Alternative 3'UTRs play a widespread role in translation-independent mRNA association with endoplasmic reticulum

Transcripts encoding membrane and secreted proteins are known to undergo translation on endoplasmic reticulum (ER). Translation-independent ER association (TiERA) has been reported for certain mRNAs, but the phenomenon is poorly understood. Here, using cell fractionation, polysome profiling, and 3' end sequencing, we examine TiERA of poly(A)+ RNAs in mouse C2C12 myoblast cells. We identify transcript features that facilitate TiERA, including transcript size and GG and GC content. Consistent with the feature analysis, alternative polyadenylation (APA) isoforms differ substantially in TiERA, with long 3'UTR isoforms generally having a higher TiERA potential than short 3'UTR isoforms. Importantly, the widespread 3'UTR lengthening taking place in cell differentiation leads to greater transcript association with ER in differentiated myotubes, despite that the TiERA potential being generally maintained. In addition, we show that TiERA correlates negatively with mRNA stability, highlighting 3'UTR-mediated mRNA decay on ER. Together, our data indicate that sequence and size features impact ER association independent of translation, leading to distinct mRNA metabolism for different transcript groups, and APA can alter transcript stability and translation through isoform-specific ER association. Overall design: Three RNAseq samples and 26 3'READS+ samples.