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La (SS-B) is a highly expressed protein that is able to bind 3′-oligouridylate and other common RNA sequence/structural motifs. By virtue of these interactions, La is present in a myriad of nuclear and cytoplasmic ribonucleoprotein complexes in vivo where it may function as an RNA-folding protein or RNA chaperone. We have recently characterized the nuclear import pathway of the S. cerevisiae La, Lhp1p. The soluble transport factor, or karyopherin, that mediates the import of Lhp1p is Kap108p/Sxm1p. We have now determined a 113-amino acid domain of Lhp1p that is brought to the nucleus by Kap108p. Unexpectedly, this domain does not coincide with the previously identified nuclear localization signal of human La. Furthermore, when expressed in Saccharomyces cerevisiae, the nuclear localization of Schizosaccharomyces pombe, Drosophila, and human La proteins are independent of Kap108p. We have …