Component of the spliceosome and rRNA processing machinery. In association with the spliceosomal U4/U6.U5 tri-snRNP particle, required for splicing of pre-mRNA. 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 PRP38. Present with 2510 molecules/cell in log phase SD medium. Belongs to the SPP381 family.