Figure 1.

Functions of known proofreading DExD/H-box ATPases in the splicing pathway. During assembly, small nuclear ribonucleoproteins (snRNPs) (small blue circles) assemble on a pre-mRNA intron (black line) flanked by exons (yellow rectangles) in a stepwise manner. Following engagement of pre-mRNA by U2 snRNP (a), Prp5-dependent rejection (b) competes with branch site recognition by U2 (c) to antagonize splicing at introns with suboptimal branch site sequences [12, 13]. After formation of optimal branch site-U2 interactions, Prp5 activity stabilizes U2 association with the substrate (d) [13, 28]. Following addition of the U4/U6•U5 tri-snRNP, U1 and U4 are released (e), leading to formation of a catalytically active spliceosome (large blue oval). For substrates with suboptimal branch site sequences, Prp16-dependent rejection (f) competes with 5' splice site cleavage (g), resulting in formation of an intermediate that can lead to Prp43-mediated discard of the pre-mRNA substrate (h) [7, 15, 22, 23]. After 5' splice site cleavage of an optimal substrate, Prp16 promotes a transition to the exon ligation conformation of the spliceosome (i) [27]. For suboptimal substrates, Prp22-dedpendent rejection (j) competes with exon ligation (k), resulting in formation of an intermediate that can lead to discard of the 5' exon and lariat intermediate (l), again mediated by Prp43 [14, 38]. After exon ligation of an optimal substrate, Prp22 promotes release of the nascent mRNA from the spliceosome (m) [29, 30]. Following mRNA release, Prp43 promotes release of the excised intron and dissociation of the spliceosome into its component snRNPs (n) [55–58]. The canonical splicing pathway is indicated by heavy arrows.