Rosenblum J S, Pemberton L F, Blobel G
Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York 10021, USA.
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.
数量有限的转运因子，即核转运蛋白，在细胞质和核质之间运送特定的底物。基于与已知核转运蛋白有限的序列相似性，我们将酿酒酵母基因YDR395w/SXM1鉴定为一种潜在的核转运蛋白。我们从酵母胞质溶胶中分离出与几种潜在输入底物形成复合物的Sxm1p。这些底物包括Lhp1p，它是人类自身抗原La的酵母同源物，最近已被证明有助于前体tRNA的成熟，以及三种不同的核糖体蛋白Rpl16p、Rpl25p和Rpl34p。此外，我们证明Lhp1p是由Sxm1p特异性输入的。在没有Sxm1p的情况下，Lhp1p会错误定位于细胞质中。Sxm1p和Lhp1p代表了一种独特的核输入途径的核转运蛋白和同源底物，该途径在野生型细胞中位于前体tRNA成熟的主要途径上游，而前体tRNA成熟途径本身又位于tRNA输出的上游。此外，通过与核糖体蛋白的结合，Sxm1p可能在协调核糖体生物合成与tRNA加工方面发挥作用。
