A nuclear AAA-type ATPase (Rix7p) is required for biogenesis and nuclear export of 60S ribosomal subunits

Ribosomal precursor particles are assembled in the nucleolus before export into the cytoplasm. Using a visual assay for nuclear accumulation of 60S su bunits, we have isolated several conditional-lethal strains with defects in ribosomal export (rix mutants). Here we report the characterization of a m utation in an essential gene, RIX7, which encodes a novel member of the AAA ATPase superfamily. The rix7-1 temperature-sensitive allele carries a poin t mutation that causes defects in pre-rRNA processing, biogenesis of 60S ri bosomal subunits, and their subsequent export into the cytoplasm. Rix7p, wh ich associates with 60S ribosomal precursor particles, localizes throughout the nucleus in exponentially growing cells, but concentrates in the nucleo lus in stationary phase cells. When cells resume growth upon shift to fresh medium, Rix7p-green fluorescent protein exhibits a transient perinuclear l ocation. We propose that a nuclear AAA ATPase is required for restructuring nucleoplasmic 60S pre-ribosomal particles to make them competent for nucle ar export.