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PDBsum entry 1vdi
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Contractile protein
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PDB id
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1vdi
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References listed in PDB file
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Key reference
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Title
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Structural basis for ca2+-Regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin.
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Authors
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K.Murakami,
F.Yumoto,
S.Y.Ohki,
T.Yasunaga,
M.Tanokura,
T.Wakabayashi.
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Ref.
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J Mol Biol, 2005,
352,
178-201.
[DOI no: ]
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PubMed id
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Abstract
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Troponin and tropomyosin on actin filaments constitute a Ca2+-sensitive switch
that regulates the contraction of vertebrate striated muscle through a series of
conformational changes within the actin-based thin filament. Troponin consists
of three subunits: an inhibitory subunit (TnI), a Ca2+-binding subunit (TnC),
and a tropomyosin-binding subunit (TnT). Ca2+-binding to TnC is believed to
weaken interactions between troponin and actin, and triggers a large
conformational change of the troponin complex. However, the atomic details of
the actin-binding sites of troponin have not been determined. Ternary troponin
complexes have been reconstituted from recombinant chicken skeletal TnI, TnC,
and TnT2 (the C-terminal region of TnT), among which only TnI was uniformly
labelled with 15N and/or 13C. By applying NMR spectroscopy, the solution
structures of a "mobile" actin-binding domain (approximately 6.1 kDa)
in the troponin ternary complex (approximately 52 kDa) were determined. The
mobile domain appears to tumble independently of the core domain of troponin.
Ca2+-induced changes in the chemical shift and line shape suggested that its
tumbling was more restricted at high Ca2+ concentrations. The atomic details of
interactions between actin and the mobile domain of troponin were defined by
docking the mobile domain into the cryo-electron microscopy (cryo-EM) density
map of thin filament at low [Ca2+]. This allowed the determination of the 3D
position of residue 133 of TnI, which has been an important landmark to
incorporate the available information. This enabled unique docking of the entire
globular head region of troponin into the thin filament cryo-EM map at a low
Ca2+ concentration. The resultant atomic model suggests that troponin interacted
electrostatically with actin and caused the shift of tropomyosin to achieve
muscle relaxation. An important feature is that the coiled-coil region of
troponin pushed tropomyosin at a low Ca2+ concentration. Moreover, the
relationship between myosin and the mobile domain on actin filaments suggests
that the latter works as a fail-safe latch.
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Figure 7.
Figure 7. Docking of the mobile domain into the 3D cryo-EM
electron density map of actin-tropomyosin-troponin.39 (a) The
mobile domain was docked into the region corresponding to the
troponin arm in the troponin-tropomyosin part (grey contour
lines) of actin-tropomyosin-troponin reconstructed from cryo-EM
images at low [Ca^2+].39 The mobile domain (a4, light green;
other parts, yellow) is shown in ribbon format. Actin95 is shown
in wire format and residues Asp24-Asp25 (red), N terminus (light
pink), and residues Ile345-Leu349 (pinkish magenta) in the
C-terminal half of a hydrophobic helix are coloured. (b) Rotated
by 40° with respect to (a) to show the relationship between
the mobile domain and cyan actin. (c) Solid model of three actin
monomers showing their surface electrostatic potential and the
ribbon model of the mobile domain to show that major
interactions were electrostatic. The interacting residues of the
mobile domain and actin are shown in ball-and-stick model and
labelled with the same colour as in (a) and (b). The rotation
angle is the same as in (a). Amino acid residues participating
in the interaction between the mobile domain and actin are
summarized in Figure 2(b) and Table 2.
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Figure 9.
Figure 9. Docking the mobile domain and the core domain of
troponin into the 3D cryo-EM map of actin-tropomyosin-troponin39
at low Ca^2+. (a) Densities due to troponin-tropomyosin39 are
shown with contour lines (grey). The N domain of TnC in the core
domain of troponin12 (PDB code 1J1D) was replaced with the apo N
domain of TnC91 (PDB code 5TNC). The amino acid sequence of the
replaced part is described in Materials and Methods. The mobile
domain (a4, light green; other parts, yellow), TnI (blue), TnT
(orange), TnC (red), and tropomyosin (dark green) are shown in
ribbon format. Calcium ions bound to high affinity sites in the
C domain of TnC are shown as spheres. Actin95 is shown in wire
format and residues Asp24-Asp25 (red), N terminus (light pink),
and residues Ileu345-Leu349 (pinkish magenta) are coloured. The
rotation angle is the same as that in Figure 7 and Figure 8
Rotated by 40° and 100° with respect to (a),
respectively. (d) General relation of actin with troponin at low
[Ca^2+]. The colour scheme is the same as in (a)-(c). Dotted
lines indicate actin-troponin interactions. Continuous lines
indicate interactions among troponin subunits. N-terminal
region, C-terminal region, and the DNase I loop of actin are
hatched in red, dark grey (back side of cyan actin), and blue,
respectively.
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(2005,
352,
178-201)
copyright 2005.
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