolar GTPases, Nucleostemin (NS), or the dose-dependent response of breast tumor autoantigen NGP-1, to rescue grn1 implied the highly specific roles of Grn1p or GNL3L in nucleolar events. Our analysis uncovers an important role for Grn1p/GNL3L within this unique group of nucleolar GTPases.
These authors contributed equally to this work.
Address correspondence to:
David Balasundaram (davidb{at}imcb.a-star.edu.sg)
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