An MST-based assay reveals new binding preferences of IFIT1 for canonically and non-canonically capped RNAs
IFIT proteins (interferon-induced proteins with tetratricopeptide repeats) are key components of the innate immune response that bind to viral and cellular RNA targets to inhibit viral translation and replication. The RNA target recognition is guided by molecular patterns, particularly at the RNA 5 ends. IFIT1 preferably binds RNAs modified with the 7-methylguanosine (m7G) cap-0 structure, while RNAs with cap-1 structure are recognized with lower affinity. Less is known about the propensity of IFIT1 to recognize non-canonical RNA 5 ends, including hypermethylated and non-canonical RNA caps. Deciphering the structure-function relationship for IFIT1-RNA interaction may improve understanding of cellular selection of IFIT targets and guide the design of exogenously delivered therapeutic RNAs, but requires high-throughput and robust analytical methods. Here, we report a biophysical assay for quick, direct, in-solution affinity assessment of differently capped RNAs with IFIT1. The procedure, which relies on measuring microscale thermophoresis (MST) of fluorescently labelled protein as a function of increasing ligand concentration, is applicable to various RNA lengths and sequences without the need for labelling or affinity tagging. Using the assay, we examined thirteen canonically and non-canonically 5-capped RNAs, revealing new binding preferences of IFIT1. The 5 terminal m6A mark in the m7G cap had a protective function against IFIT1, which was additive with the effect observed for the 2-O position (m6Am cap-1). In contrast, an increased affinity for IFIT1 was observed for several non-canonical caps, including trimethylguanosine (TMG), unmethylated (G), and flavin-adenine dinucleotide (FAD) caps. The results suggest new potential cellular targets of IFIT1 and may contribute to broadening the knowledge on the mechanisms of the innate immune response as well as the more effective design of chemically modified mRNAs.