Figure 4

3D reconstruction of thin filaments demonstrates MyBP-C N-termini shift of tropomyosin in the absence of Ca²⁺. (A) Reconstruction of reconstituted thin filament (F-actin, tropomyosin, troponin). Actin atomic structure (yellow ribbon) has been fitted into the reconstruction (grey envelope). Red and green helices represent Tm in the known blocked and closed positions on the thin filament, respectively. In this control filament, at low Ca²⁺, Tm occupies the blocked position (grey cylinder enclosing red Tm helix). Tn is averaged out as it does not follow the helical symmetry of actin used to carry out the reconstructions. The addition of (B) ssC1C2, (C) fsC1C2, and (D) C0C2 causes a shift in Tm azimuth towards the closed position, with C0C2 causing the largest shift and fsC1C2 the smallest. These variable shifts are further revealed when each decorated reconstruction is superimposed on the actin:tropomyosin:troponin control, both in surface view (E–H) and in cross-sectional views of the reconstructions (I–O) (cf. ref.¹⁴). Actin subdomains 1–4 are marked in I. Red arrows indicate Tm in blocked position in low Ca²⁺ control filament; green arrows show shifted position of Tm in low Ca²⁺ decorated filaments.