Frac-Seq Analysis of ZFC3H1 Knockdown U2OS Cells

We have found that depletion of the zinc finger protein ZFC3H1 inhibits the nuclear retention of these RNAs. ZFC3H1 forms a complex with MTR4 and PABPN1 (the PolyAdenylation EXosome Targeting complex (Meola et al 2016, Ogami et al 2017) and likely recruits the nuclear exosome for RNA degradation. Recently, we found that depletion of U1-70K, one protein component of the U1 snRNP spliceosomal complex, inhibits nuclear retention of 5'SS motif containing RNAs. The U1 snRNP complex recognizes and binds to the 5'SS motif and in vitro studies showed that U1-70K directly interact with a similar 5'SS motif through U1 snRNA binding (Gunderson et al 1999). Interestingly, we found that U1 snRNP is involved in targeting 5'SS motif containing mRNAs to nuclear speckles shortly following RNA transcription. In contrast, ZFC3H1 is involved in nuclear speckle localization post-transcriptionally and actively retain RNAs in these structures. Together, our results suggest a model where U1 snRNP interacts with the RNA via the 5'SS motif and targets mature mRNAs to nuclear speckles. Specifically, it suggests that misprocessed RNAs can be targeted to speckles (or other nucleoplasmic foci) and that these foci serve as a platform to recruit the nuclear exosome via ZFC3H1. Our results highlight a splicing independent role of U1 snRNP and how mRNA nuclear export function in nuclear surveillance to prevent the translation of aberrant proteins. Overall design: Frac-Seq Analysis of ZFC3H1 Knockdown U2OS Cells