Nmd3p from yeast is required for the export of the
large (60S) ribosomal subunit from the nucleus (Ho et al.,
2000). Here, we show that Nmd3p forms a stable complex
with free 60S subunits. Using an epitope-tagged Nmd3p, we
show that free 60S subunits can be coimmunoprecipitated
with Nmd3p. The interaction was specific for 60S subunits;
40S subunits were not coimmunoprecipitated. Using this
coprecipitation technique and pulse-chase labeling of ribosomal
subunit proteins we showed that Nmd3p bound nascent subunits,
consistent with its role in export. However, under conditions
in which ribosome biogenesis was inhibited (e.g., inhibition
of transcription with thiolutin, inhibition of transcription
of ribosomal protein and RNA genes in a sly1-1
mutant at nonpermissive temperature, and inhibition of
translation in a conditional prt1 mutant), Nmd3p
remained associated with 60S subunits. In addition, Nmd3Δ120,
a truncated protein that lacked a nuclear localization
signal, retained 60S binding. These results suggest that
Nmd3p recruits nascent 60S subunits into the pool of free
60S subunits and exchanges on 60S subunits as they recycle
during translation.