Fig. 7. Model of the formation of 5S RNP that assembles into nascent 60S subunits or signals the MDM2–p53 pathway.

After Syo1-mediated nuclear import of uL5–uL18, the 5S RNP assembles upon binding to the nascent 5S rRNA. Under normal circumstances (for example, cell proliferation), the 5S RNP is targeted to the ribosome biogenesis pathway by the binding of Rpf2–Rrs1, thereby forming the conserved hexameric 5S RNP that eventually integrates into pre-60S particles. Under nucleolar stress (for example, impaired ribosome biogenesis), the 5S RNP instead accumulates in a free pool that can sequester the ubiquitin ligase Mdm2, which in consequence stabilizes p53 levels that can eventually cause cell cycle arrest and apoptosis (see Discussion). NPC, nuclear pore complex.