The Calcium Trigger

Setsuro Ebashi and the Troponin Molecular Switch in Tokyo

When pure actin filaments and pure myosin filaments are extracted from rabbit skeletal muscle and mixed in a glass beaker with adenosine triphosphate (ATP), the proteins precipitate into an immediate, irreversible contraction. In 1942 at the University of Szeged, Albert Szent-Györgyi had demonstrated that extracted actomyosin threads contract violently upon contact with ATP. For the next twenty years, muscle biochemists hunted for a soluble enzymatic relaxing factor that they believed forced muscle fibers to let go after a twitch.

In 1963, at the University of Tokyo Faculty of Medicine, pharmacologist Setsuro Ebashi proved that the entire field had the control logic backward. Skeletal muscle is not triggered from a passive resting state by an activating enzyme. Instead, resting actomyosin is held under a continuous mechanical lock that must be sterically lifted before myosin can touch actin. Earlier, in 1955, working in Fritz Lipmann's laboratory and in Tokyo, Ebashi had identified the true identity of the relaxing factor. It consisted of microscopic membrane vesicles—fragments of the sarcoplasmic reticulum—that used an ATP-driven pump (SERCA) to vacuum free calcium ions ($\text{Ca}^{2+}$) out of the cytoplasm.

What remained a puzzle was why highly purified actin and myosin contracted violently even when calcium was completely removed with chelating agents. In 1963, Ebashi isolated the missing regulatory latch—"native tropomyosin"—and showed that it consisted of long rod-like tropomyosin cables paired every 40 nanometers with a globular three-subunit protein complex he named troponin:

• * Troponin T (TnT): The structural bracket that clamps the complex onto the coiled-coil tropomyosin strand running along the groove of the thin actin filament.
• * Troponin I (TnI): The inhibitory tether that binds directly to actin in the resting state, holding tropomyosin directly over the myosin-binding sites so cross-bridges cannot attach.
• * Troponin C (TnC): The dumbbell-shaped calcium sensor containing four EF-hand metal-binding pockets.

In a resting muscle fiber, SERCA pumps pack calcium into the sarcoplasmic reticulum at millimolar concentrations ($[\text{Ca}^{2+}]_{\text{SR}} \approx 2 \times 10^{-3}\text{ M}$), keeping cytoplasmic calcium down near $10^{-7}\text{ M}$ ($0.1\,\mu\text{M}$). At that low baseline, the N-terminal regulatory pockets of Troponin C sit empty. Tropomyosin physically blocks the outer domain of every seven-monomer actin repeat.

When a motor nerve action potential races down the sarcolemma and plunges into the transverse tubules (T-tubules), depolarization twists voltage-sensing dihydropyridine receptors (DHPR) in the T-tubule wall. In skeletal muscle, each DHPR tetrad is mechanically coupled like a rigid plunger to a ryanodine receptor ($\text{RyR1}$) calcium release channel in the adjacent sarcoplasmic reticulum terminal cisterna. Unplugged within two milliseconds, the $\text{RyR1}$ channels dump calcium down a twenty-thousand-fold gradient into the myoplasm:

Cytoplasmic calcium surges one hundred-fold from $10^{-7}\text{ M}$ to $10^{-5}\text{ M}$ ($10\,\mu\text{M}$). Four calcium ions snap into the binding pockets of Troponin C, opening a hydrophobic cleft that yanks the inhibitory arm of Troponin I off the actin surface. That tug rolls the attached tropomyosin cable azimuthally by 25 degrees ($0.8\text{ nanometers}$) deeper into the groove of the actin helix, exposing the myosin binding patches.

Cocked myosin heads carrying bound ADP and inorganic phosphate ($\text{P}_i$) immediately latch onto the exposed actin sites. Phosphate release triggers a 70-degree rotation of the myosin lever arm, pulling the thin filament 10 nanometers toward the center of the sarcomere with 5 piconewtons of force. The instant nerve stimulation ceases, SERCA pumps strip calcium back down to $10^{-7}\text{ M}$ within thirty milliseconds. Troponin I snaps tropomyosin back over the binding sites, and the muscle relaxes.