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Cover   In eukaryotes, DNA replication, RNA transcription, and differential gene regulation all require the import of proteins into the nucleus. The process of nuclear import was beautifully visualized by Carl Feldherr using gold particles coated with the Xenopus nuclear protein nucleoplasmin (NP; J. Cell Biol. 99, 2216-2222, 1984). Feldherr and colleagues injected nucleoplasmin-coated gold into the cytoplasm of Xenopus oocytes, waited varying amounts of time, and then fixed and sectioned the oocytes for electron microscopic examination. At 0 min, the injected NP-gold was exclusively seen in the cytoplasm (top panel). However, by 15 min, electron microscopy revealed that NP-coated gold particles of up to 260 Å were being imported into the nucleus and entered through the very centers of the nuclear pores (bottom panel, low and high magnification). Feldherr found that each pore of the oocyte nucleus imported NP-gold, indicating that all the pores were capable of import and providing evidence that there are not specialized classes of nuclear pores. The multiple NP-gold particles, which appeared to be aligned on both sides of the nuclear pore, gave the suggestion that there exist short filaments or tracks leading to and from the pore on which nuclear proteins dock, a model later substantiated (Newmeyer and Forbes, Cell 52, 641-653, 1988; Richardson et al, Cell 52, 655-664, 1988). Dworetzky and Feldherr next examined the question of whether RNA export could be similarly visualized and, if so, whether a single nuclear pore was capable of both protein import and RNA export (J. Cell Biol. 106, 575-584, 1988). Small tRNA-coated gold particles (20-50 Å) were injected into the nucleus of a Xenopus oocyte, whereas large NP-coated gold particles (120-220 Å) were injected into the cytoplasm. Within 1 h, electron microscopy of fixed and sectioned oocytes revealed that the small tRNA-coated gold particles had been exported to the cytoplasmic side of the pore, whereas the large NP-coated gold had been imported to the nuclear side of the same pores. This work demonstrated that not only is RNA-coated gold perceived correctly by the pore as an export substrate, but that a single nuclear pore is capable of both RNA export and nuclear protein import. Subsequent work by Feldherr and colleagues (for example, see J. Cell Biol. 115, 933-939, 1991) has demonstrated that the functional diameter of the nuclear transport channel can be modulated: the nuclear pores of proliferating tissue culture cells are capable of importing much larger protein complexes (230 Å) than the pores of growth-arrested cells (110 Å). Feldherr's pictures continue to challenge our thinking on nuclear transport.Douglass J. Forbes