Core component of the spliceosomal U1, U2, U4 and U5 small nuclear ribonucleoproteins (snRNPs), the building blocks of the spliceosome (By similarity). May play a minor role in the splicing of cellular pre-mRNAs (PubMed:21421416). SMN-mediated assembly into core snRNPs occurs in the cytosol before SMN-mediated transport to the nucleus to be included in spliceosomes. Expressed in young seedlings, roots, leaves, flowers and immature siliques. No visible phenotype under normal growth conditions, but the double mutants smd3-a and smd3-b display embryonic lethality. Belongs to the snRNP core protein family.