CHAPTER 9
The Cytoskeleton and Cell Motility
Introduction
• The cytoskeleton is a network of
filamentous structures: microtubulues,
microfilaments, and intermediate
filaments.
Properties of cytoskeletal components
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.
Structure and functions of the cytoskeleton
Overview of the Major Functions of
the Cytoskeleton (2)
• Provides anchoring
site for mRNA.
• Serves as a signal
transducer.
• An essential
component of the
cell’s division
machinery.
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 (livecell imaging).
– Used to reveal the location of a protein
present in very low concentrations.
Applications using fluorescence imaging
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.
Using video microscopy to follow activities of
molecular motors
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 α- and ß-tubulin subunits assembled into
tubules with plus and minus ends.
The structure of microtubules
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.
MAPs
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.
Microtubules (3)
• Microtubules as
Structural
Supports and
Organizers
– Microtubules
function in axonal
transport.
– Microtubules play
a role in axonal
growth during
embryogenesis.
Microtubules (4)
• Microtubules as Structural Supports and
Organizers
– In plant cells, microtubules help maintain cell shape by
influencing formation of the cell wall.
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.
Axonal transport
Axonal transport
Visualizing axonal transport
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.
Microtubules (7)
• Kinesins
– Kinesin—member 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.
Kinesin
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 cell’s plasma
membrane.
Kinesin-mediated organelle transport
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.
Cytoplasmic dynein
Cytoplasmic dynein
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.
The centrosome
The centrosome
Microtubules (11)
• Centrosomes
(continued)
– Responsible for
initiation and
organization of the
microtubular
cystoskeleton.
– Microtubules
terminate in the
PCM.
Microtubules (11)
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.
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.
The role of -tubulin in centrosome function
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.
Four arrays of microtubules in a plant cell
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.
Nucleation of plant microtubules
Nucleation of plant microtubules
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.
Microtubule assembly in vitro
Structural cap model of dynamic instability