Figure 7.

Prp28p proofreads splicing at 5′SS during the association of 5′SS with U6 snRNA. By either an unwinding activity or an RNPase activity, Prp28p disrupts the 5′SS–U1 interaction, then facilitates the 5′SS–U6 interaction and proofreads 5′SS before the first step of splicing. It is unclear whether this is a concerted one-step U1 release/U6 association or two distinct steps; therefore, disruption of 5′SS–U1 snRNA interaction by Prp28p is shown in dash. (A) Productive splicing pathway facilitated by Prp28p. Duplex formation between optimal 5′SS substrates and U6 snRNA is faster than the rate of conformational change by either WT-Prp28p or selected mutant Prp28p. (B) Conformational change by WT-Prp28p is faster than the rate of duplex formation between suboptimal 5′SS substrates and U6 snRNA, resulting in Prp28p-mediated transition prior to 5′SS:U6 pairing and thus rejecting suboptimal substrates. However, mutant Prp28p with a slower activity reduces the rate of conformational change, allowing longer temporal opportunity to achieve stable 5′SS:U6 pairing and thus progression of splicing.