Abstract: We previously showed that p21Cip1 transits through the nucleolus on its way from the
nucleus to the cytoplasm and that DNA damage inhibits this transit and induces the formation
of p21Cip1-containing intranucleolar bodies (INoBs). Here, we demonstrate that these
INoBs also contain SUMO-1 and UBC9, the E2 SUMO-conjugating enzyme. Furthermore,
whereas wild type SUMO-1 localized in INoBs, a SUMO-1 mutant, which is unable to conjugate
with proteins, does not, suggesting the presence of SUMOylated proteins at INoBs.
Moreover, depletion of the SUMO-conjugating enzyme UBC9 or the sumo hydrolase
SENP2 changed p21Cip1 intracellular distribution. In addition to SUMO-1 and p21Cip1, cell
cycle regulators and DNA damage checkpoint proteins, including Cdk2, Cyclin E, PCNA,
p53 and Mdm2, and PML were also detected in INoBs. Importantly, depletion of UBC9 or
p21Cip1 impacted INoB biogenesis and the nucleolar accumulation of the cell cycle regulators
and DNA damage checkpoint proteins following DNA damage. The impact of p21Cip1
and SUMO-1 on the accumulation of proteins in INoBs extends also to CRM1, a nuclear
exportin that is also important for protein translocation from the cytoplasm to the nucleolus.
Thus, SUMO and p21Cip1 regulate the transit of proteins through the nucleolus, and that disruption
of nucleolar export by DNA damage induces SUMO and p21Cip1 to act as hub proteins
to form a multiprotein complex in the nucleolus.
