Involved in pre-mRNA splicing and its phosphorylated form (by SRPK2) is required for spliceosomal B complex formation. Independently of its spliceosome formation function, required for the suppression of incorrect R-loops formed during transcription; R-loops are composed of a DNA:RNA hybrid and the associated non-template single-stranded DNA. ATP + H2O = ADP + H(+) + phosphate The phosphorylated form (by SRPK2) is a component of the U4/U6-U5 tri-snRNP complex composed of the U4, U6 and U5 snRNAs and at least PRPF3, PRPF4, PRPF6, PRPF8, PRPF31, SNRNP200, TXNL4A, WDR57, SNRNP40, DDX23, CD2BP2, PPIH, SNU13, EFTUD2, SART1 and USP39. Identified in the spliceosome C complex. Interacts with ERBB4. Interacts with ERCC6. During transcription, accumulates at chromatin loci where unscheduled R-loops form and colocalizes with paused 'Ser-5'-phosphorlyated POLR2A/RNA polymerase II and kinase SRPK2. In vitro phosphorylated by CLK1 and U1 snRNP-associated protein kinase. Phosphorylated by SRPK2 and this phosphorylation is required for its association with the tri-snRNP (U4/U6-U5 tri-small nuclear ribonucleoproteins) and subsequent spliceosomal B complex formation. May be phosphorylated by SRPK2 on Ser residues in the SR domain; the phosphorylation is required for the removal of inappropriate R-loops during transcription. Belongs to the DEAD box helicase family. DDX23/PRP28 subfamily.