PDBsum entry 1vdi

Contractile protein

PDB id: 1vdi

Contents

Protein chain

52 a.a. *

* Residue conservation analysis

PDB id:

1vdi

Name:

Contractile protein

Title:

Solution structure of actin-binding domain of troponin in ca2+-free state

Structure:

Troponin i, fast skeletal muscle. Chain: a. Fragment: residues 131-182. Synonym: troponin i, fast-twitch isoform. Engineered: yes

Source:

Gallus gallus. Chicken. Organism_taxid: 9031. Expressed in: escherichia coli. Expression_system_taxid: 562.

NMR struc:

20 models

Authors:

K.Murakami,F.Yumoto,S.Ohki,T.Yasunaga,M.Tanokura,T.Wakabayashi

Key ref:

K.Murakami et al. (2005). Structural basis for Ca2+-regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin. J Mol Biol, 352, 178-201. PubMed id: 16061251 DOI: 10.1016/j.jmb.2005.06.067

Date:

22-Mar-04 Release date: 06-Sep-05

Protein chain

P68246  (TNNI2_CHICK) -  Troponin I, fast skeletal muscle from Gallus gallus

Seq: 183 a.a.

Struc: 52 a.a.

DOI no: 10.1016/j.jmb.2005.06.067

J Mol Biol 352:178-201 (2005)

PubMed id: 16061251

Structural basis for Ca2+-regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin.

K.Murakami, F.Yumoto, S.Y.Ohki, T.Yasunaga, M.Tanokura, T.Wakabayashi.

ABSTRACT

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.

Selected figure(s)

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.

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.

The above figures are reprinted by permission from Elsevier: J Mol Biol (2005, 352, 178-201) copyright 2005.

Figures were selected by an automated process.