Involved in pre-mRNA splicing as component of the spliceosome. Required for the assembly of the U4/U5/U6 tri-snRNP complex, one of the building blocks of the spliceosome. Identified in the spliceosome B complex. Component of the U4/U6-U5 tri-snRNP complex. Component of some MLL1/MLL complex. Predominantly found in speckles and in Cajal bodies. Interacts with the snRNP via the Nop domain. The coiled coil domain is formed by two non-contiguous helices. Belongs to the PRP31 family.