Component of the PRP19-CDC5L complex that forms an integral part of the spliceosome and is required for activating pre-mRNA splicing. Participates in AID/AICDA-mediated Ig class switching recombination (CSR) (By similarity). 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 directly with CWC15 and CDC5L in the complex. Interacts with AICDA; the interaction is important for the antibody diversification activity of AICDA. Interacts with PRPF31 (via its NLS). Interacts (via its N-terminal NLS) with KPNA1 and KPNA2 (By similarity). The surface residues of the concave side of the superhelical ARM repeat region contribute to, but are not essential for NLS binding. Truncated N-terminus.