XL-MS of 5S RNP preribosomal complexes

The 5S RNP is assembled from its three components (5S rRNA, Rpl5/uL18, and Rpl11/uL5) before being incorporated into the pre-60S subunit. However, when ribosome synthesis is disturbed, a free 5S RNP can enter the MDM2–p53 pathway to regulate cell cycle and apoptotic signaling. Here we reconstitute and determine the cryo-EM structure of the conserved hexameric 5S RNP with fungal or human factors. This reveals how the nascent 5S rRNA associates with the initial nuclear import complex Syo1–uL18–uL5, and upon further recruitment of two nucleolar factors Rpf2–Rrs1 develops into the 5S RNP precursor that can assemble into the pre-ribosome. In addition, we elucidate the structure of another 5S RNP intermediate, carrying the human ubiquitin ligase Mdm2, which unraveled how this enzyme can be sequestered from its target substrate p53. Our data provide molecular insight into how the 5S RNP can mediate between ribosome biogenesis and cell proliferation.