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Ch. 16 - Cytoskeleton - Part 1 A summary of chapter 16 and lecture notes on the cytoskeleton. Includes illustrations and graphics from the textbook.

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York University
BIOL 2021
Julie Clark

BIOL 2021- April 23 2009 CHAPTER 16: CYTOSKELETON CYTOSKELETON - Gives cell strength, shape, movement of cells 3 filaments - Intermediate filaments:  Mechanical strength - Microtubules:  Intracellular transport  Organelle location - Actin filaments  Shape  Locomotion Accessory proteins:  Control assembly Motor proteins:  Movement *Pannel 16-1 ACTIN FILAMENTS (microfilaments): - 5-9nm thick - Found in all eukaryotes - Gene sequences are conserved (gene sequences are similar in all species) - Flexible networks, bundles - Mostly in cortex (beneath plasma membrane) - Actin in microvilli, muscles MICROTUBULES - Found in all eukaryotes - Conserved - 25nm think (thickest) - Forms a hollow tube - Rigid, forms trackways which vesicles move along - Attached to MTOC (microtubule organising center) INTERMEDIATE FILAMENTS - Only in some metazoans (not in all eukaryotes); only in some cell types (specialized functions) - 10 nm thick - Make rope like fibers - Span across a cell and gives it strength and toughness GENERAL PRINCIPLES OF DYNAMICS - Dynamic structure; constantly in flux - Assembled from small subunits - Assembled with non covalent interactions so rapid changes can occur in cytoskeleton - Disable and reassemble filaments to change directions - Accessory proteins control assembly and disassembly - Small subunits make protofilaments - Line protofilaments together to make helical filaments (like a cable) - Nucleation is rate limiting - In nucleation you first need a few subunits to attach to something (like in recrystallization) - Special nucleation proteins - Fig 16-11 MICROTUBULES - Subunits= tubulin - Globular, two kinds of tubulin: α and β  Use non covalent bonding - Each binds GTP - Alpha- GTP never leaves - Beta- GTP will hydrolyse and exchange (where energy is generated) - 13 protofilaments = microtubule - Protofilament= alpha and beta alternating - Always have alpha at one end called the “-“ end  Beta is at the other end, called the “+” end  So there is polarity ACTIN FILAMENTS - Subunit is actin - One globular protein - Binds ATP - Protofilmanet= actin subunits lined up head to head in a row - Filament= 2 protofilaments - Polarity -> + and – end (actin is not symmetric) POLYMERIZATION - Making long filaments from small subunits - Panel 16-2 - Plus end: rapid addition of monomers - Minus end: slow addition of monomers because the shape is different and must undergo conformational change - Conformational changes make them different at either end - Binds spontaneous to “+” side, and high affinity for T form - Nucleotide hydrolysis (after sitting for a while)  ATP or GTP hydrolyze after binding  Causes T to go into D form; this lowers affinity, D form leaves - T form adds at high monomer concentration (at + end) - Lowering monomer concentration causes change to D form and spontaneously falls off ACTIN TREADMILLING - Intermediate concentration - D form comes off one end and T form adds at one end (occurs at same time so filament doesn’t change in length) MICROTUBULES DYNAMIC INSTABILITY - Rapid shrinking at one end - Goes through a period of rapid growth and then T-> D; D spontaneously disassembles from +
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