Michaud N, Goldfarb D
Department of Biology, University of Rochester, New York 14627.
J Cell Biol. 1992 Feb;116(4):851-61. doi: 10.1083/jcb.116.4.851.
The inhibitory effects of wheat germ agglutinin and mAb 414 on the nuclear import of all types of U snRNAs indicate that they cross the nuclear envelope through the nuclear pore complex. However, the import of different U snRNAs occurs by kinetically distinct targeting pathways that can be distinguished from one another by the competitive effects of free trimethylguanosine cap dinucleotide (m3GpppG) and P(Lys)-BSA, an efficient synthetic karyophile based on the nuclear localization signal of SV40 large T antigen. The import of U snRNAs that contain 5' m3GpppN caps and are complexed by Sm proteins (U1, U2, U4, and U5) is competed by coinjection with free m3GpppG, indicating a shared transport factor, but not by P(Lys)-BSA. The import of U6 snRNA, which lacks a m3GpppN cap and is not complexed by the Sm proteins, is competed by P(Lys)-BSA but not by free m3GpppG. Thus, by the criterion of kinetic competition, U6 snRNA import is identical to that of the karyophilic proteins P(Lys)-BSA and nucleoplasmin. Uniquely, the import of U3 snRNA, which contains a m3GpppN cap but does not bind Sm proteins is not competed by either free m3GpppG or P(Lys)-BSA. Thus, U3 snRNA appears to be imported by a novel third kinetic pathway.
麦胚凝集素和单克隆抗体414对所有类型U小核RNA核输入的抑制作用表明，它们通过核孔复合体穿过核膜。然而，不同U小核RNA的输入通过动力学上不同的靶向途径发生，这些途径可通过游离三甲基鸟苷帽二核苷酸（m3GpppG）和P（Lys）-BSA（一种基于SV40大T抗原核定位信号的高效合成亲核蛋白）的竞争效应相互区分。含有5'm3GpppN帽并与Sm蛋白（U1、U2、U4和U5）复合的U小核RNA的输入与游离m3GpppG共注射时会受到竞争，这表明存在共享的转运因子，但不受P（Lys）-BSA的竞争。缺乏m3GpppN帽且未与Sm蛋白复合的U6小核RNA的输入受到P（Lys）-BSA的竞争，但不受游离m3GpppG的竞争。因此，根据动力学竞争标准，U6小核RNA的输入与亲核蛋白P（Lys）-BSA和核质蛋白的输入相同。独特的是，含有m3GpppN帽但不结合Sm蛋白的U3小核RNA的输入既不受游离m3GpppG也不受P（Lys）-BSA的竞争。因此，U3小核RNA似乎通过一种新的第三条动力学途径输入。
