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PDBsum entry 1iq1
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Protein transport
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PDB id
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1iq1
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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Biophysical characterization of interactions involving importin-Alpha during nuclear import.
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Authors
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B.Catimel,
T.Teh,
M.R.Fontes,
I.G.Jennings,
D.A.Jans,
G.J.Howlett,
E.C.Nice,
B.Kobe.
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Ref.
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J Biol Chem, 2001,
276,
34189-34198.
[DOI no: ]
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PubMed id
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Abstract
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Proteins containing the classical nuclear localization sequences (NLSs) are
imported into the nucleus by the importin-alpha/beta heterodimer. Importin-alpha
contains the NLS binding site, whereas importin-beta mediates the translocation
through the nuclear pore. We characterized the interactions involving
importin-alpha during nuclear import using a combination of biophysical
techniques (biosensor, crystallography, sedimentation equilibrium,
electrophoresis, and circular dichroism). Importin-alpha is shown to exist in a
monomeric autoinhibited state (association with NLSs undetectable by biosensor).
Association with importin-beta (stoichiometry, 1:1; K(D) = 1.1 x 10(-8) m)
increases the affinity for NLSs; the importin-alpha/beta complex binds
representative monopartite NLS (simian virus 40 large T-antigen) and bipartite
NLS (nucleoplasmin) with affinities (K(D) = 3.5 x 10(-8) m and 4.8 x 10(-8) m,
respectively) comparable with those of a truncated importin-alpha lacking the
autoinhibitory domain (T-antigen NLS, K(D) = 1.7 x 10(-8) m; nucleoplasmin NLS,
K(D) = 1.4 x 10(-8) m). The autoinhibitory domain (as a separate peptide) binds
the truncated importin-alpha, and the crystal structure of the complex resembles
the structure of full-length importin-alpha. Our results support the model of
regulation of nuclear import mediated by the intrasteric autoregulatory sequence
of importin-alpha and provide a quantitative description of the binding and
regulatory steps during nuclear import.
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Figure 5.
Fig. 5. Crystal structure of the complex between Imp  (44-54) and
Imp (70-529).
A, stereoview of the electron density (drawn with the program
BOBSCRIPT (52)) in the region of the peptide bound to the major
binding site of Imp (70-529).
All peptide residues were omitted from the model and simulated
annealing run with the starting temperature of 1000 K. The
electron density map was calculated with coefficients 3|F[obs]|
 2|F[calc]|
and data between 30 and 2.8 Å resolution and contoured at
1.3 standard deviations. Superimposed is the refined model of
the peptide. B, schematic diagram of the complex. Importin- is shown as
a ribbon diagram (yellow; drawn with program RIBBONS (53)). The
superhelical axis of the repetitive part of the molecule is
approximately horizontal. The two peptides are shown in a
ball-and-stick representation; the peptide bound to the major
site is colored cyan, and the peptide bound to the minor site is
colored red. C, superposition of the Imp (44-54)
peptide (cyan) and the corresponding region of full-length
importin (magenta)
bound to the major NLS-binding site of importin- . The C
atoms of
residues 70-496 were used in the superposition (drawn with the
program RIBBONS (53)).
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Figure 7.
Fig. 7. Schematic diagram of the NLS-dependent nuclear
import pathway. Yellow, importin- ; green,
importin-  ; cyan,
NLS-containing cargo protein; magenta, Ran-GTP. For simplicity,
other factors involved in the pathway such as nuclear transport
factor-2, the nuclear export receptor for importin- , and
Ran-binding proteins have been omitted from the diagram.
Dissociation constants for the different binding events are
shown.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2001,
276,
34189-34198)
copyright 2001.
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