If solutions of actin and myosin are mixed, a great increase in viscosity occurs, due to the formation of a complex called actomyosin. Actomyosin is an ATPase which is activated by magnesium ions. 'Pure' actomyosin (a mixture of purified actin and purified myosin) will split ATP in the absence of calcium ions. However 'natural' actomyosin (an actomyosin-like complex which can be extracted from minced muscle with strong salt solutions, and which also contains tropomyosin and troponin) can only split ATP if there is a low concentration of calcium ions present. In the absence of calcium ions, addition of ATP to a solution of natural actomyosin results in a decrease in viscosity, suggesting that the actin—myosin complex becomes dissociated.
Fig. 10.16. Diagram to show an actin filament moving on a lawn of myosin S1 heads in an in vitro motility assay. The direction of sliding is determined by the polarity of the actin filament. ATP is split in the process. From H. E. Huxley (1990), reproduced with permission of the American Society for Biochemistry & Molecular Biology.
We can use these observations to make plausible suggestions about how actin and myosin interact within the filament array in which they exist in the living muscle. In the resting condition there is an adequate concentration of ATP and a very low concentration of calcium ions, so there is no interaction between the actin and myosin and no ATP splitting. On activation the calcium ion concentration rises and so cross-bridges are formed between the two sets of filaments, ATP is split and sliding occurs.
In recent years we have learnt much more about the myosin motor and its interaction with actin from some remarkable experiments involving in vitro motility assays. These use purified actin and myosin in systems that allow the movement of single filaments to be seen by light microscopy. One arrangement is shown in Fig. 10.16: an actin filament to which a fluorescent dye has been attached is placed on a 'lawn' of myosin S1 heads. In the absence of ATP the actin filament is bound to the S1 heads but there is no movement. On adding ATP the actin moves across the lawn at a speed comparable with the sliding of filaments in whole muscle.
The primary source of the movement is probably a change in shape of the myosin S1 head, brought about by the splitting of ATP, so that the lever arm section swings through about 10 nm and thereby pulls on the S2 link and so on the whole myosin filament (Fig. 10.17). Fine evidence for this 'swinging lever' model comes from some experiments by Spudich and his colleagues (1995) in which S1 mutants with lever arms longer or shorter than usual were used in a motility assay: the velocities of the actin filaments were proportional to the lengths of the lever arm.
Cross-bridge action is thus a cyclical process. Each cross-bridge will attach to the adjacent actin filament, its lever arm will swing so as to pull the actin and myosin filaments past each other, then it will detach from the actin filament. The cross-bridge is then ready to attach to a new site on the actin filament and so repeat the cycle. The energy for each turn of the cycle is provided by the breakdown of one molecule of ATP to ADP and inorganic phosphate.
Actin-binding site ^P_Loop 2
Fig. 10.17. The swinging lever arm model for S1 action. A change in shape of the molecule near to the ATP-binding pocket produces a movement of about 10 nm at the end of the lever arm. This pulls on the S2 link, which is attached to the myosin filament backbone. Redrawn after Spudich (1994), with permission from the author and Nature, Macmillan Magazines Limited.
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