LENG8 mediates RNA nuclear retention and degradation in eukaryotes.

In eukaryotes, incompletely processed and misprocessed mRNAs as well as numerous noncoding RNAs are retained in the nucleus and often degraded. However, the mechanisms for this critical quality control pathway remain poorly understood. Here we identify LENG8 as a conserved RNA nuclear retention factor. We showed that LENG8 is recruited to pre-mRNAs by splicing factors, including the U1 snRNP. LENG8 binds to PCID2 and SEM1 to form the REX (Repressor of EXport) complex, which is conserved from yeast to human, and causes RNA nuclear retention by acting as a dominant negative factor for the essential mRNA export factor TREX-2. LENG8 depletion leads to the leakage of misprocessed mRNAs, including intronically polyadenylated and intronretained mRNAs, as well as noncoding RNAs into the cytoplasm. Finally, LENG8 promotes RNA degradation by recruiting PAXT and the RNA exosome. Thus our study revealed a conserved quality control mechanism for eukaryotic gene expression that ensures only fully and correctly processed RNAs are exported from the nucleus.