Quantitative imaging and photobleaching were used to measure ER/Golgi recycling of GFP-tagged Golgi proteins in interphase cells and to monitor the dissolution and reformation of the Golgi during mitosis. In interphase, recycling occurred every 1.5 hr, and blocking ER egress trapped cycling Golgi enzymes in the ER with loss of Golgi structure. In mitosis, when ER export stops, Golgi proteins redistributed into the ER as shown by quantitative imaging in vivo and immuno-EM. Comparison of the mobilities of Golgi proteins and lipids ruled out the persistence of a separate mitotic Golgi vesicle population and supported the idea that all Golgi components are absorbed into the ER. Moreover, reassembly of the Golgi complex after mitosis failed to occur when ER export was blocked. These results demonstrate that in mitosis the Golgi disperses and reforms through the intermediary of the ER, exploiting constitutive recycling pathways. They thus define a novel paradigm for Golgi genesis and inheritance.

Nucleoporin 98 (Nup98), a glycine-leucine-phenylalanine-glycine (GLFG) amino acid repeatcontaining nucleoporin, plays a critical part in nuclear trafficking. Injection of antibodies to Nup98 into the nucleus blocks the export of most RNAs. Nup98 contains binding sites for several transport factors; however, the mechanism by which this nucleoporin functions has remained unclear. Multiple subcellular localizations have been suggested for Nup98. Here we show that Nup98 is indeed found both at the nuclear pore complex and within the nucleus. Inside the nucleus, Nup98 associates with a novel nuclear structure that we term the GLFG body because the GLFG domain of Nup98 is required for targeting to this structure. Photobleaching of green fluorescent protein-Nup98 in living cells reveals that Nup98 is mobile and moves between these different localizations. The rate of recovery after photobleaching indicates that Nup98 interacts with other, less mobile, components in the nucleoplasm. Strikingly, given the previous link to nuclear export, the mobility of Nup98 within the nucleus and at the pore is dependent on ongoing transcription by RNA polymerases I and II. These data give rise to a model in which Nup98 aids in direction of RNAs to the nuclear pore and provide the first potential mechanism for the role of a mobile nucleoporin. INTRODUCTIONThe nuclear pore complex is a massive structure that conducts all traffic between the nucleus and cytoplasm (reviewed by Ohno et al., 1998;Gorlich and Kutay, 1999;Ryan and Wente, 2000;Vasu and Forbes, 2001). The pore has been studied intensely at the structural level in both yeast and vertebrate systems (Stoffler et al., 1999a;Allen et al., 2000). Although the vertebrate pore is larger and thought to contain a greater number of constituent proteins, the pores of both yeast and vertebrates share a similar structural organization. The central mass of the nuclear pore displays eightfold symmetry around a central axis perpendicular to the nuclear envelope. Two distinct sets of fibers extend out from the cytoplasmic and nuclear faces of the pore. On the nuclear side, the fibers are joined together at their distal ends by a ring to form the nuclear basket of the pore. Additionally, the nuclear basket of both yeast and vertebrate pores has associated filaments that extend for considerable distances into the nuclear interior and may serve to direct transport cargoes to and/or from the pore.A subset of the nuclear pore complex proteins (nucleoporins or Nups) each contain a domain with multiple, nontandem repeats of the amino acid sequences FG, FXFG, or GLFG (glycine-leucine-phenylalanine-glycine). These domains provide docking sites for a family of nuclear transport signal receptor proteins (known as importins, exportins, and transportin, or collectively as karyopherins). The repeat domain nucleoporins are found in multiple substructures of the nuclear pore and thus are thought to facilitate the moveArticle published online ahead of print. Mol. Biol. Cell 10.1091/ mbc.01-11-0538. Article and p...

Multidrug resistance (MDR) is a significant problem in the treatment of cancer. Chemotherapeutic drugs distribute through the cyto- and nucleoplasm of drug-sensitive cells but are excluded from the nucleus in drug-resistant cells, concentrating in cytoplasmic organelles. Weak base chemotherapeutic drugs (e.g., anthracyclines and vinca alkaloids) should concentrate in acidic organelles. This report presents a quantification of the pH for identified compartments of the MCF-7 human breast tumor cell line and demonstrates that (a) the chemotherapeutic Adriamycin concentrates in acidified organelles of drug-resistant but not drug-sensitive cells; (b) the lysosomes and recycling endosomes are not acidified in drug-sensitive cells; (c) the cytosol of drug-sensitive cells is 0.4 pH units more acidic than the cytosol of resistant cells; and (d) disrupting the acidification of the organelles of resistant cells with monensin, bafilomycin A1, or concanamycin A is sufficient to change the Adriamycin distribution to that found in drug-sensitive cells, rendering the cell vulnerable once again to chemotherapy. These results suggest that acidification of organelles is causally related to drug resistance and is consistent with the hypothesis that sequestration of drugs in acidic organelles and subsequent extrusion from the cell through the secretory pathways contribute to chemotherapeutic resistance.