Dahlberg J E, Lund E
Department of Biomolecular Chemistry, University of Wisconsin-Madison, WI 53706, USA.
Curr Opin Cell Biol. 1998 Jun;10(3):400-8. doi: 10.1016/s0955-0674(98)80017-3.
Significant and exciting advances in the field of RNA and protein export have been made recently, due in large part to discovery of the roles played by Ran, a small, soluble GTPase present in both the nucleus and cytoplasm of all eukaryotic cells. Ran is thought to be primarily bound to GTP in the nucleus and to GDP in the cytoplasm, as a result of the assymetric distribution of factors that interact with Ran to promote guanine nucleotide exchange (in the nucleus) and GTP hydrolysis (in the cytoplasm). A key function of the nuclear Ran.GTP is to support formation of complexes containing an export receptor (an exportin) and cargos such as RNAs, RNPs or proteins that are destined for export. In the cytoplasm, removal of the Ran.GTP from the complex results in its destabilization and release of the export cargo. Although Ran.GTP is required for formation of the export complex, GTP hydrolysis does not appear to be necessary for translocation through the nuclear pore complex or cytoplasmic release. Nevertheless, the GTPase of Ran does appear to be required in as yet unidentified intranuclear steps prior to export of some, but not all, RNAs.
最近，RNA和蛋白质输出领域取得了重大且令人兴奋的进展，这在很大程度上归功于Ran的作用被发现，Ran是一种小型可溶性GTP酶，存在于所有真核细胞的细胞核和细胞质中。由于与Ran相互作用以促进鸟嘌呤核苷酸交换（在细胞核中）和GTP水解（在细胞质中）的因子的不对称分布，Ran在细胞核中主要与GTP结合，在细胞质中主要与GDP结合。细胞核中Ran·GTP的一个关键功能是支持形成包含输出受体（一种输出蛋白）和诸如RNA、核糖核蛋白或注定要输出的蛋白质等货物的复合物。在细胞质中，从复合物中去除Ran·GTP会导致其不稳定并释放输出货物。虽然形成输出复合物需要Ran·GTP，但GTP水解似乎对于通过核孔复合物转运或细胞质释放并非必需。然而，在一些但不是所有RNA输出之前的尚未确定的核内步骤中，Ran的GTP酶似乎是必需的。
