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PDBsum entry 2bto
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Cytoskeletal protein
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PDB id
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2bto
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Contents |
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413 a.a.
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423 a.a.
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103 a.a.
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References listed in PDB file
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Key reference
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Title
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Structure of bacterial tubulin btuba/b: evidence for horizontal gene transfer.
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Authors
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D.Schlieper,
M.A.Oliva,
J.M.Andreu,
J.Löwe.
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Ref.
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Proc Natl Acad Sci U S A, 2005,
102,
9170-9175.
[DOI no: ]
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PubMed id
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Abstract
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alphabeta-Tubulin heterodimers, from which the microtubules of the cytoskeleton
are built, have a complex chaperone-dependent folding pathway. They are thought
to be unique to eukaryotes, whereas the homologue FtsZ can be found in bacteria.
The exceptions are BtubA and BtubB from Prosthecobacter, which have higher
sequence homology to eukaryotic tubulin than to FtsZ. Here we show that some of
their properties are different from tubulin, such as weak dimerization and
chaperone-independent folding. However, their structure is strikingly similar to
tubulin including surface loops, and BtubA/B form tubulin-like protofilaments.
Presumably, BtubA/B were transferred from a eukaryotic cell by horizontal gene
transfer because their high degree of similarity to eukaryotic genes is unique
within the Prosthecobacter genome.
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Figure 2.
Fig. 2. BtubA/B polymers have a longitudinal repeat similar
to  ,  -tubulin indicating
protofilament formation. (A) Low-magnification micrograph
showing BtubA/B filaments after polymerization in the presence
of GTP. Protein at 10 µM was incubated for 30 min at
ambient temperature with 100 mM Pipes·NaOH (pH 6.8), 5 mM
MgCl[2], 200 mM KCl, and 1 mM GTP and was negatively stained
with 2% uranyl acetate. (B-D) BtubA/B double filaments. These
are the most commonly formed filaments, presumably consisting of
two BtubA/B protofilaments. Most filaments twist (B and C),
indicated by arrowheads at the crossover points. Filament C has
an average width of  109 Å. (Scale bar:
100 nM.) (E) Computed diffraction pattern of filament B. Layer
lines are clearly visible at 42 Å, representing
the subunit repeat along the protofilament axis. This repeat
matches the repeat seen in the BtubA/B crystal structure and is
close to that of tubulin (40 Å).
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Figure 3.
Fig. 3. Crystal structures of BtubA and BtubA/B. (A)
Crystal structure of BtubA at 2.5-Å resolution. BtubA's
structure is closely related to tubulin. The fold is divided
into the N-terminal nucleotide-binding domain (blue), separated
by helix H7 (yellow) from the intermediate domain (orange) and
two large helices forming the C-terminal domain (red) (4). (B)
BtubA contains the C-terminal tubulin domain. Shown is a view
rotated 90° around the y axis from A. The two large helices
(red) at the C terminus of tubulin form the outside of
microtubules (7) and make the biggest difference between tubulin
and FtsZ. (C) Crystal structure of BtubA/B heterodimer
(asymmetric unit of the crystals) at 3.2-Å resolution.
BtubA/B form the same heterodimer as tubulin (24, 25). The
protofilament axis is vertical. BtubA is situated at the plus
(+) end (red), and BtubB is at the minus (-) end (blue). In the
crystals, BtubA contains GDP, whereas BtubB has a sulfate ion in
the nucleotide-binding site. The heterodimer is not completely
straight; the two subunits are rotated by 15° around the z axis
[same direction as in the tubulin-stathmin complex (26)],
tangential to the microtubule wall. (D) The BtubA/B crystals
contain a continuous double filament. The 6[5]22 space group
symmetry produces an antiparallel double filament with repeating
BtubA/B units in the crystal packing. The bend per heterodimer
is 60°, divided into 15° between BtubA and -B
(intradimer; see C) and 45° between B and A (interdimer).
(E) BtubA/B are very closely related to tubulin. Shown is the
superposition of BtubA (black) (rmsd to BtubB, 1.34 Å; 36%
sequence identity; 82% aligned) with -tubulin (25) (green;
rmsd 1.5 Å; 37% sequence identity; 85% aligned; Protein
Data Bank ID code 1JFF [PDB]
), -tubulin (25) (red; rmsd
1.71 Å; 35% sequence identity; 85% aligned; Protein Data
Bank ID code 1JFF [PDB]
), and subunit B from the tubulin-stathmin complex (26) (blue;
rmsd 1.3 Å; 35% sequence identity; 85% aligned; Protein
Data Bank ID code 1SA0 [PDB]
). Differences are small and mainly located in the T7-loop, the
M-loop, which is involved in microtubule formation for tubulin
(7), helix H6, and loop H1-S2, which are part of the
protofilament contact. BtubA/B have a short S9-S10 loop that in
-tubulin covers the
Taxol-binding pocket completely. (Figure was generated with
PYMOL.
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