Three-dimensional image analysis of the complex of thin filaments and myosin molecules from skeletal muscle. V. Assignment of actin in the actin-tropomyosin-myosin subfragment-1 complex.

To assign the actin molecule in the three-dimensional image of the actin-tropomyosin-myosin subfragment-1 (actin-TM-S1) complex, the three-dimensional image of the actin-tropomyosin complex was correlated to that of actin-TM-S1. To assess the similarity of two structures in a quantitative manner, we used a normalized cross-correlation function ("similarity function"). The calculation of similarity indicated that domain A and domain B defined in (1, 2) correspond to actin-tropomyosin. This assignment indicates that one S1 molecule strongly interacts with only one actin molecule, but at least two regions of S1 contribute to the binding. Comparison of the reconstituted models of thin filaments with those of decorated thin filaments suggested a change in the shape of the actin molecule.