Topography of actin with tropomyosin. (A) The top-down view and (B) side profile view of Tm (the black coils) in the blocked state on a single actin monomer. The red, white, and blue scale is elevation; therefore, the peaks that are creating a channel for Tm are red, and the lower portion is blue. The peaks appear to trap the Tm in a valley. The angled and longitudinal transitions move along the valley, while the azimuthal transition has to move over the left ridge that results in an increased initial energy barrier.