Fig. 2: The transition from mature ED pre-B to late ED pre-B.
From: Structural insights into human exon-defined spliceosome prior to activation
a The ACAGA box of U6 snRNA is highly flexible in the mature ED pre-B complex. The local environment for the disordered ACAGA box is shown. b The ACAGA box already forms a duplex with 5′SS in the late ED pre-B complex. The surrounding protein components and RNA elements are color-coded. c A close-up view on the double-sandwich structure of pre-mRNA recognition in the late ED pre-B complex. The 3′-end sequence of the exon forms a duplex with U5 loop I. The nucleobase of C–3 is flipped out of the duplex registry and sandwiched by Arg535 and Phe1551, both from PRP8. The nucleotide G1 is sandwiched by Phe1551 on one side and Gly1564/Lys1565 on the other side. The bases C–4 and A–5 have weak EM density and are modeled with low occupancies in the final atomic model. d A close-up view on the recognition of U6 C37 and 5′SS. His505 of SF3a120 interacts with C37 of U6 snRNA. The Endo-loop grasps G1 of 5′SS; Asn99/Asn100 of DIM1 recognizes U2. e Changes of the RNA elements during the transition from mature ED pre-B to late ED pre-B.
