Plays a role in pre-mRNA splicing as a core component of the spliceosomal U1, U2, U4 and U5 small nuclear ribonucleoproteins (snRNPs), the building blocks of the spliceosome (By similarity). Interacts with the SMN complex. Methylated on arginine residues by Art5 and Art7; methylation is not required for assembly and biogenesis of snRNPs. Belongs to the snRNP core protein family.