A specific messenger ribonucleoprotein (RNP) particle, Balbiani ring (BR) granules in the dipteran Chironomus tentans, can be visualized during passage through the nuclear pore complex (NPC). We have now examined the transport through the nuclear basket preceding the actual translocation through the NPC. The basket consists of eight fibrils anchored to the NPC core by nucleoprotein Nup153. On nuclear injection of anti-Nup153, the transport of BR granules is blocked. Many granules are retained on top of the nuclear basket, whereas no granules are seen in transit through NPC. Interestingly, the effect of Nup153 seems distant from the antibody-binding site at the base of the basket. We conclude that the entry into the basket is a two-step process: an mRMP first binds to the tip of the basket fibrils and only then is it transferred into the basket by a Nup153-dependent process. It is indicated that ribosomal subunits follow a similar pathway.
INTRODUCTIONRNA molecules are exported from nucleus to cytoplasm as ribonucleoprotein (RNP) complexes (Daneholt, 2001;Dreyfuss et al., 2002;Lei and Silver, 2002). The translocation of RNPs through nuclear pores has been visualized in the electron microscope both for messenger RNPs (Stevens and Swift 1966;Mehlin et al., 1992;Kiseleva et al., 1998) and for ribosomes/ribosomal subunits (Franke and Scheer, 1974). The pores contain a specific supramolecular assembly, the nuclear pore complex (NPC), and the passage occurs through the central channel of the complex (Dworetzky and Feldherr, 1988). During the last decade, it has been revealed that the translocation apparently takes place in discrete steps (Daneholt, 1997). The molecular mechanisms involved are currently being revealed.The NPC is a three-layered structure with a central spoke assembly sandwiched between a nuclear and cytoplasmic ring (Allen et al., 2000;Vasu and Forbes, 2001;Fahrenkrog and Aebi, 2003;Suntharalingam and Wente, 2003). In the center, there is a 25-nm-wide channel filled with a meshwork of very thin fibrils, often designated as the central plug. Additional auxiliary components are attached to the NPC periphery. On the nuclear side, eight longer fibrils emanate from the nuclear ring. At their distal ends, these fibrils seem to bifurcate and laterally interdigitate with their neighboring fibrils, thereby forming another ringlike structure referred to as the terminal ring. Together, nuclear fibrils and terminal ring are considered a structural and functional entity, called the nuclear basket. On the cytoplasmic side, eight short fibrils extend from the cytoplasmic ring into cytoplasm.The NPC contains ϳ30 proteins designated nucleoporins, the majority of which are symmetrically organized in the core of the NPC (Rout et al., 2000;Cronshaw et al., 2002). The central plug material is composed of a group of nucleoporins containing multiple repeats of the dipeptide sequence FG. These repeats are involved in the actual translocation of macromolecules and supramolecular assemblies through the pore (see further below i...