Most protein machines are made of complexes of noncovalently bound polypeptides. Characterization of the mechanical properties of these multimolecular structures is also an important challenge for "single-molecule" force spectroscopy. Supramolecular complexes with an integral structure and behavior, such as the long coiled-coil structure of the myosin II tail (Fig. 8.8b), have also been characterized using this technique (Table 8.1; Schwaiger et al. 2002). tte myosins are a large family of motor proteins that move along actin filaments as they hydrolyze ATP. Myosin II is a hexamer formed by four light chains and two heavy chains that have a globular N-terminal motor and a tail that dimerizes into a 150 nm long parallel coiled coil (Fig. 8.8). tte typical force-extension curve of a myosin tail presents a characteristic molecular fingerprint. Ms suggests a mechanism for myosin elasticity in which the folded coiled coil initially extends by entropic elasticity up to 20 pN, and then at 20-25 pN, short coiled-coil segments start to unfold, leading to a very long force plateau (which marks a structural transition of the tail). At the end of the process the completely unfolded coiled coils additionally extend as the force rises rapidly. During the force plateau the coiled coil extends to about 2.5 times its original length. Furthermore, the force-extension profile of relaxation almost exactly follows the trace observed during pulling, indicating that the myosin coiled coil is a true elastic protein structure, able to refold against forces of up to 30 pN in much less than 1 s.
tte comparison of the unfolding/refolding cycles shows that the transition presents little hysteresis, ttus, very little energy is dissipated in the complete cycle as the process is basically in equilibrium, tterefore, evolution seems to have selected this simple coiled-coil fold to confer nearly pure entropie elasticity to the tail of the myosin II machine (Fig. 8.6a, panel 3).
Other attempts have been made to measure supramolecular mechanics in a few other protein systems, but the results are often unclear and must be treated with caution. Future experimentation in such complex systems should ensure that all the appropriate controls are included in order to rule out the spurious interactions present in the system (intermolecular interactions and single-molecule events).
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