Involved in pre-mRNA splicing as component of the spliceosome (PubMed:28502770, PubMed:28076346). Component of the PRP19-CDC5L complex that forms an integral part of the spliceosome and is required for activating pre-mRNA splicing (PubMed:11101529, PubMed:11544257). Identified in the spliceosome C complex (PubMed:12176931, PubMed:28502770, PubMed:28076346). Component of the PRP19-CDC5L splicing complex composed of a core complex comprising a homotetramer of PRPF19, CDC5L, PLRG1 and BCAS2, and at least three less stably associated proteins CTNNBL1, CWC15 and HSPA8 (PubMed:11101529, PubMed:20176811). Interacts (via its WD40 repeat domain) directly with CDC5L (via its C-terminal); the interaction is required for mRNA splicing but not for spliceosome assembly (PubMed:11544257). Also interacts directly in the complex with BCAS2 and PRPF19 (PubMed:20176811). Interacts with USB1 (PubMed:23022480). May be due to a competing donor splice site. Belongs to the WD repeat PRL1/PRL2 family.