Rrb1p, a Yeast Nuclear WD-Repeat Protein Involved in the Regulation of Ribosome Biosynthesis

Abstract

Ribosome biogenesis is regulated by environmental cues that coordinately modulate the synthesis of ribosomal components and their assembly into functional subunits. We have identified an essential yeast WD-repeat-containing protein, termed Rrb1p, that has a role in both the assembly of the 60S ribosomal subunits and the transcriptional regulation of ribosomal protein (RP) genes. Rrb1p is located in the nucleus and is concentrated in the nucleolus. Its presence is required to maintain normal cellular levels of 60S subunits, 80S ribosomes, and polyribosomes. The function of Rrb1p in ribosome biogenesis appears to be linked to its association with the ribosomal protein rpL3. Immunoprecipitation of Rrb1p from nuclear extracts revealed that it physically interacts with rpL3. Moreover, the overproduction of Rrb1p led to increases in cellular levels of free rpL3 that accumulated in the nucleus together with Rrb1p. The concentration of these proteins within the nucleus was dependent on ongoing protein translation. We also showed that overexpression of RRB1 led to an increase in the expression of RPL3 while all other examined RP genes were unaffected. In contrast, depletion of RRB1 caused an increase in the expression of all RP genes examined except RPL3. These results suggest that Rrb1p regulates RPL3 expression and uncouples it from the coordinated expression of other RP genes.

ACKNOWLEDGMENTS

We are especially grateful to Günter Blobel in whose lab this work was initiated. We thank David Dilworth and all the members of the Wozniak lab for helpful discussions. We thank the Protein/DNA Technology Center at the Rockefeller University (New York, N.Y.), especially Joseph Fernandez, for peptide sequencing. We kindly thank Mike Rout, Ed Hurt, Jonathan Warner, and David Goldfarb for providing reagents listed in the text.

R. W. Wozniak and J. Aitchison are supported by salary awards from the Alberta Heritage Foundation for Medical Research and the Medical Research Council of Canada. Support for this work is provided by an operating grant from the Medical Research Council of Canada.