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PDBsum entry 2b9c
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Contractile protein
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PDB id
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2b9c
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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 the mid-Region of tropomyosin: bending and binding sites for actin.
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Authors
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J.H.Brown,
Z.Zhou,
L.Reshetnikova,
H.Robinson,
R.D.Yammani,
L.S.Tobacman,
C.Cohen.
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Ref.
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Proc Natl Acad Sci U S A, 2005,
102,
18878-18883.
[DOI no: ]
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PubMed id
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Abstract
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Tropomyosin is a two-chain alpha-helical coiled coil whose periodic interactions
with the F-actin helix are critical for thin filament stabilization and the
regulation of muscle contraction. Here we deduce the mechanical and chemical
basis of these interactions from the 2.3-A-resolution crystal structure of the
middle three of tropomyosin's seven periods. Geometrically specific bends of the
coiled coil, produced by clusters of core alanines, and variable bends about
gaps in the core, produced by isolated alanines, occur along the molecule. The
crystal packing is notable in signifying that the functionally important fifth
period includes an especially favorable protein-binding site, comprising an
unusual apolar patch on the surface together with surrounding charged residues.
Based on these and other results, we have constructed a specific model of the
thin filament, with the N-terminal halves of each period (i.e., the so-called
"alpha zones") of tropomyosin axially aligned with subdomain 3 of each
monomer in F-actin.
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Figure 2.
Fig. 2. Isolated core alanines create holes in the coiled
coil. Shown is a series of space-filling models of consecutive
stretches of the structure, each one viewing the broad face of
the coiled coil. The region from residue 141 to the C terminus
of the fragment is well packed [average gap volume = 30 Å3
per residue pair (see Table 4, which is published as supporting
information on the PNAS web site)] whereas that from residues
113-141 contains significant holes in the core (average gap
volume = 67 Å3 per residue pair). The quality of the
packing between the helices of the coiled coil relates to the
sequential pattern of the sizes of the side chains at the
interface between the helices (see Results and letters in Fig.
3).
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Figure 3.
Fig. 3. Specific bends from alanine staggers at low
coiled-coil radii, and holes and variable bends from isolated
core alanines at high radii. The graph shown is the coiled-coil
radius at the location of each residue of MidTm. The letters on
the graph are the one-letter code of the local core side chain;
the printed size of the letter correlates with the mass of the
side chain, and the color of the graph indicates whether the
 -helices have a local
axial stagger greater than (blue) or less than (red) 1.0
Å. The cartoons depict different bends in the coiled coil
(see Results); -helices are depicted
by straight or curved rectangles, and their coiling around one
another is not shown for clarity. See also Fig. 6 and ref. 22.
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