A nuclear import pathway for a protein involved in tRNA maturation

J Cell Biol. 1997 Dec 29;139(7):1655-61. doi: 10.1083/jcb.139.7.1655.


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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Cell Nucleus / metabolism*
  • Fungal Proteins / genetics
  • Fungal Proteins / metabolism*
  • Gene Deletion
  • Karyopherins
  • Molecular Sequence Data
  • Nuclear Envelope / metabolism
  • Nuclear Proteins / chemistry
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism*
  • RNA Processing, Post-Transcriptional
  • RNA, Transfer / metabolism*
  • RNA-Binding Proteins / metabolism
  • Saccharomyces cerevisiae / metabolism*
  • Saccharomyces cerevisiae Proteins*
  • Sequence Homology, Amino Acid
  • beta Karyopherins


  • Fungal Proteins
  • Karyopherins
  • LHP1 protein, S cerevisiae
  • Nuclear Proteins
  • RNA-Binding Proteins
  • SXM1 protein, S cerevisiae
  • Saccharomyces cerevisiae Proteins
  • beta Karyopherins
  • RNA, Transfer