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© The Rockefeller University Press, 0021-9525/1998//887 $5.00
The Journal of Cell Biology, Volume 143, Number 4, , 1998 887-899

Regular Articles

Nuclear Import and the Evolution of a Multifunctional RNA-binding Protein



Jonathan S. Rosenblum, Lucy F. Pemberton, Neris Bonifaci, and Günter Blobel

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 {alpha}2, karyopherin β1, 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.

Key Words: nuclear transport • karyopherin • protein evolution • RNA biogenesis • RNA-binding proteins

Abbreviations used in this paper: aa, amino acid; GFP, green fluorescent protein; IRES, internal ribosome entry site; Kap, karyopherin; NLS, nuclear localization sequence; NPC, nuclear pore complex; RRM, RNA recognition motif.

J.S. Rosenblum was supported by a National Institutes of Health postdoctoral fellowship.

2. Throughout the text, depending on context, nonstandard nomenclature of La proteins will be used to facilitate the discussion. Specifically, the S. cerevisiae La, Lhp1p, will sometimes be referred to as scLa and the S. pombe La, Sla1, will be referred to as spLa. Drosophila and human La proteins will be referred to as dmLa and hsLa, respectively.


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