BIO1140 CLASS NOTES - TOPIC 9 MICROTUBULES MOTOR PROTEINS CILIA AND FLAGELLA Microtubules Are the largest of the cytoskeletal elements(25 nm in

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TOPIC 9: MICROTUBULES, MOTOR PROTEINS, CILIA AND FLAGELLA Microtubules Are the largest of the cytoskeletal elements (25 nm in diameter) Two general groups, which differ in both degree of organization and structural stability: axonemal microtubules and cytoplasmic microtubules Axonemal microtubules is a highly organized, stable MT that is found in specific subcellular structures associated with cellular movement Cytoplasmic microtubules is the more loosely organized group which is known to encompass the cytosol of most eukaryotic cells (ex. maintain axons, form the mitotic spindles, organelle and their place) Tubulin Heterodimers Are the Protein Building Blocks of Microtubules Outer diameter of 25 nm and an inner diameter of 15 nm The MT wall consists of longitudinal arrays of linear polymers called protofilaments The Heterodimers that form the bulk of protofilaments are composed of alpha- tubulins and beta-tubulins As soon as the Heterodimers synthesize they bind non covalently to each other that does not disassociate under normal conditions Individual alpha and beta tubulins have a diameter of 4-5 nm and a molecular weight of 42 000 Daltons Each protein folds into three domains: a GTP-binding domain at the N-terminus, a domain in the middle to which MT poison colchicine can bind, and a third domain at which the C-terminus that interacts with MT-associated proteins (MAPs) Within the MT all the tubulin dimers are oriented in the same direction, giving the protofilaments an natural polarity MT itself is also a polar structure Microtubules Form by the Addition of Tubulin Dimers at Their Ends A critical step in the polymerization of MTs is the aggregation of tubulin dimers into clusters called oligomers These oligomers serve as “nuclei” from which new MTs can grow, and hence this process is called nucleation Once MT has been nucleated, it grows by addition of subunits at either end, a process called elongation MT formation is initially slow ( lag phase ) nucleation The elongation phase of MT assembly is relatively faster than nucleation Eventually the mass of MT increases till the tubulin concentration becomes limiting, this leads to the plateau phase , where MT assembly is balanced by disassembly Tubulin polymerize when tubulin concentration is high and depolymerize when tubulin concentration is low When assembly is exactly balanced with disassembly , at this point it is called the overall critical concentration
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Addition of Tubulin Dimers Occurs More Quickly at the Plus Ends of Microtubules The rapid growing end of the microtubule is called the plus end , and the other end is the minus end The different growth rates of the plus and minus ends of the MT reflect the different critical concentrations required for assembly at the two ends of the MT The critical concentration for the plus end is lower than that of the minus end If the free tubulin concentration is higher than the critical concentration for the
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This note was uploaded on 06/17/2008 for the course BIO 1140 taught by Professor Fenwick during the Winter '07 term at University of Ottawa.

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BIO1140 CLASS NOTES - TOPIC 9 MICROTUBULES MOTOR PROTEINS CILIA AND FLAGELLA Microtubules Are the largest of the cytoskeletal elements(25 nm in

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