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Lecture

BIOL 2021 Lecture Notes - Intermediate Filament, Globular Protein, Cytoskeleton


Department
Biology
Course Code
BIOL 2021
Professor
Julie Clark

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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

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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

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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)
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