Significance of the first transcribed nucleoside of capped RNA for ligand-induced folding of the cap-binding complex

Many proteins, including those that bind RNA, change conformation upon binding a ligand, a phenomenon known as induced fit. CBP20, the small subunit of the nuclear cap-binding complex (CBC), recognizes specifically the 5' cap of eukaryotic mRNA and snRNA. The N- and C-terminal regions of the CBP20 subunit of the human nuclear cap-binding complex only acquire a proper fold in complex with capped RNA. The cap is composed of 7-methylguanosine linked by a 5-to-5triphosphate bridge to the first transcribed nucleoside of the RNA. The significance of the latter for the capped RNA-CBC association and local folding of CBC has been characterized by emission spectroscopy. Fluorescence titration of CBC has been performed for three selected, mono- and dinucleotide mRNA 5' cap analogues. The measured values of the equilibrium association,constant and the corresponding Gibbs free energy depend on the type of the first transcribed nucleoside (purine or pyrimidine), and decrease similar to 10-fold in the case of a mononucleotide analogue, 7-methylguanosine triphosphate. However, the total quenching of the intrinsic protein fluorescence is similar for each analogue. Changes of the solvent-accessible CBC hydrophobic surface of CBC on binding of the structurally different cap analogues have been followed using bis-ANS fluorescent probe.