Microtubules note - More on Microtubules (Lodish Chapter...

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(Lodish Chapter 18) I. Microtubules A. MT subunit is a α / β heterodimer (Lodish Fig. 17-2) B. MT works with kinesins and dyneins (Lodish Fig. 18-1) C. MT is responsible for long-range transport of organelles in interphase cells (Lodish Fig. 18-1, 2). D. In mitosis, MT form spindles. E. MT invloved movements: cilia and flagella (Lodish Fig. 18-2) II. Tubulin A. MT subunit is a α / β heterodimer (Lodish Fig. 18-3) 1. Both can bind GTP. However α -tubulin binds it irreversibly and cannot hydrolyze to GDP. β -tubulin can act as a normal G protein. B. Tubulin heterodimers polymerize longitudally to form a protofilament. Protofilaments (13x) then associate side to side to form sheets and then cylinders (Lodish Fig. 18-3) C. Most common cylinder is a singlet, but doublets and triplets are found in more specialized MT structures (Lodish Fig. 18-4) D. MTs have polarity, with the (+) end being the one with GTP-bound β - tubulin. III. MT organizing centers (MTOC) A. MTs often radiate like spokes from a centrosome (Fig. 18-15, 6), which often contains two centrioles (Fig. 18-16). Centrosome = 2x centrioles + pericentriolar matrix (mainly pericentrin & γ -tubulin). B. The (-) ends of MTs associate with MTOCs C. Another MTOC is the basal body D. Surrounding the centrioles is a dense mass of proteins called the pericentriolar (PC) matrix. 1. γ -tubulin and pericentrin are major components E. Most γ -tubulin is found in a complex called the γ -tubulin ring complex ( γ -tuRC) (Lodish Fig. 18-7) 1. The γ -tuRC is about 25 nm in diameter and can stimulate MT assembly at concentrations below the Cc. 2. Localized to the (-) end of MTs (Lodish Fig. 18-15) F. Plant and fungi do not have centrosomes, but they do have γ -tubulin IV. MT assembly (Lodish Fig. 18-8, 9, 10, 11, 12) A. Like actin filaments, there is a Cc (critical concentration, ~0.03 µM). B. Polymerization usually occurs at the (+) end treadmill C. GTP/ β -tubulin form assembles more rapidly D. Preexisting MT fragments greatly enhance polymerization nucleating or seeding. E. When MTs are polymerizing, the (+) can be jagged, due to unequal addition of subunits
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This note was uploaded on 09/03/2009 for the course MCDB 428 taught by Professor Wang during the Winter '08 term at University of Michigan.

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Microtubules note - More on Microtubules (Lodish Chapter...

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