Cytoplasmic Recycling of 60S Preribosomal Factors Depends on the AAA Protein Drg1

Abstract

Allelic forms of DRG1/AFG2 confer resistance to the drug diazaborine, an inhibitor of ribosome biogenesis in Saccharomyces cerevisiae. Our results show that the AAA-ATPase Drg1 is essential for 60S maturation and associates with 60S precursor particles in the cytoplasm. Functional inactivation of Drg1 leads to an increased cytoplasmic localization of shuttling pre-60S maturation factors like Rlp24, Arx1, and Tif6. Surprisingly, Nog1, a nuclear pre-60S factor, was also relocalized to the cytoplasm under these conditions, suggesting that it is a previously unsuspected shuttling preribosomal factor that is exported with the precursor particles and very rapidly reimported. Proteins that became cytoplasmic under drg1 mutant conditions were blocked on pre-60S particles at a step that precedes the association of Rei1, a later-acting preribosomal factor. A similar cytoplasmic accumulation of Nog1 and Rlp24 in pre-60S-bound form could be seen after overexpression of a dominant-negative Drg1 variant mutated in the D2 ATPase domain. We conclude that the ATPase activity of Drg1 is required for the release of shuttling proteins from the pre-60S particles shortly after their nuclear export. This early cytoplasmic release reaction defines a novel step in eukaryotic ribosome maturation.

SUPPLEMENTAL MATERIAL

We thank Günther Daum, Sabine Rospert, Franco Fasiolo, François Lacroute, Johannes H. Hegemann, Sepp D. Kohlwein, and Ed Hurt for providing antibodies and strains. We are indebted to Samantha Clay for critically reading the manuscript. The expert help of Ron Booij, Christina Morgenstern, Michael Durchschlag, and Eva Wehrschütz-Sigl during early phases of this work is kindly acknowledged.

This work was supported by grants P15458 (G.H.) and P19791 (H.B.) from the Austrian Science Foundation (FWF) and grant GIP ANR-06-BLAN-0037 (M.F.-R.) from the Ministère de la Recherche, France.