Ubiquitin-protein ligase which is a core component of several complexes mainly involved pre-mRNA splicing and DNA repair. Required for pre-mRNA splicing as component of the spliceosome. Core component of the PRP19C/Prp19 complex/NTC/Nineteen complex which is part of the spliceosome and participates in its assembly, its remodeling and is required for its activity. During assembly of the spliceosome, mediates 'Lys-63'-linked polyubiquitination of the U4 spliceosomal protein PRPF3. Ubiquitination of PRPF3 allows its recognition by the U5 component PRPF8 and stabilizes the U4/U5/U6 tri-snRNP spliceosomal complex. Recruited to RNA polymerase II C-terminal domain (CTD) and the pre-mRNA, it may also couple the transcriptional and spliceosomal machineries. The XAB2 complex, which contains PRPF19, is also involved in pre-mRNA splicing, transcription and transcription-coupled repair. Beside its role in pre-mRNA splicing PRPF19, as part of the PRP19-CDC5L complex, plays a role in the DNA damage response/DDR. It is recruited to the sites of DNA damage by the RPA complex where PRPF19 directly ubiquitinates RPA1 and RPA2. 'Lys-63'-linked polyubiquitination of the RPA complex allows the recruitment of the ATR-ATRIP complex and the activation of ATR, a master regulator of the DNA damage response. May also play a role in DNA double-strand break (DSB) repair by recruiting the repair factor SETMAR to altered DNA. As part of the PSO4 complex may also be involved in the DNA interstrand cross-links/ICLs repair process. In addition, may also mediate 'Lys-48'-linked polyubiquitination of substrates and play a role in proteasomal degradation (By similarity). May play a role in the biogenesis of lipid droplets (By similarity). May play a role in neural differentiation possibly through its function as part of the spliceosome (PubMed:16352598). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Homotetramer. Component of activated, catalytic and post-catalytic spliceosomes. Component of the Prp19 complex/PRP19C/Nineteen complex/NTC and related complexes described as PRP19-CDC5L splicing complex and PSO4 complex. A homotetramer of PRPF19, CDC5L, PLRG1 and BCAS2 constitute the core of those complexes. The interaction with CDC5L, PLRG1 and BCAS2 is direct within this core complex. At least three less stably associated proteins CTNNBL1, CWC15 and HSPA8 are found in the Prp19 complex. The Prp19 complex associates with the spliceosome during its assembly and remodeling recruiting additional proteins. Component of the XAB2 complex, a multimeric protein complex composed of XAB2, PRPF19, AQR, ZNF830, ISY1, and PPIE. Interacts with CWC22 and EIF4A3 in an RNA-independent manner. Interacts with RPA1 and RPA2; the PRP19-CDC5L complex is recruited to the sites of DNA repair where it interacts with the replication protein A complex (RPA). Interacts with SETMAR; required for SETMAR recruitment to site of DNA damage. Interacts with U2AF2; the interaction is direct and recruits the Prp19 complex to RNA polymerase II C-terminal domain (CTD) and the pre-mRNA. Interacts with PRPF3. Interacts with APEX1, DNTT and PSMB4. Interacts with KNSTRN (By similarity). Interacts with PSMC5 (By similarity). Interacts (via N-terminus) with CDC5L (PubMed:16352598). Interacts with KHDC4 (By similarity). Interacts with USB1 (By similarity). Nucleoplasmic in interphase cells. Irregularly distributed in anaphase cells. In prophase cells, uniformly distributed, but not associated with condensing chromosomes. Found in extrachromosomal regions in metaphase cells. Mainly localized to the mitotic spindle apparatus when chromosomes segregate during anaphase. When nuclei reform during late telophase, uniformly distributed in daughter cells and displays no preferred association with decondensing chromatin. Recruited on damaged DNA at sites of double-strand break (By similarity). The 7 WD repeats are necessary and sufficient to support interaction with the RPA complex. Belongs to the WD repeat PRP19 family.