The Pescadillo protein was identified via a developmental defect
and implicated in cell cycle progression. Here we report that
human Pescadillo and its yeast homolog (Yph1p or Nop7p) are
localized to the nucleolus. Depletion of Nop7p leads to nuclear
accumulation of pre-60S particles, indicating a defect in subunit
export, and it interacts genetically with a tagged form of the
ribosomal protein Rpl25p, consistent with a role in subunit
assembly. Two pre-rRNA processing pathways generate alternative
forms of the 5.8S rRNA, designated 5.8SL and
5.8SS. In cells depleted for Nop7p, the 27SA3
pre-rRNA accumulated, whereas later processing intermediates
and the mature 5.8SS rRNA were depleted. Less depletion
was seen for the 5.8SL pathway. TAP-tagged Nop7p
coprecipitated precursors to both 5.8SL and
5.8SS but not the mature rRNAs. We conclude that
Nop7p is required for efficient exonucleolytic processing of
the 27SA3 pre-rRNA and has additional functions in
60S subunit assembly and transport. Nop7p is a component of
at least three different pre-60S particles, and we propose that
it carries out distinct functions in each of these complexes.