Figure 1. (a) The filament array of one sarcomere: the myosin filaments occupy the centre of the sarcomere with actin filaments extending in from either end. In a muscle, many sarcomeres are connected end to end, forming a long structure which shortens to make the muscle contract. (b) The zone of overlap between myosin and actin filaments in insect flight muscle in the absence of nucleotides is visualized by electron microscopy. The prominent cross bridges seen between myosin filaments and actin filaments are each composed of myosin heads. As can be seen, these cross bridges appear to make different angles relative to the filament axes, an observation that helped to first establish the hypothesis that myosin acted as a rowing oar, propelling the actin filaments past the myosin filaments. (c) A kinesin motor is shown pulling a load down a microtubule.

Figure 2. (a) A schematic of the myosin and kinesin molecules. The motor domains of each are shown in red, with the neck regions in blue. The coiled-coil that connects the head regions to the load is shown in cyan. The regions for attachment to the load are shown in green. (b) High resolution structures for the head regions of myosin and ncd. The two structures are shown with the conserved regions surrounding the nucleotide in the same alignment. The binding sites for the polymers on both proteins are on the right-hand side. The motor domains are shown in red, the neck regions in blue, and the nucleotides are shown in cyan. The -helices that help transmit conformational changes from the nucleotide to the neck are shown as ribbons. Drawn using Midas.

Figure 3. (a) The structure of myosin (motor domain red, neck blue, coiled-coil cyan) is shown docked to a polymer of actin (yellow). Two structures are shown for the light chain region, with one representing the end of the power stroke and the other one representing the beginning of the power stroke. (b) The structure of a dimer of ncd (motor domain red, neck region blue, coiled-coil cyan) is shown docked onto a microtubule (yellow). Two dimers of tubulin are shown, with the plus end of the microtubule on the left. As shown both motors move to the right along their polymers. Drawn using Midas.

Figure 4. The kinetic cycles for (a) myosin and (b) kinesin. (a) In state 1 myosin has been released from actin by the tight binding of ATP to the myosin active site. The orientation of the neck is the same in this state as it was at the end of the power stroke in state 4. States 1 and 2 represent a collection of states, which are in relatively rapid equilibrium. In these states, myosin is either dissociated from, or is forming a weak bond with, actin. ATP is hydrolysed by myosin, producing a swing of the light chain domain described above, represented as the transition from state 1 to state 2. This change in the conformation of myosin is thought to represent a reversal of the power stroke. The myosin then rebinds weakly to actin, to form state 3, and the release of phosphate, or isomerizations accompanying the release of phosphate, allow myosin to form a force producing interaction with actin. In the transition from states 3 to 4 the orientation of the neck region returns to 45° to the filaments, which causes the translation of the actin filaments to the left by about 10 nm. This transition is known as the power stroke, which is defined as that part of the cycle where motion, force and mechanical work are produced. The power stroke ends with state 4, when the release of ADP from myosin leads to the tight bond between actin and myosin known as the rigor complex. ATP then binds to this complex, releasing the myosin from actin and leading back to state 1. (b) In state 1, one of the kinesin heads is bound to the microtubule, with the other head detached, in the conformation that immediately follows detachment. In the transition from state 1 to state 2 a conformational change in the neck region of the attached head moves the second detached head towards the plus end of the microtubule for kinesin. The neck region was disordered before this transition, and it binds down to its motor domain with a ‘zipper-like’ motion, moving the unattached head forward as shown. In the transition from state 2 to state 3 this detached head is carried across the 8 nm to the next tubulin site. Thermal fluctuations are responsible for translating the kinesin head across this distance in a process known as directed diffusion. Once at the site, the detached head now attaches to the tubulin. The net effect of this transition is the binding of the detached kinesin head to the adjacent tubulin dimer. In the transition from state 3 to back to state 1 the first head is released from the microtubule and the cycle repeats.