Thin filament structure and activation

Skeletal muscle contraction is triggered by the release of calcium ions from the sarcoplasmic reticulum of the myofibrils. Troponin and tropomyosin in the thin (actin-containing) filaments act together as the transducer of this signal. Evidence from reconstruction of electron images of negatively stained samples and from x-ray diffraction studies of intact myofibrils (e.g. 2) have led to the steric blocking model of regulation by the tropomyosin troponin complex. In this model, tropomyosin is believed to occupy a defined location on the myosin binding site in the absence of calcium. When calcium binds to the regulatory complex, tropomyosin is thought to change its location on actin, moving to occupy a second site adjacent to the myosin binding site. Time resolved x-ray diffraction studies on muscle during activation show that movement of tropomyosin precedes crossbridge attachment by 12-17ms, suggesting that it is a prerequisite for myosin binding. In contrast, biochemical experiments suggest that tropomyosin may not affect the ability of the myosin head to bind to actin but rather may interfere with a subsequent kinetic step in the crossbridge cycle.