Note Feb 4, 2013 recitation and lecture

F actin can also be organized into tight bundles anti

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F-actin can also be organized into tight bundles, anti-parallel bundles (for contraction), or randomly linked as a gel (loose, fills space, resists compression). The cross-linking proteins used for each type of organized structure have two F- actin binding sites positioned near each other or at a distance, depending on the structure being assembled. Severing and capping proteins regulate filament dynamics The rate of filament assembly (growth) is dictated by the availability of ends, and the ratio of plus-ends to minus-ends. Katanin severs microtubules, gelsolin severs F-actin.
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Cell Biology (020.306) CYTOSKELETON Weeks 2 and 3 6 Motor proteins are another class of filament-binding protein. These are protein complexes that use ATP hydrolysis to trigger a conformational change in the motor which leads to force generation, i.e., movement of the motor relative to the filament (“stepping” or “walking”). F-actin-based motors: MYOSINS (a multi-gene family) All myosins possess a similar ATP-binding "head". This head is connected to a tail that can take many forms. Myosin II and muscle: Myosin II is the protein responsible for muscle contraction but is also found in other cells. Your body contains 3 types of muscles: cardiac (heart), smooth (involuntary) and skeletal (voluntary). Muscle myosin assembles into very large antiparallel bundles, called thick filaments , that have many motor heads at each end. These cause muscle contraction by walking along muscle actin filaments (thin filaments) to pull the two ends of the contractile unit (sarcomere) together. This is referred to as the sliding filament model of muscle contraction, because the thin and thick filaments slide past each other as the muscle contracts. In skeletal muscle, contraction is triggered by Ca 2+ , which is released into the cytoplasm of the muscle cell when the cell is stimulated by a motor neuron. Muscle contains a number of actin-binding proteins that stabilize (tropomyosin) and crosslink ( α -actinin) the actin filaments, link them to the surrounding membrane (CapZ) and control the access of the myosin head to the actin filaments (troponin and tropomyosin). The myosin crossbridge cycle: Coupling of ATP hydrolysis and product release to actin-myosin binding, stepping, and release In the absence of ATP (after release of the hydrolysis products, ADP and Pi) the myosin is tightly bound to actin. This state is called rigor. ATP binding causes the myosin head to release from the actin filament. ATP hydrolysis causes the head to undergo a conformational change, which causes it to become "cocked" and then bind a new actin subunit. As the
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Cell Biology (020.306) CYTOSKELETON Weeks 2 and 3 7 myosin head releases the reaction products (first Pi, then ADP) it undergoes a second conformational change in which it pulls on the actin filament, causing it to slide along. This is referred to as the "power stroke".
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