BIS104 Note11

BIS104 Note11 - Lecture 11 Bio Sci 104 Molecular motor I....

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Lecture 11 1 Bio Sci 104 Winter 2010 Molecular motor I. Motor proteins that move along cytoskeleton The cytoskeleton is the source of all cell movements, and of movements of organelles and particles within a cell. In order for the cytoskeleton to produce force, small molecular motors must act on the cytoskeleton to move things. A. Myosins Molecular motors that work on actin filament a. large family of muscle and non-muscle myosins b. have globular head (or heads) and long α -helical tails c. hydrolyze ATP to move along actin filaments from - to + end Myosins hydrolyze ATP to produce a conformational change, called the power stroke, which bends the myosin, moving the associated filament. The myosins form a large family of actin filament motors. At least 18 subfamilies of myosins function in various different cell types. Here we focus on Myosin II (the first identified myosin), a two-headed myosin consisting of two heavy chains and 4 associated light chains. Heavy chains contain a motor domain head and a long coiled rod domain that dimerizes into a coiled-coil. Myosin II bundles end to end to form myosin thick filaments . These thick filaments then align with actin thin filaments in muscle cells to produce the contractile forces. B. Kinesins and Dyneins Molecular motors also utilize microtubules as tracks for organelle movement. Molecular motors generally have a motor "head " and a cargo-binding "tail ." Two classes of motors exist for movement along microtubules. Like myosins, these motors use ATP to produce conformational changes in their necks to move along microtubules. 1. Kinesins - fam i ly of mo lecu lar motors that use ATP to move cargo prote ins; typically the majority of kinesins move from - to + ends of microtubules. Cargos can be organelles , secretory vesicles (e.g. neurotransmitters in nerve cells), or the motor can be involved in chromosome movements on spindle during cell division. (FYI: Conventional kinesin was first described in extracts from the giant axon of a squid. Kinesin is the founding member of a family of kinesin related motors. Some have their motor domains at the N terminus like conventional kinesin, but others have the motor domain at the C terminus, and yet others in the middle.) Outside the motor domain, kinesins are not conserved, although they usually contain a coiled-coil dimerizing domain. Conventional kinesin
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Lecture 11 2 proteins from + to - ends of microtubules, Also involved in aspects of organelle transport and mitosis. All characterized dyneins move towards the minus end of MTs (towards centrosomes). The dyneins are a relatively small family. Many organisms have just two dyneins, a cytoplasmic form and a axonemal form (in flagella and cilia). Since there usually is only one cytoplasmic dynein, associated proteins, including the
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BIS104 Note11 - Lecture 11 Bio Sci 104 Molecular motor I....

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