Cytoskeletal Systems II

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Cytoskeletal Systems II

• Reading Becker, ch. 22, pp. 754-765

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Microfilaments

• Smallest of the major cytoskeletal elements
  (diameter 7 nm)
• Assembled in manner similar to MTs
• Have an inherent polarity (plus and minus end)
• Thinner, more flexible, shorter than MTs
• More numerous than MTs
• Found in bundles or in meshlike networks

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Major Functions of MFs

• Maintenance of cell shape
• Movement
• Cell-Matrix and Cell-Cell Adhesion

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MFs and Maintenance of Cell Shape

• Cell Cortex
• Meshlike network of MFs underlying PM
  provides rigidity to cell
• Microvilli
• Fingerlike extensions found on surface of cells
  specialized for absorption
• Lamellipodia
• Flattened sheet-like extensions that spread
  out from the cell body of migratory cells or
  parts of cells
• Filopodia
• Fingerlike extensions at the leading edge of a
  lamellipodium

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MFs and Movement

• Muscle contraction
• Migration of cells across a surface
• Neurite outgrowth
• Cytoplasmic streaming
• Formation of cleavage furrows to divide cytoplasm
  during cytokinesis

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MFs and Adhesion

• Cell-Matrix Adhesion Cell-Cell Adhesion
• Focal Adhesion Adherens Junction

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Actin The Monomers of MFs

• G-actin
• Globular actin
• Monomeric actin
• Single polypeptide 375 aas
• Folds into U-shaped conformation
• Binds 1 molecule of ATP
• F-actin
• Filamentous actin
• Polymers of G-actin
• Either form can bind actin-binding proteins,
  which
• regulate actin function

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Different Types of Actin

• a-actins
• muscle-specific
• b- and g-actins
• non-muscle actins
• b- and g-actins are differentially localized
  within the cell
• Actin-related proteins (ARPs)
• 50 sequence similarity to actins (aa level)
• Involved in actin polymerization

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Actin Polymerization

• Similar to MT polymerization
• MFs have plus and minus ends
• Monomers add preferentially to plus end
• Monomers are usually ATP-bound
• Presence of ATP stabilizes the MF
• ATP hydrolysis to ADP destabilizes

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Actin Polymerization
Notice that the MF is actually formed from 2
intertwined linear polymers of F-actin
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Microfilament Polarity

• Myosin a protein that binds to actin
• S1 fragment of myosin retains ability to
  bind actin

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Other Molecules That Regulate Actin Polymerization

• 50 G-actin monomers in the cell are polymerized
  at any given time
• Intracellular actin is high
• What keeps 100 of the G-actin monomers from
  polymerizing into MFs?
• Actin-Binding Proteins

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Actin-Binding Proteins That Regulate Actin
Polymerization

• Thymosin b4
• binds free G-actin and prevents its addition to
  the growing end of MF
• Profilin
• transfers G-actin from thymosin b4 to growing
  end of MF
• ADF/cofilin
• increases depolymerization at the minus end
• CapZ
• capping protein
• binds to plus end and prevents further addition

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Drugs That Regulate Actin Polymerization

• Cytochalasins
• prevent addition to the plus end, leading to
  eventual depolymerization
• Latrunculin A
• prevents polymerization by sequestering
  G-actin monomers

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Specialized Structures Comprised of Actin

• Microvilli
• Cell Cortex
• Lamellipodia
• Filopodia
• Contractile Bundles
• Contractile Ring during cytokinesis

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Microvilli

• Protrusions found predominantly on apical surface
  of intestinal epithelial cells
• 1-2 mm long
• 0.1 mm in diameter
• Function to increase surface area of cell for
  absorption of nutrients

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Microvilli

• Contain bundles of MFs, with plus ends
• pointing toward tip
• MFs attached to each other by fimbrin and
• villin
• MFs attached to PM by myosin I and
• calmodulin
• Terminal web found at base of
• microvillus contains myosin and spectrin
• serves to interconnect MFs, connect MFs to PM,
• connect MFs to IFs coursing through the rest
• of the cell anchor MF bundles and give
• microvillus rigidity

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Cell Cortex

• Meshwork of MFs and associated proteins
  underlying PM
• Supports PM provides rigidity to cell surface
• Changes in cell cortex allow for changes in cell
  shape and movement
• MFs linked together by filamin
• Network is disrupted by gelsolin

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Filamin, Gelsolin, and MFs of the Cell Cortex

• Filamin exists as dimers
• Each monomer has a
• binding site for actin
• Binding of an MF to each
• monomer causes 2 MFs to
• crisscross one another to form a network
• Gelsolin breaks MFs and caps their ends, making
• the cortex less rigid and more fluid

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Lamellipodia

• Flattened sheet-like extensions spreading out
  from the cell body
• Present in migratory cells and in other motile
  parts of cells (neuronal growth cones)

Neuronal growth cone
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Branched Networks of MFs in Lamellipodia
Actin Polymerization Actin Depolymerization
Site of nucleation of new MF
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Filopodia

• Fingerlike extensions at the leading edge of the
  lamellipodium
• Contains loose bundle of 10-20 MFs with plus ends
  pointing outward

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MFs and Cell Migration
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Actin Polymerization at Leading Edge of
Lamellipodium Allows for Cell Migration or Growth
Cone Extension
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Actin Polymerization at Leading Edge of
Lamellipodium Allows for Cell Migration or Growth
Cone Extension
Actin Polymerization Actin Depolymerization
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Types of Actin-Binding Proteins
Thymosin b4
Arp2/3
Fimbrin Villin
Gelsolin
Myosin
Filamin
CapZ
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Intermediate Filaments (IFs)

• Intermediate in size (8-12 nm)
• Strong and ropelike
• Tension-bearing role in cell
• Most stable cytoskeletal element
• Scaffolding of cell maintenance of cell shape
• Tissue-specific

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Tissue Specificity of IFs
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IF Structure

• Protein monomers of IFs are fibrous rather than
  globular
• Monomers of different IFs are similar in
  structure and size
• Central fibrous domain (rodlike and helical)
• Flanking N- and C-terminal globular domains

310-318 aas
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Assembly of IFs
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Functions of IFs

• Tension-bearing allow cell to withstand
  mechanical forces
• Provides scaffolding of cell
• Provides scaffolding of nucleus

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Nuclear Lamins (IFs in the Nucleus)

• Nuclear lamina IF scaffolding of nucleus
• Comprised of nuclear lamins A, B, and C

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