Locating muscle proteins: scientists bring the basis of muscle movement into sharper focus
21 Jul 2012
Muscle contraction and many other movement processes are controlled by the interplay between myosin and actin filaments. Two further proteins, tropomyosin and troponin, regulate how myosin binds to actin. While theoretical models have in fact described exactly how these muscle proteins interact, this interaction has never previously been observed in detail.
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| Schematic representation of the sarcomere. The z line is depicted in black, myosin in green/grey, actin in red and tropomyosin in blue © MPI of Molecular Plant Physiology |
Stefan Raunser and Elmar Behrmann from the Max Planck Institute of Molecular Physiology in Dortmund have now managed to image the actin-myosin-tropomyosin complex with an unprecedented accuracy of 0.8 nanometres, which amounts to a resolution of less than one-millionth of a millimetre. This has, for the first time, made it possible to correctly identify the location of proteins within the complex and to analyse the processes involved in muscle contraction.
These findings could help determine the impact of genetically determined modifications to the actin-myosin-tropomyosin complex in certain types of hereditary heart disease.
The basic functional unit of a muscle, known as the sarcomere, consists of actin, myosin and tropomyosin proteins. If a muscle is to be able to contract, the myosin must slide along filamentous actin molecules.
Working together with troponin, tropomyosin regulates muscle contraction by controlling when myosin binds to actin. In the resting state, tropomyosin and troponin block the binding site for myosin on the actin filament.
At this point, the myosin head is at a 90-degree position. Only after an influx of calcium, which docks onto the regulating proteins, is the binding site on the actin filament exposed. The myosin head docks onto this site, changes its conformation and bends in an articulated manner, thereby pulling the actin along with it. As the filaments slide over one another, the sarcomere shortens and the muscle thus contracts.
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