Preparation of RNA and protein samples

The SRSF1 RRM2 ORF corresponding to amino acids 107–203 was cloned in the pET24 expression vector. A GB1 tag was fused at the N-terminal extremity of the protein to increase its solubility and stability (11). The protein was overexpressed at 37°C in Escherichia coli BL21 (DE3) codon plus cells in minimal M9 medium containing 1 g/l ¹⁵NH₄Cl and 4 g/l glucose. The protein was purified by two successive nickel affinity chromatography (QIAGEN) steps using an N-terminal 6x His tag, dialysed against NMR buffer (20 mM phosphate buffer at pH 5.5, 50 mM l-Glu, 50 mM l-Arg, 0.05% β-mercaptoethanol) and concentrated to 0.4 mM with a 10-kDa molecular mass cutoff Centricon device (Vivascience). The GB1 tag was kept for all NMR and ITC titrations performed in the presence of SRSF1 RRM2, as it was previously reported that its presence does not influence the protein interaction with RNA (11). The RNA oligonucleotide was purchased from Dharmacon, deprotected according to the manufacturer's instructions, lyophilised and resuspended in NMR buffer. The NMR titrations were performed in the NMR buffer at 40°C.