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J. Cell Biol.,
Volume 143, Number 4, November 16, 1998 887-899
Laboratory of Cell Biology, Howard Hughes Medical Institute and Rockefeller University, New York, New York 10021
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 been able to reconstitute the nuclear import of human La into permeabilized HeLa
cells using the recombinant human factors karyopherin
2, karyopherin 
1, Ran, and p10. As such, the yeast
and human La proteins are imported using different sequence motifs and dissimilar karyopherins. Our results
are consistent with an intermingling of the nuclear import and evolution of La.
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