Large Step with Short Leg
Motor proteins transport cargos from one place in living cells to another, for example cell components along the long axons of nerve cells. A motor protein, such as myosin VI, has to "walk" or "run" along the cellular highway of actin filaments to perform the transport. In the case of myosin VI, snapshots from crystallography revealed "legs" are too short to explain the step size taken. Computational and experimental biophysicists have now solved the mystery of how myosin VI dimers realize their large step size despite their short legs. The investigation based on the program NAMD and reported recently demonstrates that the answer lies in the flexibility of the legs made by domains. Myosin VI is able to extend each leg made through a short bundle of up-down-up connected segments, so-called alpha-helices, to stretched-out down-down-down connected segments, thereby tripling the bundle's length. In the telescoping process, myosin VI also gets help from its well-known binding partners, namely calmodulins. The calmodulins not only direct the telescoping of the protein legs, but apparently also strengthen the extended legs by binding to it. Together with an earlier study of the "neck" region of the molecule (see December 2010 highlight on Opposites Attract in a Motor Protein), the scientists have established how walking myosin VI achieves its wide stride. More information can be found on our motor protein website.