Functional characterization of nuclear localization signals in yeast Sm proteins

In mammals, nuclear localization of U-snRNP particles requires the snRNA hy permethylated cap structure and the Sm core complex. The nature of the sign al 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 carboxyl-terminal domains reveals the presence of basic regions that are r eminiscent 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 localiz ation properties. Accordingly, deletions or mutations in the NLS-like motif s of SmB and SmD1 dramatically reduce nuclear fluorescence of the GFP-Sm mu tant fusion alleles. Phenotypic analyses indicate that the NLS-like motifs of SmB and SmD1 are functionally redundant: each NLS-like motif can be dele ted without affecting yeast viability, whereas a simultaneous deletion of b oth 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 ba sic amino acid-rich protuberance that functions as a nuclear localization d eterminant.