3-DIMENSIONAL VISUALIZATION OF THE ROUTE OF PROTEIN IMPORT - THE ROLEOF NUCLEAR-PORE COMPLEX SUBSTRUCTURES

The three-dimensional localization of nucleoplasmin and wheat germ agg lutinin (WGA) at the nuclear envelope of Xenopus oocytes is demonstrat ed by microinjecting protein coated gold colloids and examining their distribution using both stereo transmission electron microscopy and fi eld emission in-lens scanning electron microscopy. Localization of man y WGA gold particles and nucleoplasmin gold particles at the same nucl ear pore complex (NPC) following coinjection is demonstrated. Binding of the WGA gold in the central region of the NPCs appears to form a ba rrier, preventing the import of nucleoplasmin gold and includes centra l localization along radial ''tracks'' which correspond to the interna l filaments connecting the cytoplasmic ring and the central region of the NPC. We suggest that these filaments may in some way be involved i n opening and closing of the central channel of the NPC for transport. Transport of nucleoplasmin through the central region of the NPCs app ears to be in ''single file'' regardless of the size of the colloidal gold, and distribution into the nucleoplasm appears to be through the basket rings with no association of the nucleoplasmin gold with the ba sket filaments being observed. (C) 1997 Academic Press.