Structural basis of m7GpppG binding to the nuclear cap-binding protein complex

The 7-methyl guanosine cap structure of RNA is essential for key aspects of RNA processing, including pre-mRNA splicing, 3 end formation, U snRNA transport, nonsense-mediated decay and translation. Two cap-binding proteins mediate these effects: cytosolic eIF-4E and nuclear cap-binding protein complex (CBC). The latter consists of a CBP20 subunit, which binds the cap, and a CBP80 subunit, which ensures high-affinity cap binding. Here we report the 2.1 Angstrom resolution structure of human CBC with the cap analog m 7 GpppG, as well as the structure of unliganded CBC. Comparisons between these structures indicate that the cap induces substantial conformational changes within the N-terminal loop of CBP20, enabling Tyr 20 to join Tyr 43 pi-pi stacking interactions with the methylated guanosine base. CBP80 stabilizes the movement of the N-terminal loop of CBP20 and locks the CBC into a high affinity cap-binding state. The structure for the CBC bound to m(7)GpppG highlights interesting similarities and differences between CBC and eI-4E, and provides insights into the regulatory mechanisms used by growth factors and other extracellular stimuli to influence the cap-binding state of the CBC.