Structures of aberrant spliceosome intermediates on their way to disassembly

Intron removal during pre-mRNA splicing is of extraordinary complexity and its disruption causes a vast number of genetic diseases in humans ¹ . While key steps of the canonical spliceosome cycle have been revealed by combined structure-function analyses ^2,3 , structural information on an aberrant spliceosome committed to premature disassembly is not available. Here, we report two cryo-EM structures of post-B ^act spliceosome intermediates from S. pombe primed for disassembly. We identify the DEAH-box helicase – G patch protein pair (Gih35-Gpl1, homologous to human DHX35-GPATCH1) and show how it maintains catalytic dormancy. In both structures, Gpl1 recognizes a remodeled active site introduced by an over-stabilization of the U5 loop I interaction with the 5’ exon leading to a single nucleotide insertion at the 5’splice site. Remodeling is communicated to the spliceosome surface and the Ntr1 complex that mediates disassembly is recruited. Our data pave the way for a targeted analysis of splicing quality control.