The influence of microRNAs and poly(A)-tail length on endogenous mRNA-protein complexes

All mRNAs are bound in vivo by proteins to form mRNA–protein complexes (mRNPs), but changes in the composition of mRNPs during post-transcriptional regulation remain largely unexplored. Here, we have analyzed, on a transcriptome-wide scale, how microRNA-mediated repression modulates the associations of core components (eIF4E, eIF4G, and PABP) and of the decay factor DDX6 in human cells. Despite the transient nature of repressed intermediates, we detected significant changes in mRNP composition, marked by dissociation of eIF4G and PABP, and by recruitment of DDX6. Furthermore, although poly(A)-tail length has been considered critical in post-transcriptional regulation, differences in steady-state tail length explained little of the variation in either PABP association or mRNP organization more generally. Instead, relative occupancy of core components correlated best with gene expression. Together, these results indicate that posttranscriptional regulatory factors influence the associations of PABP and other core factors, and do so without substantially affecting steady-state tail length. Overall design: Total and immunoprecipitated samples analyzed by RNA-seq and PAL-seq