Chemoproteomic capture of RNA binding activity in living cells
Proteomic methods for RNA interactome capture (RIC) rely principally on crosslinking native or labeled cellular RNA to enrich and investigate RNA-binding protein (RBP) composition and function in cells. The ability to measure RBP activity at individual binding sites by RIC, however, has been more challenging due to the heterogenous nature of peptide adducts derived from the RNA-protein crosslinked site. Here, we present an orthogonal strategy that utilizes clickable electrophilic purines to directly quantify protein-RNA interactions on proteins through photoaffinity competition with 4-thiouridine (4SU)-labeled RNA in cells. Our photo-activatable-competition and chemoproteomic enrichment (PACCE) method facilitated detection of >5,500 cysteine sites across ~3,000 proteins displaying RNA-sensitive alterations in probe binding. Importantly, PACCE enabled functional profiling of canonical RNA-binding domains as well as discovery of moonlighting RNA binding activity in the human proteome. Collectively, we present a chemoproteomic platform for global quantification of protein-RNA binding activity in living cells. Overall design: Transcriptome of HEK293T was monitored for changes as the result of 4-thiouridine treatment (4SU, 16 hr, 37 C, 100uM), clickable electrophilic purine  (CEP, 1 hr, 37 C, 25 uM) treatment and a combination of both treatments. We then performed gene expression profiling of three indepdent replicates in a single cell line.