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PDBsum entry 3k3f
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Transport protein
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PDB id
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3k3f
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References listed in PDB file
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Key reference
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Title
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Crystal structure of a bacterial homologue of the kidney urea transporter.
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Authors
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E.J.Levin,
M.Quick,
M.Zhou.
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Ref.
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Nature, 2009,
462,
757-761.
[DOI no: ]
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PubMed id
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Abstract
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Urea is highly concentrated in the mammalian kidney to produce the osmotic
gradient necessary for water re-absorption. Free diffusion of urea across cell
membranes is slow owing to its high polarity, and specialized urea transporters
have evolved to achieve rapid and selective urea permeation. Here we present the
2.3 A structure of a functional urea transporter from the bacterium
Desulfovibrio vulgaris. The transporter is a homotrimer, and each subunit
contains a continuous membrane-spanning pore formed by the two homologous halves
of the protein. The pore contains a constricted selectivity filter that can
accommodate several dehydrated urea molecules in single file. Backbone and
side-chain oxygen atoms provide continuous coordination of urea as it progresses
through the filter, and well-placed alpha-helix dipoles provide further
compensation for dehydration energy. These results establish that the urea
transporter operates by a channel-like mechanism and reveal the physical and
chemical basis of urea selectivity.
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Figure 2.
Figure 2: Fold and oligomeric structure of dvUT. a, Cartoon
representation of the dvUT protomer. The two-fold
pseudo-symmetry axis, marked as a black oval, is normal to the
plane of the figure. Colour of helices matches that in the
topology diagram (Supplementary Fig. 1b). b, Cartoon
representation of the full dvUT trimer. The crystallographic
three-fold symmetry axis is marked as a black triangle.
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Figure 4.
Figure 4: Schematic view of the selectivity filter. The
selectivity filter is shown from two angles. The predicted
locations of three urea molecules and their hydrogen-bonding
partners are on the left. In the perpendicular direction, the
filter is compressed by phenylalanine and leucine side chains
lining the walls of the pore (right). Helices contributing
residues to the selectivity filter are represented as grey
cylinders.
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The above figures are
reprinted
by permission from Macmillan Publishers Ltd:
Nature
(2009,
462,
757-761)
copyright 2009.
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