Conformational Changes in Reconstituted Skeletal Muscle Thin Filaments Observed by Fluorescence Spectroscopy

Abstract

The cyclic interaction of myosin and actin coupled ATP hydrolysis generates the mechanical force of muscle contraction. During this process, the system passes through several steps. One of these is thought to be identical to the stable rigor complex formed by myosin and actin in the absence of ATP. This cyclic interaction is regulated by changes in tropomyosin (Tm) and troponin (Tn) located on the actin filament in response alterations in intracellular Ca2+ concentration (). Tm contains seven quasi-equivalent regions, each of which has a pair of putative actin-binding motifs. Tn comprises three different subunits, TnC, TnI, and TnT. TnI alone inhibits actomyosin ATPase activity which is removed on adding TnC, irrespective of Ca2+ concentration. TnT is required for full Ca2+-regulation of the ATPase activity of a reconstituted system (). The globular part of the Tn complex (TnC, TnI and the C-terminal region of TnT) is located on residues 150–180 of Tm (), and the elongated part, composed of the N-terminal region, covers an extensive region of the C-terminal half of Tm. The binding of Ca2+ to TnC induces a series of conformational changes in the other components of the thin filament. This allows the effective association of myosin with actin, thus producing force. Although numerous studies have characterized the interaction between these thin filament proteins, the molecular mechanism whereby the Ca2+-trigger is propagated from TnC to the rest of the thin filament is still not well understood.