p38^MAPK/MK2-mediated phosphorylation of RBM7 regulates the human nuclear exosome targeting complex

The nuclear exosome targeting complex (NEXT) directs a major 3′–5′ exonuclease, the RNA exosome, for degradation of nuclear noncoding (nc) RNAs. We identified the RNA-binding component of the NEXT complex, RBM7, as a substrate of p38^MAPK/MK2-mediated phosphorylation at residue S136. As a result of this phosphorylation, RBM7 displays a strongly decreased RNA-binding capacity, while inhibition of p38^MAPK or mutation of S136A in RBM7 increases its RNA association. Interestingly, promoter-upstream transcripts (PROMPTs), such as proRBM39, proEXT1, proDNAJB4, accumulated upon stress stimulation in a p38^MAPK/MK2-dependent manner, a process inhibited by overexpression of RBM7^S136A. While there are no stress-dependent changes in RNA-polymerase II (RNAPII) occupation of PROMPT regions representing unchanged transcription, stability of PROMPTs is increased. Hence, we propose that phosphorylation of RBM7 by the p38^MAPK/MK2 axis increases nuclear ncRNA stability by blocking their RBM7-binding and subsequent RNA exosome targeting to allow stress-dependent modulations of the noncoding transcriptome.