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Unformatted text preview: Each myosin head binds and hydrolyses ATP, using the energy of TAP hydrolysis to walk toward the plus end of an actin filament. The head of the myosin controls motor activity. Myosin VI is the only one that moves towards the minus end. Myosin II is always associated with contractile activity in muscle and nonmuslce cells. It is also required for cytokinesis. Myosin I involve in intracellular organization and the protrusion of actin-rich structures at the cell surface. Myosin V is involved in vesicle and organelle transport while Myosin VII is found in the inner ear in vertebrates and can cause deafness in mice and humans. Since the motor domains of kinesins are essentially identical, how can they move in opposite directions? --> answer seems to lie in the way in which the heads are connected. Conventional kinesin has its motor domain at the protein's N-terminus and moves toward the plus end of the mt. Whereas, a minus-end-directed motor with the motor domain at the C-terminus can walk the opposite direction . Myosin walk: the binding of ATP causes change in shape conformation of the myosin therefore it also leads to the head losing its affinity for the actin filament. Hydrolysis occurs while the myosin head is detached from the filament, causing the head to assume a cocked conformation. ADP and Pi are still attached to the head at this point. Then Pi is released first which triggers the power stroke, and then in the course of the power stroke, the head loses its bound ADP which will then restart the whole cycle. Myosin is tightly bound to the actin filament when there is no ATP. Whereas kinesin binds tightly to the microtubule filament when ATP is present. Kinesin is processive which means they can travel for a long distance without leaving the mt completely. In other words, the kinesin is never released from the mt, there's always at least one motor domain on the microtubule filament. Myosin cannot do that, such as myosin II. This is because skeletal muscle myosin never operates as a single molucule but rather as part of huge array of myosin II molecules. Processivity would actually inhibit biological function, since efficient muscle contraction requires that each myosin head perform its power stroke and then quickly get of the way, to avoid interfering with the actions of the other heads attached to the same actin filament. Myosin loses processivity but gains in speed. ERM-family proteins are used to attach actin filaments to the PM. Regulated unfolding of an ERM-family protein, caused by phosphorylate or binding of PIP2, exposes two binding sites. One for an actin filament and the other is for a transmembrane glycoprotein. Activation of ERM-family proteins can thereby generate and stabilize cell- surface protrusions that form in response to extracellular signals. Focal adhesions, adherens junctions, desmosomes, and hemidesmosomes....
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- Summer '08