Structural role of tropomyosin in muscle regulation: Analysis of the X-ray diffraction patterns from relaxed and contracting muscles

Myosin-binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism

“…At low Ca 2+ , Tm lies on SD1, where it sterically blocks the binding of myosin to actin (the "blocked" position); the consequent inhibition of actin-myosin interaction leads to muscle relaxation. On activation, Tn binds Ca 2+ , causing Tm to move onto actin SD3 (the "closed" position), exposing myosin binding sites on SD1 and initiating crossbridge cycling and contraction (29)(30)(31)(32)(33)(34)(35). When a model of Tm in its low Ca 2+ (blocked) state is positioned on the reconstruction of F-actin decorated with C0C3, it appears to clash with cMyBP-C's C0 and C1 domains, suggesting that cMyBP-C and Tm might compete for binding to SD1 in the relaxed thin filament (25,27,28).…”

Structural changes in troponin in response to Ca 2+ and myosin binding to thin filaments during activation of skeletal muscle

“…In resting muscle tropomyosin covers the myosin binding sites on actin. On activation, Ca 2ϩ binding to troponin leads to an azimuthal motion of tropomyosin around the actin filament that uncovers the myosin binding sites, permitting the actin-myosin interaction that drives contraction (5)(6)(7).…”

Localization of [alpha]-actinin and tropomyosin in muscle and nonmuscle systems

“…The other two strands are the approaching and departing legs of a looping actin strand from the opposite side (see Figure 2). Kelly (1967) is also able to construct a second model, the looping tropomyosin strand model, by in corporating the fact that tropomyosin is known to be part of the Ifilament (Pepe, 1966;Ebashi and Kodama, 1966) and that two strands of tropomyosin lie within the grooves of the right-handed actin doublehelical strand (Parry and Squire, 1973). The looping tropomyosin strand model assumes that actin filaments terminate at the I-Z boundary; this allows for an actin filament length of 1 ym as opposed to the 2 ym fila ment length required for the looping actin strand model and is therefore more consistent with the fact that actin filaments isolated by homogenization of muscle fibers generally measure less than 1 ym in length (Allen and Tepe, 1565).…”