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Molecular and Cellular Biology, May 2005, p. 3802-3813, Vol. 25, No. 9
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.9.3802-3813.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Defining the Order in Which Nmd3p and Rpl10p Load onto Nascent 60S Ribosomal Subunits

Matthew West,{dagger} John B. Hedges,{dagger} Anthony Chen, and Arlen W. Johnson*

Section of Molecular Genetics and Microbiology and Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712

Received 17 December 2004/ Returned for modification 20 January 2005/ Accepted 28 January 2005

The large ribosomal subunit protein Rpl10p is required for subunit joining and 60S export in yeast. We have recently shown that Rpl10p as well as the cytoplasmic GTPase Lsg1p are required for releasing the 60S nuclear export adapter Nmd3p from subunits in the cytoplasm. Here, we more directly address the order of Nmd3p and Rpl10p recruitment to the subunit. We show that Nmd3p can bind subunits in the absence of Rpl10p. In addition, we examined the basis of the previously reported dominant negative growth phenotype caused by overexpression of C-terminally truncated Rpl10p and found that these Rpl10p fragments are not incorporated into subunits in the nucleus but instead sequester the WD-repeat protein Sqt1p. Sqt1p is an Rpl10p binding protein that is proposed to facilitate loading of Rpl10p into the 60S subunit. Although Sqt1p normally only transiently binds 60S subunits, the levels of Sqt1p that can be coimmunoprecipitated by the 60S-associated GTPase Lsg1p are significantly increased by a dominant mutation in the Walker A motif of Lsg1p. This mutant Lsg1 protein also leads to increased levels of Sqt1p in complexes that are coimmunoprecipitated with Nmd3p. Furthermore, the dominant LSG1 mutant also traps a mutant Rpl10 protein that does not normally bind stably to the subunit. These results support the idea that Sqt1p loads Rpl10p onto the Nmd3p-bound subunit after export to the cytoplasm and that Rpl10p loading involves the GTPase Lsg1p.

* Corresponding author. Mailing address: Section of Molecular Genetics and Microbiology, ESB 325, The University of Texas at Austin, Austin, TX 78712-1095. Phone: (512) 475-6350. Fax: (512) 471-7088. E-mail: arlen{at}mail.utexas.edu.

{dagger} These authors contributed equally to this work.

Molecular and Cellular Biology, May 2005, p. 3802-3813, Vol. 25, No. 9
0022-538X/05/$08.00+0     doi:10.1128/MCB.25.9.3802-3813.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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