The Cytoskeleton and Cell Motility

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CHAPTER 9

• The Cytoskeleton and Cell Motility

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Introduction

• The cytoskeleton is a network of filamentous
  structures microtubulues, microfilaments, and
  intermediate filaments.

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Properties of cytoskeletal components
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9.1 Overview of the Major Functions of the
Cytoskeleton (1)

• The cytoskeleton has many roles
• Serves as a scaffold providing structural support
  and maintaining cell shape.
• Serves as an internal framework to organize
  organelles within the cell.
• Directs cellular locomotion and the movement of
  materials within the cell.

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Structure and functions of the cytoskeleton
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Overview of the Major Functions of the
Cytoskeleton (2)

• Provides anchoring site for mRNA.
• Serves as a signal transducer.
• An essential component of the cells division
  machinery.

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9.2 Study of the Cytoskeleton (1)

• The Use of Live-Cell Fluorescence Imaging
• Can be used to locate fluorescently-labeled
  target proteins.
• Molecular processes can be observed (live-cell
  imaging).
• Used to reveal the location of a protein present
  in very low concentrations.

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Applications using fluorescence imaging
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Study of the Cytoskeleton (2)

• The Use of In Vitro Single-Molecule Assays
• They make possible to detect the activity of an
  individual protein molecule in real time.
• Can be supplement with atomic force microscopy to
  measure the mechanical properties of cytoskeletal
  elements.

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Using video microscopy to follow activities of
molecular motors
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9.3 Microtubules (1)

• Structure and Composition
• Microtubules are hollow, cylindrical structures.
• The microtubule is a set of globular proteins
  arranged in longitudinal rows called
  protofilaments.
• Microtubules contain 13 protofilaments.
• Each protofilament is assembled from dimers of a-
  and ß-tubulin subunits assembled into tubules
  with plus and minus ends.

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The structure of microtubules
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Microtubules (2)

• Microtubule-Associated Proteins (MAPs)
• MAPs comprise a heterogeneous group of proteins.
• MAPs attach to the surface of microtubules to
  increase their stability and promote their
  assembly.
• MAPs are regulated by phosphorylation of specific
  amino acid residues.

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MAPs
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Microtubules (2)

• Microtubules as Structural Supports and
  Organizers
• The distribution of microtubules determines the
  shape of the cell.
• Microtubules maintain the internal organization
  of cells.

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Microtubules (3)

• Microtubules as Structural Supports and
  Organizers
• Microtubules function in axonal transport.
• Microtubules play a role in axonal growth during
  embryogenesis.

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Microtubules (4)

• Microtubules as Structural Supports and
  Organizers
• In plant cells, microtubules help maintain cell
  shape by influencing formation of the cell wall.

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Microtubules (5)

• Microtubules as Agents of Intracellular Motility
• They facilitate movement of vesicles between
  compartments.
• Axonal transport
• Movement of neurotransmitters across the cell.
• Movement away from the cell body (anterograde)
  and toward the cell body (retrograde).
• Mediate tracks for a variety of motor proteins.

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Axonal transport
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Axonal transport
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Visualizing axonal transport
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Microtubules (6)

• Motor Proteins that Traverse the Microtubular
  Cytoskeleton
• Molecular motors convert energy from ATP into
  mechanical energy.
• Molecular motors move unidirectionally along
  their cytoskeletal track in a stepwise manner.
• Three categories of molecular motors
• Kinesin and dynein move along microtubule tracks.
• Myosin moves along microfilament tracks.

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Microtubules (7)

• Kinesins
• Kinesinmember of a superfamily called KLPs
  (kinesin-like proteins).
• A kinesin is a tetramer of two identical heavy
  chains and two identical light chains.
• Each kinesin includes a pair of globular heads
  (motor domain), connected to a rod-like stalk.
• Kinesin is a plus end-directed microtubular motor
  based on its movement.

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Kinesin
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Microtubules (8)

• Kinesins (continued)
• They move along a single protofilament of a
  microtubule at a velocity proportional to the ATP
  concentration.
• Movement is processive, motor protein moves along
  an individual microtubule for a long distance
  without falling off.
• KLPs move cargo toward the cells plasma
  membrane.

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Kinesin-mediated organelle transport
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Microtubules (9)

• Cytoplasmic Dynein
• Dynein responsible for the movement of cilia
  and flagella.
• Cytoplasmic dynein Huge protein with a
  globular, force-generating head.
• It is a minus end-directed microtubular motor.
• Requires an adaptor (dynactin) to interact with
  membrane-bounded cargo.

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Cytoplasmic dynein
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Cytoplasmic dynein
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Microtubules (10)

• Microtubule-Organizing Centers (MTOCs)
• MTOCs specialized structures for the nucleation
  of microtubules.
• Centrosome structures responsible for
  initiating microtubules in animal cells.
• It contains two barrel-shaped centrioles
  surrounded by pericentriolar material (PCM).
• Centrioles are usually found in pairs.

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The centrosome
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The centrosome
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Microtubules (11)

• Centrosomes (continued)
• Responsible for initiation and organization of
  the microtubular cystoskeleton.
• Microtubules terminate in the PCM.

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Microtubules (11)
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Microtubules (12)

• Basal Bodies and Other MTOCs
• Basal body structure where outer microtubules
  in a cilia and flagella are generated.
• Plant cells lack MTOCs and their microtubules are
  organized around the surface of the nucleus.

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Microtubules (13)

• Microtubule Nucleation
• MTOCs control the number of microtubules, their
  polarity, the number of protofilaments, and the
  time and location of their assembly.
• The protein ?-tubulin is found in all MTOCs and
  is critical for microtubule nucleation.

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The role of ?-tubulin in centrosome function
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Microtubules (14)

• The Dynamic Properties of Microtubules
• There are four distinct arrays of microtubules in
  a dividing plant cell
• Widely distributed throughout the cortex.
• Making a single transverse band.
• In the form of a mitotic spindle.
• As a phargmoplast assisting in the formation of
  the cell wall of daughter cells.

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Four arrays of microtubules in a plant cell
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Microtubules (15)

• The Dynamic Properties of Microtubules
• Newly formed microtubules branch at an angle of
  pre-existing microtubules.
• The changes in spatial organization of
  microtubules are a combination of two mechanisms
• Rearrangement of existing microtubules.
• Disassembly of existing microtubules and
  reassembly of new one in different locations.

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Nucleation of plant microtubules
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Nucleation of plant microtubules
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Microtubules (16)

• The Underlying Basis of Microtubule Dynamics
• Insight into factors that influence microtubule
  assembly and disassembly came from studies in
  vitro.
• GTP is required for microtubule assembly.
• Hydrolysis of GTP leads to a replacement of bound
  GDP by new GTP to recharge the tubulin dimer.

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Microtubule assembly in vitro
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Structural cap model of dynamic instability