Orientational information of troponin C within the thin filaments obtained by neutron fiber diffraction

Fujiwara, Satoru; Matsumoto, Fumiko

In striated muscles, contraction is regulated by the thin filament-based proteins, troponin consisting of three subunits (TnC, TnI, and TnT), and tropomyosin. Knowledge of in situ structures of these proteins is indispensable for elucidating this Ca-sensitive regulatory mechanism. We found from neutron scattering experiments that TnC within the thin filaments assumes extended dumbbell-like structures and moves toward the filament axis by binding of Ca. Here, in order to obtain more detailed in situ structural information of TnC, neutron fiber diffraction measurements were performed. Neutron fiber diffraction patterns were obtained from the oriented samples of native thin filaments and the thin filaments containing deuterated TnC in the absence and presence of Ca. Analysis of the meridional reflections due to Tn-complex with aid of model calculation showed that the angle between the thin filament axis and the long axis of TnC was estimated to be 67(7) and 49(17), in the absence and presence of Ca, respectively, suggesting that TnC, which assumes orientations rather perpendicular to the filament axis in the absence of Ca, tilts toward the filament axis and the orientational and positional disorder increases by binding Ca. It also showed that the relative position of the TnC moved by about 22 , and this apparent movement was concomitant with the movements of other Tn-subunits. This implies that by binding Ca, significant structural rearrangements of Tn-subunits occur.