1. Volume 87, Issue 1, Pages 21-32 (October 1996)
Importin Provides a Link between Nuclear Protein Import and U snRNA Export 
Dirk Görlich, Regine Kraft, Susanne Kostka, Frank Vogel, Enno Hartmann, Ronald A Laskey, Iain W Mattaj, Elisa Izaurralde 
Cell 
Volume 87, Issue 1, Pages (October 1996)
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2. Figure 1 SRP1p-Containing Complexes
(A) Identification of SRP1p-associated proteins. Protein complexes were bound to an anti-SRP1p immuno-affinity column. The column was washed and then eluted with a linear 0–2 M MgCl2 gradient followed by a pH 2.2 step. Eluted proteins were as follows: 1, yeast homolog of importin-β; 2, Nup1p; 3, yeast CBP80 homolog; 4, the yeast CBP20 homolog; 5, Nup2p; 6, SRP1p.
(B) SRP1p is part of several different complexes. Indicated antibodies were used for immunoprecipitations from an S. cerevisiae postribosomal supernatant. Bound proteins were analyzed by SDS–PAGE and Coomassie staining or Western blotting with the indicated antibodies. Nup2p was detected by MAb 414 (Davis and Blobel 1986).
(C) SRP1p and associated proteins immunoprecipitated from yeast high speed supernatant or high speed supernatant depleted of y–importin-β.
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3. Figure 2 The 25 kDa SRP1p-Associated Protein Is Similar to CBP20
The figure shows the aligned protein sequences of CBP20 from S. cerevisiae (yCBP20) and human (hCBP20) in single-letter code.
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4. Figure 3 SRP1p and CBP80 Are Predominantly Nuclear in Yeast
Frozen sections of S. cerevisiae wild-type cells (Kaergel et al. 1996) were stained with affinity-purified anti-SRP1p or -yCBP80 antibodies followed by Protein A–10 nm gold. For quantitation, see Table 1.
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5. Figure 4 Yeast CBC Binds to Capped RNA
An m7GpppG-capped RNA probe was subjected to native gel electrophoresis either alone (lanes 1, 9, and 17), together with recombinant human CBC (lanes 2–8), or purified yeast CBC (lanes 10–16). Cap dinucleotide analogs were added to the mixtures as indicated above the lanes.
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6. Figure 5
The CBC–Importin-α (SRP1p) Complex Binds to Capped RNA, But Importin-β Dissociates the Complex from the RNA
(A) A yCBC-capped RNA complex was formed and analyzed as in Figure 4 (lanes 1 and 2). Purified recombinant SRP1p was added to the CBC–RNA mixture either alone (lanes 3 and 4, 1 or 2 μg) or together with increasing amounts of y–importin-β (lanes 5–8, 1–4 μg). The same quantities of y–importin-β were also added in the absence of SRP1p (lanes 9–12).
(B) As in (A), except that purified recombinant hCBC, Rch1p, and h–importin-β were used. Lane 1, probe alone; lane 2, hCBC (400 ng); lanes 3–4, hCBC plus recombinant Rch1p (1, 1.5μg); lanes 5–8, hCBC plus 1.5 μg Rch1p and increasing amounts of h–importin-β (1.4, 2, 2.8, and 3.5 μg, respectively). The same quantities of h–importin-β were also added in the absence of Rch1p (lanes 9–12).
(C) Yeast CBP20, CBP80, SRP1p, and importin-β form a tetrameric complex. Yeast importin-β was prebound to immobilized anti-yeast importin-β antibodies and incubated with SRP1p, yCBC, or both. Bound fractions were washed, eluted with SDS, and analyzed by SDS–PAGE and Coomassie staining.
(D) RanGTP protects the CBC–importin-α–RNA complex against dissociation by importin-β. A mixture of hCBC, Rch1p, and RNA was incubated to allow complex formation. Subsequently, either importin-β or a mixture of importin-β and RanQ69L was added. The reaction was then fractionated as in Figure 4. Protein mixtures contained 400 ng of hCBC, 1.25 μg of Rch1, 6 μg of importin-β, and 6 μg of RanQ69L (GTP form) as indicated.
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7. Figure 6
Importin-α Is Bound to CBC and Capped RNAs in the Nuclei of Xenopus Oocytes
(A) Soluble nuclear fractions from oocytes were immunoprecipitated with anti–importin-α antiserum. Proteins were eluted, fractionated by SDS–PAGE, and analyzed by Western blotting. Lanes 1 and 2, bound fractions from four and eight nuclei, respectively. Lanes 3–5, total protein extracted from one, two, and four nuclei, respectively.
(B) A mixture of U1ΔSm, U6Δss, and tRNA was microinjected into Xenopus oocyte nuclei. After 1 hr, RNA was extracted from either total or fractionated oocytes (lanes 1–3). Extract from 20 isolated nuclei was prepared and selected on beads either with or without anti–importin-α antibodies. Bound RNA was extracted from control (lane 4) or anti–importin-α beads (lane 5) corresponding to eight oocyte nuclei. RNA corresponding to two nuclei from the supernatants (lanes 6 and 7) was also loaded.
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8. Figure 7 U snRNA Export Requires Importin-Dependent Protein Import
Xenopus laevis oocytes were injected into the cytoplasm with truncated (lanes 7–9) or functional (lanes 10–12) IBB domains or with phosphate-buffered saline alone (lanes 1–6). After 1 hr, U1ΔSm, U5ΔSm, U6Δss, and tRNA were injected into their nuclei. In lanes 4–12, RNA was extracted 180 min after injection; in lanes 1–3, RNA was extracted immediately after injection. T, C, and N indicate RNA extracted from total oocytes or from cytoplasmic or nuclear fractions, respectively.
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9. Figure 8 A Model of the CBC Nuclear Transport Cycle
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