In mammals, nuclear localization of U-snRNP particles requires the snRNA hypermethylated cap structure and the Sm core complex. The nature of the signal located within the Sm core proteins is still unknown, both in humans and yeast. Close examination of the sequences of the yeast SmB, SmD1, and SmD3 carboxylterminal domains reveals the presence of basic regions that are reminiscent of nuclear localization signals (NLSs). Fluorescence microscopy studies using green fluorescent protein (GFP)-fusion proteins indicate that both yeast SmB and SmD1 basic amino acid stretches exhibit nuclear localization properties. Accordingly, deletions or mutations in the NLS-like motifs of SmB and SmD1 dramatically reduce nuclear fluorescence of the GFP-Sm mutant fusion alleles. Phenotypic analyses indicate that the NLS-like motifs of SmB and SmD1 are functionally redundant: each NLS-like motif can be deleted without affecting yeast viability whereas a simultaneous deletion of both NLS-like motifs is lethal. Taken together, these findings suggest that, in the doughnut-like structure formed by the Sm core complex, the carboxyl-terminal extensions of Sm proteins may form an evolutionarily conserved basic amino acid-rich protuberance that functions as a nuclear localization determinant.Splicing of nuclear pre-mRNAs in yeast and mammals occurs in the multicomponent complex called the spliceosome whose formation involves the ordered assembly of the U1, U2, and U4/U6.U5 snRNPs, together with a multitude of nonsnRNP-associated protein factors on the pre-mRNA substrate (27, 67). In both systems, except for the U6 snRNP, each particle is composed of a U snRNA, a set of specific proteins associated with one particular snRNP, and a set of common (or core) proteins shared by all snRNPs. This last group is composed of the Sm proteins B, BЈ (in mammals), D1, D2, D3, E, F, and G, which assemble around the Sm site of the snRNA (34). In addition to the common Sm core proteins, the yeast and metazoan U6, U4/U6, and U4/U6.U5 snRNPs also contain seven distinct proteins called Lsm (like Sm) exhibiting clear homology to the Sm proteins (1,7,19,21,39,55,62,68).The Sm and Lsm proteins are highly conserved in all eukaryotic organisms. They contain the Sm domain, which consists of two conserved regions separated by a linker of variable length. Seven residues are highly conserved, and at many positions, the physicochemical property of the amino acid is maintained (7,21,62). The Sm motif is important for Sm protein function. Indeed, mutations at various positions in this motif, and mostly the hydrophobic conserved residues, abolish protein-protein interactions between Sm partners and hinder Sm core RNP complex formation (5, 21). Based on the crystal structures of two human Sm protein complexes (SmB/SmD3 and SmD1/SmD2), a model in which the seven Sm proteins form a doughnut-like structure has been proposed (26).Assembly of the eukaryotic U snRNPs is a multistep process following an ordered pathway (36, 47). After transcription by RNA polymerase II, the snRNAs ...