The 12-site CG model contains four main sites, representing the cores of the subdomains of actin, and eight minor sites, representing regions that are highly conformationally mobile, functionally important, or solvent exposed. Mapping from the all-atom (a) to CG (b) representation is shown, along with the two collective variables that are biased during US: the dihedral twist and the cleft width. To see this figure in color, go online.