Visualizing nuclear export of different classes of RNA by electron microscopy.

Export of RNA from the cell nucleus to the cytoplasm occurs through nuclear pore complexes (NPCs). To examine nuclear export of RNA, we have gold-labeled different types of RNA (i.e., mRNA, tRNA, U snRNAs), and followed their export by electron microscopy (EM) after their microinjection into Xenopus oocyte nuclei. By changing the polarity of the negatively charged colloidal gold, complexes with mRNA, tRNA, and U1 snRNA can be formed efficiently, and gold-tagged RNAs are exported to the cytoplasm with kinetics and specific saturation behavior similar to that of unlabeled RNAs. U6 snRNA conjugates, in contrast, remain in the nucleus, as does naked U6 snRNA. During export, RNA-gold was found distributed along the central axis of the NPC, within the nuclear basket, or accumulated at the nuclear and cytoplasmic periphery of the central gated channel, but not associated with the cytoplasmic fibrils. In an attempt to identify the initial NPC docking site(s) for RNA, we have explored various conditions that either yield docking of import ligands to the NPC or inhibit the export of nuclear RNAs. Surprisingly, we failed to observe docking of RNA destined for export at the nuclear periphery of the NPC under any of these conditions. Instead, each condition in which export of any of the RNA-gold conjugates was inhibited caused accumulation of gold particles scattered uniformly throughout the nucleoplasm. These results point to the existence of steps in export involving mobilization of the export substrate from the nucleoplasm to the NPC.