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Download Lecture 17 Outline Cell Motility: Encompasses both changes in cell
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Transcript
Lecture 17 Outline Cell Motility: Encompasses both changes in cell location and more limited movement of parts of the cell. Requires interactions of cytoskeleton with motor proteins, utilizes ATP Motor Proteins: Bind and hydrolyze ATP- Results in Conformational Change within the Motor Protein. Unidirectional movement involves coupling of mechanical events with chemical events. Bind Polarized Filaments Function by: 1. Movement along Tracks ( i.e. Movement of Mitochondria) 2. Cause Filaments to Slide against One Another (i.e. Muscle Contraction) Different Motor Proteins Differ in: The Type of Filament They Associate (Actin or Microtubule) The Direction of Movement (Plus End or Minus End Directed), The “Cargo” they carry Microtubule Based Movement Motors: Kinesin ( Plus end directed) and Dynein (Minus end directed motors) Kinesin- plus End Directed Away from Centrosome. Two Globular heads- bind / hydrolyze ATP and bind MT. Middle Flexible domain, mediates dimerization, Tail for binding cargo- Specific. Highly processive. Roles in Axonal transport, cell polarization, Arrangement of ER. Dynein- Processive -Fastest Motor Protein, Large Motor Protein. Similar Two globular heads bind ATP and MT. Role in Motion of Cilia and Flagella, Localize Golgi. Mechanism of Dynein walking along MT.1) Leading Head Attaches to a beta Tubulin (No ATP). Now Leading Head Binds ATP Trailing Head Propelled Forward Past Leading Head. ADP Release From New Leading Head and Hydrolysis of ATP by New Lagging Head. Cycle Can Begin Again- has Moved One Step. Roles of Kinesin and Dynein: Cell Polarization is a Reflection of the Polarized System of Microtubules in the Cell Interior. Movement along MT tracks takes different cargos- vesicles for release from axon. Requires processive Kinesin . Microtubule Motor Proteins Help Arrange Membrane Enclosed Organelles in Eukaryotes- Inhibition of MT polymerization- ER ends up by centrosome and Golgi falls apart. Association of motor proteins with proteins on outside of vesicle membrane allows for interactions. Cilia and Flagella-unique 9+2 arrangement of MT. MTOC is basal body at base of cilia or flagella. Undulating movement of flagella and rowing oar like movement of cilia different but mechanism same. Key is axonemal dynein that can bind MT at head and tail. Cross bridges between the neighboring tubule pairs ( via Nexin protein) allows movement of ciliary dyneins to not cause sliding of one filament over other, instead, bending of cilia or flagella. Microfilament based Movement Motors: Myosins Superfamily of myosin proteins- globular head binds ATP and actin. Myosin II (conventional myosin) involved in contraction of skeletal muscle. Myosin I – single head- nonprocessive – movement of cargo, or associate with cortical actin to influence cell shape. Myosin II – important in muscle contraction, forms bipolar myosine filament- as many as 300 myosin heads per filament. Shortening of Sarcomere ( Z-Disc to Z-Disc) Upon Contraction, Length of Filaments Remains Unchanged, Thin (Actin) Filaments And Thick ( Myosin) Filaments Slide Across One Another, Bipolar Myosin Filament Walks to the Plus End of Actin Filament (Z-Disc) Structural Components of Sarcomere Titin, Nebulin alpha- actinin, CapZ, Tropomodulin. Regulatory Components Tropomyosin, Troponin, Ca2+. Actin Filament Stability Requires: CAP Z- Cap Plus End of Actin Filament, Tropomodulin- Cap Minus Ends of Actin Filament. Nebulin- stabilize actin filament- determine length. Alpha Actinin- Component of Z Disc - Actin Bundling, Note Banding Pattern- be familiar. Calcium released by voltage sensitive channels in SR, allow for all myofibrils to spontaneously contract in muscle cell. Tropomyosin- rod shaped Actin binding protein blocks myosin binding site. Calcium binding to regulatory troponin complex, facilitates rearrangement of tropomyosin, now myosin can bind actin. Absence of Calcium- no contraction. Steps in myosin movement – see slide for details. Actin based motility in non muscle cells. Cell with ability for whole cell locomotion. Example: Fibroblast moving toward site of wound. Cell needs to extend. Attach to surface , and contract back end to make way along. Actin organized in bundles in Filopodia structures (thin protrusions) and thin sheets in Lamellipodia. Actin concentration at leading edge- a result of nucleation of actin by ARP proteins. ARP function as nucleation sites- attach onto + end of Actin filaments, now formation of new actin filament branch from site. Small Family of Rho G proteins ( monomeric like Ras) activated in intracellular signaling pathway and target ARP proteins for cytoskeletal rearrangement. .