New mechanistic insights into Prp22-mediated exon ligation and mRNA release.

The DExD/H-box RNA helicase Prp22 catalyzes messenger RNA (mRNA) release from the spliceosome, and has also been implicated in proofreading the 3' splice site (3'SS), preventing exon ligation of mutant pre-mRNAs through an ATP-dependent mechanism. However, here we reveal an unexpected role for Prp22 in promoting exon ligation of both wild-type and mutant pre-mRNAs by stabilizing Slu7's association with the spliceosome prior to exon ligation. Notably, ATP binding, rather than hydrolysis, by Prp22 inhibits exon ligation of 3'SS mutant pre-mRNA. Following exon ligation, Prp22-mediated ATP hydrolysis facilitates the dissociation of both Slu7 and mRNA from the spliceosome. Remarkably, Prp22 and Cwc22, which bind the 3'- and 5'-exons respectively, remain associated with the released mRNA, whereas Slu7 and Fyv6 dissociate independently. We propose that Prp22 facilitates exon ligation by stabilizing Slu7 binding, with binding of ATP by Prp22 potentially destabilizing that interaction, thereby weakening contacts between the 5'-exon and the 3'SS to inhibit exon ligation. After exon ligation, Prp22-driven ATP hydrolysis induces a conformational change in Prp8 that disrupts its interdomain interactions, enabling mRNA release through the domain interfaces, with Prp22 and Cwc22 remaining associated with the released mRNA.