Lecture 14, microtubule regulators and motors, 19F.pdf - Regulators of microtubule Microtubule-based motility Many proteins affect the dynamic

Lecture 14, microtubule regulators and motors, 19F.pdf -...

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Regulators of microtubule Microtubule-based motility
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Many proteins affect the dynamic properties and functions of microtubules in a living cell. Here are just some examples. DON’T memorize this figure.
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Examples of MTOC: Plant cells are unique in that they have no well defined MTOC. In most animal cells, the minus ends of most microtubules are NOT free. Instead, most microtubules are assembled from m icrot ubule o rganizing c enters (MTOCs), where they also become capped. Differentiated animal cells, which no longer proliferate or undergo mitosis , often have n on-c entrosomal MTOCs ( ncMTOC s) instead of centrosomes. interphase mitosis Basal body in some (proliferating or not) animal cells with cilia or flagella . Centrosome in all proliferating animal cells , which undergo mitosis; cilia or flagella on cell surface
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Pair of centrioles (arranged perpendicular to each other) are important parts of the centrosome (and basal body, but not other forms of MTOC) 9 triplet microtubules; each triplet has 13 + 10 + 10 protofilaments (cyclinder of cylinders) 9 molecules of SAS-6 dimers form a hub and spokes structure to organize the triplet MTs (with many different proteins) Figure 16-48 Molecular Biology of the Cell (© Garland Science 2015) cross section of a centriole cartwheel- like
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The centrosome is the major m icrot ubule o rganizing c enter (MTOC) of proliferating animal cells. It is found near the cell center next to the nucleus and Golgi clusters in most such cells. Figure 16-47 Molecular Biology of the Cell (© Garland Science 2015) At the centrosome, microtubules are nucleated from a large number (>50) of g -tu bulin r ing c omplexes ( g - TuRCs ) . During M-phase, additional molecules of g - TuRC (~5X) are recruited to each centrosome, resulting in the nucleation of more microtubules per centrosome.
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Based on what you have learned so far from the nucleation of F-actin assembly – for example, by the ARP complex – what do you want in a microtubule nucleating factor?
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7 molecules of g -TuSC ( g -t ubulin s mall c omplex) and other accessory proteins organize into the g -TuRC ( g -tu bulin r ing c omplex) minus end, capped g -TuRC nucleates microtubule assembly at the minus end, with elongation at the dynamic distal plus end. It also caps this minus end, making it no longer dynamic. g -tubulin is similar in sequence and structure to a - and b -tubulins, with the exposed end of g -tubulin in the g -TuRC especially resembling the exposed end of b -tubulin in a microtubule (i.e., plus end). Figure 16-46 Molecular Biology of the Cell (© Garland Science 2015) 13 g -tubulin molecules exposed at end dynamic end-on view +
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Microtubules are severed and released in some cells The new minus ends of such released microtubules are dynamic (mostly disassembly) unless they become capped by other proteins.
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