Troponin and its interactions with tropomyosin

Flicker, Paula; Phillips, George N.; Cohen, Carolyn

doi:10.1016/0022-2836(82)90540-x

Cited by 178 publications

(122 citation statements)

References 22 publications

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“…The exact stagger was determined as part of the averaging procedure. Troponin has been described as a "comma-shaped" molecule (7,43). This appearance is similar to the shape of the two regions of density we see in the reconstruction.…”

Section: F-actin Fitting In a Helicalsupporting

confidence: 78%

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Structure and Orientation of Troponin in the Thin Filament

Paul

Morris

Kensler

et al. 2009

Journal of Biological Chemistry

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The troponin complex on the thin filament plays a crucial role in the regulation of muscle contraction. However, the precise location of troponin relative to actin and tropomyosin remains uncertain. We have developed a method of reconstructing thin filaments using single particle analysis that does not impose the helical symmetry of actin and is independent of a starting model. We present a single particle three-dimensional reconstruction of the thin filament. Atomic models of the F-actin filament were fitted into the electron density maps and troponin and tropomyosin located. The structure provides evidence that the globular head region of troponin labels the two strands of actin with a 27.5-Å axial stagger. The density attributed to troponin appears tapered with the widest point toward the barbed end. This leads us to interpret the polarity of the troponin complex in the thin filament as reversed with respect to the widely accepted model.Regulation of actin filament function is a fundamental biological process with implications ranging from cell migration to muscle contraction. Skeletal and cardiac muscle thin filaments consist of actin and the regulatory proteins troponin and tropomyosin. Contraction is initiated by release of Ca 2ϩ into the sarcomere and the consequent binding of Ca 2ϩ to regulatory sites on troponin. Troponin is believed to undergo a conformational change leading to an azimuthal movement of tropomyosin, which allows myosin heads to interact with actin, hydrolyze ATP, and generate force. The molecular basis by which troponin acts to regulate muscle contraction is only partly understood. It is essential that the structure of troponin in the thin filament at high and low Ca 2ϩ is determined to properly understand the mechanism of regulation.The basic structure of the thin filament was described by Ebashi in 1972 (1). In this structure each tropomyosin molecule covers seven actin monomers, and there is a 27.5-Å stagger between troponin molecules. The 7-Å tropomyosin structure (2), the atomic model of F-actin (3), and the troponin "core domain" (4) have recently been used to generate atomic models of the thin filament in low and high Ca 2ϩ states (5). While the position of troponin in these models was constrained by known distance measurements between filament components, the exact arrangement of the complex on the filament has not been determined a priori. Although recently published crystal structures of partial troponin complexes (4, 6) have provided valuable insights into the arrangement of the globular head or core domain, the complex in its entirety has not been crystallized.Troponin is believed to consist of a globular core domain with an extended tail (7). The globular core contains the Ca 2ϩ -binding subunit (TnC), 2 the inhibitory subunit (TnI), and the C-terminal part (residues 156 -262) of the tropomyosin-binding subunit (TnT). The extended tail consists of the N-terminal part of TnT (residues 1-155). A structural rearrangement associated with Ca 2ϩ dissociation from the troponin cor...

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Section: F-actin Fitting In a Helicalsupporting

confidence: 78%

“…Troponin is believed to consist of a globular core domain with an extended tail (7). The globular core contains the Ca 2ϩ -binding subunit (TnC), 2 the inhibitory subunit (TnI), and the C-terminal part (residues 156 -262) of the tropomyosin-binding subunit (TnT).…”

mentioning

confidence: 99%

Structure and Orientation of Troponin in the Thin Filament

Paul

Morris

Kensler

et al. 2009

Journal of Biological Chemistry

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“…Electron micrographs and crystallographic studies of tropomyosin͞troponin complexes showed troponin to consist of a globular head and an elongated tail (51,52). The globular portion consists of all of the three subunits, and the tail is comprised of the C-terminal part of TnT.…”

Section: Models Of Binary Complexmentioning

confidence: 99%

Structure of the inhibitory region of troponin by site directed spin labeling electron paramagnetic resonance

Brown

Sale

Hills

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

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Site-directed spin labeling EPR (SDSL-EPRtroponin I ͉ spin labels ͉ Fourier transform electron paramagnetic resonance ͉ DEER ͉ dipolar R egulation of striated muscle contraction is associated with Ca 2ϩ -dependent structural transitions in the muscle thin filament, which is composed of the troponin complex, tropomyosin, and actin. Troponin is composed of three components: TnC, which binds Ca 2ϩ ; TnI, which inhibits actomyosin activity; and TnT, which anchors TnC and TnI to tropomyosin. Muscle contraction is initiated by the binding of Ca 2ϩ to the N-lobe regulatory sites of TnC. The N lobe then undergoes a structural transition from a closed to an open form, which then facilitates the release of the inhibitory region of TnI from actin, binding to TnC and stimulation of the actomyosin ATPase. The mechanism of this signaling pathway is still tentative (for review, see ref. 1).Crystal and NMR structures are available for TnC and several complexes of TnC with TnI fragments. The crystal structure of TnC reveals a dumbbell-shaped protein consisting of two globular domains, joined by a 22-residue central ␣-helix (2-4). Each domain contains a hydrophobic cleft and two helix-loop-helix EF-hand metal binding motifs; two high-affinity Ca 2ϩ ͞Mg 2ϩ sites in the C-terminal domain (sites III and IV) and two low-affinity Ca 2ϩ specific sites in the N-terminal domain (sites I and II). In skeletal TnC, sites III and IV are permanently occupied by Mg 2ϩ and facilitate the structural binding of TnC to the contractile apparatus (5). Binding of Ca 2ϩ to sites I and II is the physiological trigger for muscle contraction. In cardiac TnC, binding site I is inactive and Ca 2ϩ binding to site II does not induce as large a structural change as observed in skeletal TnC (6). TnI is capable of inhibiting actomyosin ATPase in the absence of other subunits, but Ca 2ϩ -dependent regulation requires TnC, TnT, and tropomyosin. The inhibitory region (skTnI 96-117) alone can fully inhibit actomyosin ATPase activity (7) possibly by binding either to actin or to TnC in the ''on'' or ''off'' states (8-10). The corresponding residues for the cardiac inhibitory region are 129-150 because of a unique Ϸ32 residue N-terminal extension of cTnI.Structural information for the intact troponin complex is limited to low-resolution neutron diffraction and electron microscopy studies (11-13), though several high-resolution structures of TnC with bound TnI peptides are available. Two computational models have been recently proposed for the binary complex of TnC and TnI (14,15). Both models have TnI and TnC in an antiparallel arrangement with multiple interaction sites between the two subunits. NMR, crystallography, and neutron scattering was used by Tung et al. (15) to develop a computational model of the binary complex in which TnI winds around TnC in either a left-handed manner (model ''L'') or a right-handed manner (model ''R''). In both structures, the inhibitory region of TnI is modeled as a flexible ␤-hairpin in close proximity to the central helix of TnC. ...

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“…Several pertinent observations support this interpretation. Electron microscopic images of troponin-tropomyosin polymers show that the globular head region of troponin is often distinctly separated from the tropomyosin filament (Flicker et al, 1982). Co-crystals of tropomyosin and troponin show that troponin is most ordered near the carboxyl terminus of tropomyosin and less ordered near tropomyosin residues 190 -235 (White et al, 1987;White, 1988).…”

Section: Effect Of Tnt Truncation On Troponin Binding To Tropomyosin-mentioning

confidence: 99%

“…In this widely held scheme of thin filament function, troponin remains anchored to the thin filament by relatively Ca 2ϩ -insensitive binding of the aminoterminal region of TnT to the carboxyl-terminal region of tropomyosin (Pato, et al, 1981;Mak and Smillie, 1981;Tanokura, et al, 1983;Cho and Hitchcock-DeGregori, 1990;Ishii and Lehrer, 1991). Indeed, rotary-shadowed electron micrographs of troponin and of TnT (Flicker, et al, 1982) show a highly extended molecule with a narrow tail mostly attributable to the amino-terminal portion of TnT (White et al, 1987) and a distinctly more globular head region that is likely to include portions of all three subunits.…”

mentioning

confidence: 99%