Ytm1, Nop7, and Erb1 Form a Complex Necessary for Maturation of Yeast 66S Preribosomes

Abstract

The essential, conserved yeast nucleolar protein Ytm1 is one of 17 proteins in ribosome assembly intermediates that contain WD40 protein-protein interaction motifs. Such proteins may play key roles in organizing other molecules necessary for ribosome biogenesis. Ytm1 is present in four consecutive 66S preribosomes containing 27SA₂, 27SA₃, 27SB, and 25.5S plus 7S pre-rRNAs plus ribosome assembly factors and ribosomal proteins. Ytm1 binds directly to Erb1 and is present in a heterotrimeric subcomplex together with Erb1 and Nop7, both within preribosomes and independently of preribosomes. However, Nop7 and Erb1 assemble into preribosomes prior to Ytm1. Mutations in the WD40 motifs of Ytm1 disrupt binding to Erb1, destabilize the heterotrimer, and delay pre-rRNA processing and nuclear export of preribosomes. Nevertheless, 66S preribosomes lacking Ytm1 remain otherwise intact.

ACKNOWLEDGMENTS

We thank Seiji Matsumoto for generously providing ytm1-1 and YTM1 strains and for communicating unpublished results. We thank Stan Fields and Tony Hazbun (Yeast Resource Center, University of Washington) for plasmids containing YTM1, ERB1, or NOP7 cloned into pOAD. We also thank Michael McAlear, Jonathan Warner, David Goldfarb, Janine Maddock, Jennifer Fuentes, and Bernard Trumpower for antibodies against ribosomal proteins or ribosome assembly factors. We are grateful to Susan Dowd and Mark Bier (Center for Molecular Analysis, Carnegie Mellon University) for assistance with mass spectrometry. We thank Jon Minden, Peter Berget, Jeff Brodsky, Elizabeth Jones, and members of our laboratory for fruitful discussions and for comments on the manuscript. We thank Brooke McCartney for use of her microscope.

This work was supported by National Institutes of Health grants RO1 GM28301 to J.L.W., F31 GM65067 to T.D.R., and F31 GM19937 to E.W.H. P.H. was supported by the government of Thailand.