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UNC-Chapel Hill BIOL 205 - Lecture 11 - The microtubule cytoskeleton

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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 3502.11.09Lecture 11 - The microtubule cytoskeletonThe cytoskeleton•Gives the cell its shape•Allows the cell to organize its components•Produces large-scale movements (I.e. muscle contraction, cell crawling, propulsion via cilia and flagella)The cytoskeleton is composed of networks of 3 different filamentsCytoskeletal filaments exhibit different physical propertiesThe cytoskeleton is dynamicMicrotubules are organized to perform specific functionsWhat do microtubules do?•Establish an internal polarity to movements and structures in the interphase cell•Participate in chromosome segregation during cell division•Establish cell polarity during cellular movement•Produce extracellular movement via beating of cilia and flagellaMicrotubule structureMicrotubules exhibit a behavior termed dynamic instability• Total mass of polymerized tubulin remains constant, but individual microtubules are dynamic• Growth: assembly of microtubule• Shrinkage: disassembly of microtubule• Catastrophe: switching from growth to shrinking• Rescue: switching from shrinking to growthQuickTime™ and aGraphics decompressorare needed to see this picture.Tubulin subunit addition takes place predominantly at the plus endGrowing microtubules have a “cap” of GTP at the plus endMicrotubule-associated proteins•MAPs can function as cross-bridges connecting microtubules.•They can affect microtubule rigidity and assembly rate.The centrosome is the primary microtubule nucleation site in most cellsCentrosomes act to polarize the microtubule network•Plus end - fast growing, usually in the cytoplasm•Minus end - slow growing, anchored at the centrosome in most cellsCentrosome duplication occurs once per cell cycleCentrosomes are often abnormal in cancer cellsWhy are microtubules dynamic?•Microtubule dynamics allow the cell to quickly reorganize the network when building a mitotic spindle•Dynamics also allow microtubules to probe the cytoplasm for specific objects and sites on the plasma membrane - search and captureSearch and capture modelSearch & capture during cell polarizationSearch & capture during mitosisMotor proteins •Enzymes that convert ATP hydrolysis directly into movement along cytoskeletal filaments•Some motors move towards the plus end, others move to the minus end•Carry cargo (organelles, protein complexes, RNA) and mediate microtubule/microtubule slidingFirst evidence of microtubule motors came from study of axonal transportExtruded axoplasm assays - Cytosolis squeezed from the axon with aroller onto a glass coverslip.Addition of ATP shows movementby videomicroscopyVesicle movement in this systemis about 1-2um/s similar to fast axonaltransport.Motor proteinsQuickTime™ and aMPEG-4 Video decompressorare needed to see this picture.There are two families of microtubule motors•Kinesins–Move cargo to the plus end–In mitosis, participate in mitotic spindle dynamics–Usually dimers of 2 heavy chains and 2 light chains•Dyneins–Move cargo to the minus end–In mitosis, participate in mitotic spindle dynamics–Power beating of cilia and flagella–Large protein complex with many subunitsStructure of kinesin•2 heavy chains + 2 light chains•Microtubule and ATP binding sites in the head•Cargo-binding site in the tail and light chainsKinesin “walks” along microtubulesKinesin “walks” along microtubulesQuickTime™ and aPhoto - JPEG decompressorare needed to see this picture.Dynein is a large complex of many proteinsThere are two classes of dyneins•Cytoplasmic dynein–Carries cargo in the cytoplasm–Involved in mitotic spindle dynamics•Axonemal dyneins–Localized exclusively in cilia and flagella–The motors that power cilliary and flagellar beatingGeneral model for kinesin- and dynein-mediated transportFlagella and cilia are specialized microtubule-based cellular structuresCilia and flagella•Cilia line the epithelial tissue of the respiratory tract to sweep particulate matter out of the airways•Cilia line the oviduct to push the egg•Non-motile cilia detect signals•Flagella allow sperm to swim•Flagella are essential for left-right asymmetry during development (Kartagener syndrome: situs inversus, sinusitis, brochiectasis)QuickTime™ and aYUV420 codec decompressorare needed to see this picture.Cilia in the respiratory tractStructure of a motile axonemeDynein movement causes flagella to bendMutations that disrupt cilia cause multiple diseases•Fertility (sperm motility, ectopic pregnancy)•Polycystic kidney disease•Respiratory infection•Retinal degeneration•Hearing/balance loss (Usher syndrome)QuickTime™ and aYUV420 codec decompressorare needed to see this picture.Sinus invertus: left-right body asymmetry•Defects affecting placement of lungs, heart, liver stomach and spleen•Morphogens secreted on the right side of the embryo aretransported to the left side by ciliary beating•Immotile cilia fail to establish proper morphogen


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UNC-Chapel Hill BIOL 205 - Lecture 11 - The microtubule cytoskeleton

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