Involved in pre-mRNA splicing as component of the spliceosome. Component of the PRP19-CDC5L complex that forms an integral part of the spliceosome and is required for activating pre-mRNA splicing. Identified in the spliceosome C complex. 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. 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. Also interacts directly in the complex with BCAS2 and PRPF19. Interacts with USB1 (By similarity). Belongs to the WD repeat PRL1/PRL2 family.