Cell 167, 512–524 (2016)


Ribosomal proteins are organized into 60S and 40S subunits, which require protein assembly in the nucleolus. In particular, the maturation and nuclear export of the 60S subunit requires the removal of the ribosome factors such as Rsa4 and the Ytm1–Erb1–Nop7 complex at distinct times by the AAA+ ATPase Midasin (Rea1 in Saccharomyces cerevisiae and Mdn1 in Schizosaccharomyces pombe). However, it remains unclear which assembly steps require Mdn1 activity. Kawashima et al. identified mdn1 mutations in the yeast S. pombe that confer resistance and sensitivity to ribozinoindole-1 (Rbin-1), a compound toxic to yeast through an unidentified target. These mutations were clustered near the AAA domains, and complementation analysis revealed that four of the ATPase sites are required for Mdn1 activity. Rbin-1 directly blocked the ATPase activity of recombinant Mdn1, while a Mdn1 protein variant containing a resistance mutation was unresponsive. Rbin-1 blocked nuclear export of the pre-60S subunit, which led to the accumulation of these particles in nucleoli, phenocopying the mdn1 null mutations. Kawashima et al. identified additional requirements for Mdn1 in regulating the assembly of the ribosomal particles Nsa1. Interestingly, the effects of Rbin-1 are rapidly reversible by compound washout, providing a useful tool to disrupt ribosomal assembly with efficient temporal resolution.