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.