Structural basis for alternative 3' splice site selection in the human spliceosome active center

O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=200 SRC="FIGDIR/small/741260v1_ufig1.gif" ALT="Figure 1"> View larger version (97K): org.highwire.dtl.DTLVardef@795305org.highwire.dtl.DTLVardef@13979d0org.highwire.dtl.DTLVardef@6c6a31org.highwire.dtl.DTLVardef@1afd802_HPS_FORMAT_FIGEXP M_FIG C_FIG Key findingsO_LICryo-EM reveals SDE2 as a novel factor stabilizing the spliceosome active center C_LIO_LISDE2 {Delta}N structures visualize a stalled C* state with impaired docking-factor engagement. C_LIO_LISDE2, Prp18, and FAM32A read a cis-code to stabilize weaker proximal 3'-ss. C_LIO_LISDE2 {Delta}N destabilizes 3'-ss docking and rescues BRCA1 and CFTR mis-splicing in vivo. C_LI Accurate alternative splicing requires discrimination between adjacent 3' splice sites (3'-ss) during catalysis and is disrupted by pathogenic AG-gain mutations that create competing 3'-ss. Here, we present cryo-EM structures of human spliceosomes assembled on native-sequence pre-mRNAs, revealing how the catalytic core controls alternative 3'-ss selection. SDE2 is a previously unrecognized active-center component that promotes a docking-competent spliceosome conformation. Machine learning, in vivo transcriptomics, and in vitro biochemistry show how SDE2 cooperates with FAM32A and Prp18 to act as readers of a cis-regulatory code that governs 3'-ss selection during catalysis. These factors promote weaker, proximal site use by counteracting an intrinsic distal bias generated by active-site interactions with the distal-site -4 nucleotide. Structural or genetic perturbation of these exon-ligation factors destabilizes proximal 3'-ss docking and restores canonical splicing in disease-relevant CFTR and BRCA1 AG-gain alleles. Our work establishes the spliceosome active center as a tunable regulatory hub for alternative splicing.