The mRNA-bound proteome and its global occupancy profile on protein-coding transcripts [RNA-seq]
ABSTRACT: Protein-RNA interactions are fundamental to core biological processes, such as mRNA splicing, localization, degradation and translation. We developed a photoreactive nucleotide-enhanced UV crosslinking and oligo(dT) purification approach to identify the mRNA-bound proteome using quantitative proteomics and to display the protein occupancy on mRNA transcripts by next-generation sequencing. Application to a human embryonic kidney cell line identified close to 800 proteins. Close to one third of these proteins, were neither previously annotated nor could be functionally predicted to bind RNA. Protein occupancy profiling provides a transcriptome-wide catalog of potential cis-regulatory regions on mammalian mRNAs and showed that large stretches in 3' UTRs can be contacted by the mRNA-bound proteome, with numerous putative binding sites in regions harboring disease-associated nucleotide polymorphisms. Our observations indicate the presence of a large number of unexpected mRNA-binders with novel molecular functions participating in combinatorial post-transcriptional gene-expression networks. To obtain a more detailed picture of the RNA present in the pooled precipitates of four consecutive oligo(dT)-purifications, we constructed a cDNA library by random priming of 4-thiouridine (4SU)- and 6-thioguanosine (6SG)-labeled RNA derived from UV-irradiated (365 nm)and non-irradiated cells. Digital gene expression analysis of the cDNA library of non-irradiated cells, labeled with 4SU and 6SG, was performed. To monitor the incorporation of photoreactive nucleotides into mRNA, we isolated 4SU- and 6SG-labeled RNA from the oligo(dT) precipitate of non-crosslinked cells by biotinylation and streptavidin purification (Dolken et al., 2008).