Ribosomal protein S27-like and S27 interplay with p53-MDM2 axis as a target, a substrate and a regulator.

Several ribosomal proteins regulate p53 function by modulating MDM2. We recently found that RPS27L, a RPS27-like protein, is a direct p53-inducible target. Here we showed that RPS27 itself is a p53-repressible target. Furthermore, the N-terminal region of either RPS27L or RPS27 binds to MDM2 on the central acidic domain of MDM2. RPS27L or RPS27 forms an in vivo triplex with MDM2-p53 and competes with p53 for MDM2 binding. Similar to p53, RPS27L, but not RPS27, is a short-lived protein and a novel MDM2 substrate. Degradation of RPS27L requires the RING or acidic domain of MDM2. Ectopic expression of RPS27L or RPS27 inhibits MDM-2-mediated p53 ubiquitination and increases p53 levels by extending p53 protein half-life, whereas siRNA silencing of RPS27L decreases p53 levels by shortening p53 half-life, with a corresponding reduction in p53 transcription activity. RPS27L is mainly localized in the cytoplasm, but upon p53-activating signals, a portion of RPS27L shuttled to the nucleoplasm where it colocalizes with MDM2. Both the cytoplasmic and the nuclear p53, induced by ribosomal stress, were reduced upon RPS27L silencing. Our study reveals a multilevel interplay between RPS27L/S27 and p53-MDM2 axis, with RPS27L functioning as a p53 target, a MDM2 substrate and a p53 regulator.