The splicing factor Prp18 is required for the second step of pre-mRNA splicing. The structure of a large fragment of the Saccharomyces cerevisiae Prp18 is known . This fragment is fully active in yeast splicing in vitro and includes the sequences of Prp18 that have been evolutionarily conserved. The core structure consists of five alpha-helices that adopt a novel fold. The most highly conserved region of Prp18, a nearly invariant stretch of 19 aa, forms part of a loop between two alpha-helices and may interact with the U5 small nuclear ribonucleoprotein particles .
The splicing factor Prp18 is required for the second step of pre-mRNA splicing. PRP18 appears to be primarily associated with the U5 snRNP.