Troponin, tripomyosin, and actin interactions in the Ca²⁺ion regulation of muscle contraction

Crystal structure of troponin C in complex with troponin I fragment at 2.3-Å resolution

“…Therefore, in the past years many biochemical and biophysical studies were focused on the inhibitory segment (residues 96-116) of TnI (5, 25). The inhibitory segment was shown to bind to either actin-tropomyosin or TnC, thus switching on and off the acto-myosin ATPase activity (6)(7)(8). On the other hand, the conformational changes between ''closed'' and ''open'' forms of TnC regulatory N-lobe induced by Ca 2ϩ binding also entail the regulation of muscle contraction.…”

A 1H NMR study of a ternary peptide complex that mimics the interaction between troponin C and troponin I

“…through interactions with both actin and tropomyosin-actin (Potter & Gergely, 1974;Hitchcock, 1975), and TnT binds to tropomyosin. The inhibition of the actomyosin ATPase is neutralized when calcium-saturated TnC forms a complex with TnI (Perry et al, 1972;Weeks & Perry, 1978;Chong et al, 1983).…”

Do tropomyosin and myosin compete for actin sites in the presence of calcium?

“…Fig.1 shows that the inhibition by pure tropomyosin is very similar to that caused by calciumsaturated troponin-tropomyosin suggesting that after calcium saturation troponin no longer exerts as significant effect on the actin filament. This is not surprising since calcium-saturated troponin no longer binds to actin [7]. However, the formation of rigor complexes on actin Can this tropomyosin inhibition be attributed to a com~titio~ between tropomyo~n and myosin for the same actin sites?…”

“…Also there is good evidence that, in the absence of calcium, troponin (throu~ its subunit troponin I which binds to actin) holds tropomyosin at the periphery. Calcium saturation of troponin C initiates a series of conformational changes [6] that result in the dissociation of troponin I from actin [7] and allow tropomyosin to move to its natural binding sites at or near the groove. ~thou~ movement of tropomyos~ is well established it is not clear that tropomyosin and myosin actually compete for the same binding sites on the actin filament.…”