3'READS+RIP defines differential Staufen1 binding to alternative 3'UTR isoforms and reveals structures and sequence motifs influencing binding and polysome association

Abstract

Staufen1 (STAU1) is an RNA-binding protein (RBP) that interacts with double-stranded RNA structures and has been implicated in regulating different aspects of mRNA metabolism. Previous studies have indicated that STAU1 interacts extensively with RNA structures in coding regions (CDSs) and 3'-untranslated regions (3'UTRs). In particular, duplex structures formed within 3'UTRs by inverted-repeat Alu elements (IRAlus) interact with STAU1 through its double-stranded RNA-binding domains (dsRBDs). Using 3ʹ region extraction and deep sequencing coupled to ribonucleoprotein immunoprecipitation (3’READS+RIP), together with reanalyzing previous STAU1 binding and RNA structure data, we delineate STAU1 interactions transcriptome-wide, including binding differences between alternative polyadenylation (APA) isoforms. Consistent with previous reports, RNA structures are dominant features for STAU1 binding to CDSs and 3ʹUTRs. Overall, relative to short 3ʹUTR counterparts, longer 3ʹUTR isoforms of genes have stronger STAU1 binding, most likely due to a higher frequency of RNA structures, including particular IRAlus. Nevertheless, a sizable fraction of genes express transcripts showing the opposite trend, attributable to AU-rich sequences in their alternative 3'UTRs, possibly recruiting antagonistic RBPs and/or destabilizing STAU1-binding RNA structures. Using STAU1-knockout cells, we show that strong STAU1 binding to mRNA 3'UTRs generally enhances polysome association. However, IRAlus have little impact on STAU1-mediated polysome association despite having strong interactions with the protein.Taken together, our work reveals complex interactions of STAU1 with its cognate RNA substrates. Our data also shed important light on diversity in co- and post-transcriptional regulation for the widespread APA isoforms in mammalian cells.