在转录和处理之后，转移RNA（tRNA）被从细胞核输出到细胞质中，在那里发生转录。tRNA被一个名为Xpo-t的运输因子携带穿过核膜中的小孔。现在，Xpo-t的结构已被确定。其结构是在单独情况下和与tRNA及运输所需要的另一个因子Ran-GTP结合的情况下分别被确定的。与tRNA的结合诱导大的构形变化，从而使得Xpo-t将tRNA包围起来，并与其5'端和3'端都发生相互作用。这可解释蛋白合成中质量控制的一个重要机制，按照该机制，具有未成熟端的未被处理的tRNA被阻止，而不能从细胞核中被运送出去。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 461, 60-65 (3 September 2009) | doi:10.1038/nature08394
Structures of the tRNA export factor in the nuclear and cytosolic statesnear-final version
Atlanta G. Cook1, Noemi Fukuhara2,3, Martin Jinek2,3 & Elena Conti1,2
1 Structural Cell Biology, MPI for Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany
2 EMBL, Meyerhofstrasse 1, Heidelberg D69117, Germany
3 Present addresses: Division of Cell and Molecular Biology, Biophysics section, Blackett Laboratory, Imperial College London, London SW7 2AZ, UK (N.F.); Department of Molecular Cell Biology, University of California at Berkeley, 731 Stanley Hall, Berkeley, California 94720, USA (M.J.).
Transfer RNAs are among the most ubiquitous molecules in cells, central to decoding information from messenger RNAs on translating ribosomes. In eukaryotic cells, tRNAs are actively transported from their site of synthesis in the nucleus to their site of function in the cytosol. This is mediated by a dedicated nucleo-cytoplasmic transport factor of the karyopherin- family (Xpot, also known as Los1 in Saccharomyces cerevisiae). Here we report the 3.2 ? resolution structure of Schizosaccharomyces pombe Xpot in complex with tRNA and RanGTP, and the 3.1 ? structure of unbound Xpot, revealing both nuclear and cytosolic snapshots of this transport factor. Xpot undergoes a large conformational change on binding cargo, wrapping around the tRNA and, in particular, binding to the tRNA 5' and 3' ends. The binding mode explains how Xpot can recognize all mature tRNAs in the cell and yet distinguish them from those that have not been properly processed, thus coupling tRNA export to quality control.
