ATP-dependent RNA helicase involved in mRNA splicing. May destabilize the U1/5' splice site duplex to permit an effective competition for the 5' splice site by the U6 snRNA, resulting in the switch between U1 and U6 at the 5' splice site. May also act to unwind the U4/U6 base-pairing interaction in the U4/U6/U5 snRNP, facilitating the first covalent step of splicing (By similarity). ATP + H2O = ADP + H(+) + phosphate Component of the U5 snRNP complex. The Q motif is unique to and characteristic of the DEAD box family of RNA helicases and controls ATP binding and hydrolysis. Belongs to the DEAD box helicase family. DDX23/PRP28 subfamily.