p53 localizes to intranucleolar regions distinct from the ribosome production compartments.

The tumor suppressor p53 has been implicated in the regulation of ribosome biogenesis based on its inhibitory effect on RNA polymerase I (pol I)-dependent transcription. Consistent with this, p53 has been described in nucleoli, albeit under specific experimental conditions. Since data on the intranucleolar localization of p53 are controversial, we have analyzed in detail its subnucleolar distribution. Our results show that p53 does not localize to one of the well-known structural components of the nucleolus involved in ribosome biogenesis, but rather occupies distinct intranucleolar regions that constitute nucleolar cavities. When cells were treated with the proteasome inhibitor MG132, the size and frequency of p53-containing nucleolar cavities increased, and the protein partially colocalized with inactivated proteasomes. Importantly, p53 did not colocalize with pol I at the transcription sites in fibrillar centers (FCs) as has previously been reported. The observed intranucleolar distribution and accumulation of p53 raises the question of how the protein influences rDNA transcription in vivo.