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Molecular and Cellular Biology, March 2003, p. 2042-2054, Vol. 23, No. 6
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.6.2042-2054.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Intersection of the Kap123p-Mediated Nuclear Import and Ribosome Export Pathways

Institute for Systems Biology, Seattle, Washington 98105,¹ Department of Cell Biology, University of Alberta, Edmonton, Alberta T6G 2H7, Canada²

Received 12 July 2002/ Returned for modification 28 August 2002/ Accepted 13 December 2002

Kap123p is a yeast ß-karyopherin that imports ribosomal proteins into the nucleus prior to their assembly into preribosomal particles. Surprisingly, Kap123p is not essential for growth, under normal conditions. To further explore the role of Kap123p in nucleocytoplasmic transport and ribosome biogenesis, we performed a synthetic fitness screen designed to identify genes that interact with KAP123. Through this analysis we have identified three other karyopherins, Pse1p/Kap121p, Sxm1p/Kap108p, and Nmd5p/Kap119p. We propose that, in the absence of Kap123p, these karyopherins are able to supplant Kap123p's role in import. In addition to the karyopherins, we identified Rai1p, a protein previously implicated in rRNA processing. Rai1p is also not essential, but deletion of the RAI1 gene is deleterious to cell growth and causes defects in rRNA processing, which leads to an imbalance of the 60S/40S ratio and the accumulation of halfmers, 40S subunits assembled on polysomes that are unable to form functional ribosomes. Rai1p localizes predominantly to the nucleus, where it physically interacts with Rat1p and pre-60S ribosomal subunits. Analysis of the rai1/kap123 double mutant strain suggests that the observed genetic interaction results from an inability to efficiently export pre-60S subunits from the nucleus, which arises from a combination of compromised Kap123p-mediated nuclear import of the essential 60S ribosomal subunit export factor, Nmd3p, and a RAI1-induced decrease in the overall biogenesis efficiency.

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