Promotes the association of the U4/U6.U5 tri-snRNP particle with pre-spliceosomes to form the mature spliceosomal complex. Component of the U4/U6-U5 tri-snRNP complex composed of the U4, U6 and U5 snRNAs and at least PRP3, PRP4, PRP6, PRP8, PRP18, PRP31, PRP38, SNU13, SNU23, SNU66, SNU114, SPP381, SMB1, SMD1, SMD2, SMD3, SMX2, SMX3, LSM2, LSM3, LSM4, LSM5, LSM6, LSM7, LSM8, BRR2 and DIB1. Interacts with the snRNP via the Nop domain. The coiled coil domain is formed by two non-contiguous helices. Present with 1260 molecules/cell in log phase SD medium. Belongs to the PRP31 family.