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Unformatted text preview: -1-LECTURE 33 13 November 2009 (P. J. Hollenbeck) BIOL231Molecular motors & MTs: cilia and beyond Read: pp.583-90 / DVD 17.5-7/ Probs: 82-87I. Cilia and flagella<In some cells we find moving systems that are specialized, stable, and highly organized. These haveserved as models for understanding the general principles of motor protein function and forcegeneration. We saw this in sarcomeres of muscle cells, where the functions of actin and myosin werefirst elucidated. It is the same for MT-based movement. Here the most specialized and stable movingstructure is the cilium or flagellum..>(A) General structure and function(1) Ciliaare thin (0.25 ìm diam) surfaceprotrusions found on many cells. They beat liketiny flexible oars, generating a current of liquidacross the cell surface. In some cases (such as theciliated epithelial cells that line your respiratorytract) the action of thousands of cilia per cell causesthe fluid to move over the cell surface (here,mucus) . But for a ciliated single-celled eucaryote like Tetrahymena, the same rowingaction moves the cell through the water that it lives in. A similar structure, theflagellum, beats to power the movement of protozoa or sperm cells, for example.(2) What supports this thin protrusion? Cilia and flagella contain a core of MTs andaccessory proteins. It has a characteristic pattern of 9 MTs arranged in a ring around acentral pair of 2 MTs. The MTs that form the ring have a special structure: they are“doublets,” in which one complete, or “A,” MT has a second, incomplete, or “B,” MTattached to its wall (see fig below). (3) Accessory proteins in the axoneme. The MTs of the cilium or flagellum have manyaccessory proteins that link them together, cap them, and make them extremely stable. Some of these proteins are large enough, or form complexes that are large enough, tobe clearly visible in electron micrographs. The major ones are the nexin links thatconnect adjacent MT doublets to each other, the radial spokesthat link doublet MTs tothe central region, the inner sheath that links the two central MTs to each other, andthe dynein arms that sit on the A-MTs and generate force for ciliary movement.(4) This complete assembly of proteins is very stable: you can shear the cilia off of acell, use detergents to strip off the plasma membrane, and the scaffolding of proteinsremains intact. This inner, cytoskeletal part of the cilium is called the axoneme. -2-<How are MTs, motors proteins, and accessory proteins organized to generate force...
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This note was uploaded on 12/18/2009 for the course BIOL 101 taught by Professor Wormer during the Fall '08 term at Purdue.
- Fall '08