The induction of p53 correlates with defects in the production, but not the levels, of the small ribosomal subunit and stalled large ribosomal subunit biogenesis.

Ribosome biogenesis is one of the biggest consumers of cellular energy. More than 20 genetic diseases (ribosomopathies) and multiple cancers arise from defects in the production of the 40S (SSU) and 60S (LSU) ribosomal subunits. Defects in the production of either the SSU or LSU result in p53 induction through the accumulation of the 5S RNP, an LSU assembly intermediate. While the mechanism is understood for the LSU, it is still unclear how SSU production defects induce p53 through the 5S RNP since the production of the two subunits is believed to be uncoupled. Here, we examined the response to SSU production defects to understand how this leads to the activation of p53 via the 5S RNP. We found that p53 activation occurs rapidly after SSU production is blocked, prior to changes in mature ribosomal RNA (rRNA) levels but correlated with early, middle and late SSU pre-rRNA processing defects. Furthermore, both nucleolar/nuclear LSU maturation, in particular late stages in 5.8S rRNA processing, and pre-LSU export were affected by SSU production defects. We have therefore uncovered a novel connection between the SSU and LSU production pathways in human cells, which explains how p53 is induced in response to SSU production defects.

RPS27a and RPL40, Which Are Produced as Ubiquitin Fusion Proteins, Are Not Essential for p53 Signalling.

Two of the four human ubiquitin-encoding genes express ubiquitin as an N-terminal fusion precursor polypeptide, with either ribosomal protein (RP) RPS27a or RPL40 at the C-terminus. RPS27a and RPL40 have been proposed to be important for the induction of the tumour suppressor p53 in response to defects in ribosome biogenesis, suggesting that they may play a role in the coordination of ribosome production, ubiquitin levels and p53 signalling. Here, we report that RPS27a is cleaved from the ubiquitin-RP precursor in a process that appears independent of ribosome biogenesis. In contrast to other RPs, the knockdown of either RPS27a or RPL40 did not stabilise the tumour suppressor p53 in U2OS cells. Knockdown of neither protein blocked p53 stabilisation following inhibition of ribosome biogenesis by actinomycin D, indicating that they are not needed for p53 signalling in these cells. However, the knockdown of both RPS27a and RPL40 in MCF7 and LNCaP cells robustly induced p53, consistent with observations made with the majority of other RPs. Importantly, RPS27a and RPL40 are needed for rRNA production in all cell lines tested. Our data suggest that the role of RPS27a and RPL40 in p53 signalling, but not their importance in ribosome biogenesis, differs between cell types.