The RNA exosome adaptor protein ZFC3H1 functionally competes with nuclear export activity to retain target transcripts

Mammalian genomes are promiscuously transcribed, yielding protein-coding and non-coding products. Many transcripts are short-lived due to their nuclear degradation by the ribonucleolytic RNA exosome. Here, we show that abolished nuclear exosome function causes the formation of distinct nuclear foci, containing polyadenylated (pA+) RNA secluded from nucleocytoplasmic export. We asked whether exosome co-factors could serve such nuclear retention. Co-localization studies revealed the enrichment of pA+ RNA foci with ‘pA-tail exosome targeting (PAXT) connection’ components MTR4, ZFC3H1 and PABPN1, but no overlap with known nuclear structures, such as Cajal bodies, speckles, paraspeckles or nucleoli. Interestingly, ZFC3H1 is required for foci formation, and in its absence selected pA+ RNAs, including coding and non-coding transcripts, are exported to the cytoplasm in a process dependent on the mRNA export factor AlyREF. Our results establish ZFC3H1 as a central nuclear RNA retention factor, counteracting nuclear export activity. Nuclear poly(A) selected RNA from HeLa cells was analysed by next generation sequencing upon depletion of EGFP(control) and RNA exosome core factor RRP40