RNA-binding proteins (RBPs) play essential roles in biology and are frequently associated with human disease. While recent studies have systematically identified individual RBPs, their higher order assembly into Ribonucleoprotein (RNP) complexes has not been systematically investigated. Here, we describe a proteomics method for systematic identification of RNP complexes in human cells. We identify 1,428 protein complexes that associate with RNA, indicating that over 20% of known human protein complexes contain RNA. To explore the role of RNA in the assembly of each complex, we identify complexes that dissociate, change composition, or form stable protein-only complexes in the absence of RNA. Importantly, these data also provide specific novel insights into the function of well-studied protein complexes not previously known to associate with RNA, including replication factor C (RFC) and cytokinetic centralspindlin complex. Finally, we use our method to systematically identify cell-type specific RNA-associated proteins in mouse embryonic stem cells. We distribute these data as a resource, rna.MAP (rna.proteincomplexes.org) which provides a comprehensive dataset for the study of RNA-associated protein complexes. Our system thus provides a novel methodology for further explorations across human tissues and disease states, as well as throughout all domains of life. ! 3! ! 4! upon CF-MS by comparing chromatographic separations of cellular lysate under control and RNA degrading conditions ( Figure 1A). A statistical framework is then applied to discover RNP complexes by identifying concurrent shifts of known protein complex subunits upon RNA degradation ( Figure 1A).Analysis of DIF-FRAC data answers important questions as to the role of RNA plays in macromolecular complexes. Specifically, we identify RNP complexes that 1) dissociate, 2) form stable protein-only complexes, and 3) change composition in the absence of RNA suggesting specific roles for RNA in each of these cases. Because DIF-FRAC is independent of UV crosslinking, nucleotide incorporation, genetic manipulation or poly(A) RNA capture efficiency, it can therefore be used to investigate a wide variety of cell types, tissues and species. To demonstrate this versatility, we apply DIF-FRAC to mouse embryonic stem cells (mESCs), identifying 1,165 RNA-associated proteins, to show the method is highly adaptable and can be extended to discover RNP complexes in diverse samples.Finally, we created a system-wide resource of 1,428 RNP complexes, many of which are previously unreported as having an RNA component, representing 20% of known human protein complexes. We provide our resource, rna.MAP, to the community as a fully searchable web database at rna.proteincomplexes.org.
Results and Discussion
Differential fractionation (DIF-FRAC) identifies RNP complexesThe DIF-FRAC strategy builds upon our previous strategy of Co-Fractionation Massspectroscopy (CF-MS) for identifying protein complexes in cellular lysate (Havugimana et al., 2012;Wan et al., 2015). CF-MS chromatograp...