High-throughput identification of RNA nuclear localization signals

Since the sequencing of the human genome, one of the biggest surprises has been the annotation of thousands of long noncoding RNAs (lncRNAs). Although lncRNAs and mRNAs are similar in many ways, it has become clear they differ in localization properties with lncRNAs being more nuclear enriched and in several cases exclusively nuclear. Yet the RNA based sequences that determine nuclear localization remain poorly understood. Towards the goal of systematically dissecting the lncRNA sequences that impart nuclear enrichment, we developed a massively parallel reporter assay (MPRA). Unlike previous MPRAs that determine motifs important for transcriptional regulation, we have modified this approach to identify sequences sufficient for RNA nuclear enrichment for 38 lncRNAs. Using this approach, we identified 109 distinct, conserved nuclear enrichment regions, originating from 29 distinct lncRNAs. We also discovered two shorter motifs that are enriched in our nuclear enrichment regions—one specific to XIST, and a more general motif found in 21 different lncRNAs. We further validated the autonomous functionality of the XIST motif and three nuclear enrichment regions by single molecule RNA FISH. Taken together, these results give the first glimpse of sequence elements responsible for the nuclear enrichment of this critical class of RNA molecules. Comparison of nuclear localization of different fragments of lncRNA sequences relative to whole cell lysates.