Two-headed binding of a processive myosin to F-actin

Myosins are motor proteins in cells. They move along actin by changing shap e after making stereospecific interactions with the actin subunits(1). As t hese are arranged helically, a succession of steps will follow a helical pa th. However, if the myosin heads are long enough to span the actin helical repeat (similar to 36 nm), linear motion is possible. Muscle myosin (myosin II) heads are about 16 nm long(2), which is insufficient to span the repea t(3). Myosin V, however, has heads of about 31 nm that could span 36 nm (re fs 4, 5) and thus allow single two-headed molecules to transport cargo by w alking straight 5. Here we use electron microscopy to show that while worki ng, myosin V spans the helical repeat. The heads are mostly 13 actin subuni ts apart, with values of 11 or 15 also found. Typically the structure is po lar and one head is curved, the other straighter. Single particle processin g reveals the polarity of the underlying actin filament, showing that the c urved head is the leading one. The shape of the leading head may correspond to the beginning of the working stroke of the motor. We also observe molec ules attached by one head in this conformation.