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 major role in the splicing of cellular pre-mRNAs. Required for normal plant development (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. Pleiotropic phenotypes of delayed flowering time and seed production, reduced root growth, partially defective leaf venation, abnormal numbers of trichome branches and changed numbers of floral organs. The double mutants smd3-a and smd3-b display embryonic lethality. Belongs to the snRNP core protein family.