Pre-mRNA splicing Prp18-interacting factorThe spliceosome, an assembly of snRNAs (U1, U2, U4/U6, and U5) and proteins, catalyzes the excision of introns from pre-mRNAs in two successive trans-esterification reactions. Step 2 depends upon integral spliceosome constituents such as U5 snRNA and Prp8 and non-spliceosomal proteins Prp16, Slu7, Prp18, and Prp22. ATP hydrolysis by the DEAH-box enzyme Prp16 promotes a conformational change in the spliceosome that leads to protection of the 3'ss from targeted RNase H cleavage. This change, which probably reflects binding of the 3'ss PyAG in the catalytic centre of the spliceosome, requires the ordered recruitment of Slu7, Prp18, and Prp22 to the spliceosome. There is a close functional relationship between Prp8, Prp18, and Slu7, and Prp18 interacts with Slu7, so that together they recruit Prp22 to the spliceosome. Most members of the family carry a zinc-finger of the CCHC-type upstream of this domain.