Arrangement of myosin heads between two parallel filaments. (a and b) Cross-sections of myosin upper and lower heads bound between two parallel cTFs (left, red profiles) and F-actin (right, blue profiles), scale bars, 30 nm. (c and d) The measured distance between two parallel cTFs (left) and F-actin (right) averaged in d. The difference in distance between non-regulated F-actin and regulated cTFs (P = 0.001, unpaired t test) did not affect the HMM binding and displacement analysis (Fig. 1 f and Fig. 2 c). (e and f) Two types of HMM binding between parallel cTFs or actin filaments were observed; the upper and lower heads bound towards the similar direction (right HS-AFM images in e and f) or the upper and lower heads bound towards the opposite directions (left HS-AFM images in e and f). The captured snapshots in left (e) and left (f) panels indicate the rare situation occurred due to head displacement on the filament in the presence of ATP. HMM heads between two actin filaments highlighted by red colors, S2 region of the HMM molecules highlighted by green color. The corresponding polarity of the filaments is shown. Scale bars, 30 nm.