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© The Rockefeller University Press, 0021-9525/1997//1655 $5.00
The Journal of Cell Biology, Volume 139, Number 7, , 1997 1655-1661

Article

A Nuclear Import Pathway for a Protein Involved in tRNA Maturation



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

Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York 10021

A limited number of transport factors, or karyopherins, ferry particular substrates between the cytoplasm and nucleoplasm. We identified the Saccharomyces cerevisiae gene YDR395w/SXM1 as a potential karyopherin on the basis of limited sequence similarity to known karyopherins. From yeast cytosol, we isolated Sxm1p in complex with several potential import substrates. These substrates included Lhp1p, the yeast homologue of the human autoantigen La that has recently been shown to facilitate maturation of pre-tRNA, and three distinct ribosomal proteins, Rpl16p, Rpl25p, and Rpl34p. Further, we demonstrate that Lhp1p is specifically imported by Sxm1p. In the absence of Sxm1p, Lhp1p was mislocalized to the cytoplasm. Sxm1p and Lhp1p represent the karyopherin and a cognate substrate of a unique nuclear import pathway, one that operates upstream of a major pathway of pre-tRNA maturation, which itself is upstream of tRNA export in wild-type cells. In addition, through its association with ribosomal proteins, Sxm1p may have a role in coordinating ribosome biogenesis with tRNA processing.

Abbreviations used in this paper: DAPI, 4',6-diamidino-2-phenylindole; GFP, green fluorescent protein; NLS, nuclear localization sequence; NPC, nuclear pore complex; PrA, protein A.

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

Address all correspondence to Günter Blobel, Laboratory of Cell Biology, Howard Hughes Medical Institute, 1230 York Avenue, Box 168, The Rockefeller University, New York, NY 10021. Tel.: (212) 327-8096. Fax: (212) 327-7880. E-mail: blobel{at}rockvax.rockefeller.edu


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