FIG. 7.

Schematic of the Cwc22 requirement to direct Prp2 into the productive pathway. After the activation of the spliceosome, Cwc22 and Prp2/Spp2 can bind to the spliceosome independently of each other. In the absence of Cwc22, Prp2 and Spp2 are dissociated from the spliceosome upon ATP hydrolysis with SF3a/b unreleased. In the presence of Cwc22, SF3a and SF3b are released with Prp2/Spp2 upon ATP hydrolysis. Cwc25 is then recruited to the spliceosome to promote the first catalytic reaction. Yju2 and HP-X are required for the first catalytic reaction after Prp2 action but can join the spliceosome at earlier steps, which are shown in gray to indicate their presence being functionally irrelevant. The question mark for HP-X indicates no experimental evidence for whether it can bind to the spliceosome prior to Cwc22 binding.