Microtubules (17)
• Dynamic instability
– Growing and shrinking microtubules can
coexist in the same region of a cell.
– A given microtubule can switch back and forth
between growing and shortening phases.
– It is an inherent property of the plus end of the
microtubule.
– Proteins called +TIPS regulate the rate of
growth and shrinkage.
Microtubule dynamics in living cells
Dynamic instability
Microtubules (18)
• Cilia and Flagella: Structure and Function
– Cilia and flagella are hairlike motile
organelles.
– They have similar structures but different
motility.
– Cilia tend to occur in large numbers on a cell’s
surface.
Beating movement of cilia
Beating movement of cilia
Microtubules (19)
• Cilia and flagella (continued)
– Flagella exhibit different beating patterns.
– The structure of cilia and flagella contains a
central core (axoneme) consisting of
microtubules in a 9 + 2 arrangement.
Eukaryotic flagella
Microtubules (20)
• Cilia and flagella (continued)
– The basic structure of the axoneme includes a
central sheath, connected to the A tubules of
peripheral doublets by radial spokes.
– The doublets are interconnected to one
another by an interdoublet bridge.
– A longitudinal view of the axoneme shows the
continuous nature of the microtubules
The structure of the axoneme
Longitudinal view of an axoneme
Microtubules (21)
• Cilia and flagella (continued)
– Cilia and flagella emerge from basal bodies.
– The growth of an axoneme occurs at the plus
ends of microtubules.
– Intraflagellar transport (IFT) is the process
responsible for assembling and maintaining
flagella.
– IFT depends on the activity of both plus endand minus end-directed microtubules.
Basal bodies and axonemes
Intraflagellar transport
Microtubules (22)
• The Dynein Arms
– The machinery for
ciliary and flagellar
motion resides in the
axoneme.
– Ciliary (axonemal)
dynein is required
for ATP hydrolysis,
which supplies
energy for
locomotion.
Chemical dissection
of protozoan cilia
A model of structure and function
of ciliary dynein
A model of structure and function
of ciliary dynein
Microtubules (23)
• The Mechanism of Ciliary and Flagella
Locomotion
– Swinging cross-bridges generate forces for
ciliary or flagellar movement.
– Dynein arm of an A tubule binds to a B tubule
and undergoes a conformational change that
slides tubules past each other.
– Sliding alternates from one side of axoneme
to another leading to bending.
Forces that drive ciliary or flagella motility
The Human Perspective: The Role of Cilia in
Development and Disease (1)
• Situs inversus is a syndrome in which the
left-right body symmetry is reversed.
• One cause of situs inversus is mutations in
the gene encoding ciliary proteins.
• Patients with situs inversus suffer from
respiratory infections and male infertility.
The Human Perspective: The Role of Cilia in
Development and Disease (1)
• Many cells have nonmotile primary cilia
that sense chemical and mechanical
properties of surrounding fluids.
• Mutations in primary cilia may lead to
polycystic kidney disease.
• Cilia are important in developmental
processes, and mutations lead to a range
of abnormalities.
Primary cilia
9.4 Intermediate Filaments (1)
• Intermediate filaments (IFs)–
heterogeneous group of proteins, divided
into five major classes.
• IFs classes I–IV are used in the
construction of filaments; type V (lamins)
are present in the inner lining of the
nucleus.
Distribution of major mammalian IF proteins
Intermediate Filaments (2)
• IF Assembly and Disassembly
– Assembly:
• Basic building block is a rod-like tetramer formed
by tow antiparallel dimers.
• Both the tetramer and the IF lack polarity.
– IFs are less sensitive to chemical agents than
other types of cytoskeletal elements.
A model of IF assembly and architecture
Intermediate Filaments (3)
• Assembly and disassembly of IFs are controlled
by phosphorylation and dephosphorylation
Intermediate Filaments (4)
• Types and Functions of IFs
– IFs containing keratin form the protective
barrier of the skin, and epithelial cells of liver
and pancreas.
– IFs include neurofilaments, which are the
major component of the network supporitng
neurons.
Organization of IFs within an epithelial cell
9.5 Microfilaments (1)
• Microfilaments are composed of actin
and are involved in cell motility.
• Using ATP, actin polymerizes to form actin
filaments (“F-actin”).
• The two ends of an actin filament have
different structural characteristics and
dynamic properties.
Actin filament structure
Microfilaments (2)
• One of the microfilaments appears
pointed, and the other
appears barbed.
• Orientation of the
arrowheads formed by
actin provides
information about
direction of the
microfilament
movement.
Microfilaments (3)
• Microfilament Assembly and Disassembly
– Actin assembly/disassembly in vitro depends
upon concentration of actin monomers.
– Filament assembly leads to drop in ATP-actin.
– Actin subunits are added to plus end and
removed from the minus end (steady state).
– Microfilament cytoskeleton is organized by
controlling equilibrium between assembly and
disassembly of microfilaments.
Actin assembly in vitro
Microfilaments (4)
• Actin polymerization can act as a forcegenerating mechanism in some cells.
Microfilaments (5)
• Myosin: The Molecular Motor of Actin
Filaments
– All myosins share a characteristic motor head
for binding actin and hydrolyzing ATP.
– The myosin tail is divergent.
– Myosins can be divided into two groups:
• Conventional (type II) myosins
• Unconventional myosins
Microfilaments (6)
• Conventional
(Type II) Myosins
– They generate force
in muscles and
some nonmuscle
cells.
– Each myosin II is
composed of two
heavy chains, two
light chains, and two
globular heads
(catalytic sites).
Structure of myosin II
Microfilaments (7)
• Myosin II (continued)
– All of the machinery required for motor activity
is contained in a single head.
– The tail portion plays a structural role allowing
the protein to form filaments.
Myosin II
Myosin II
Microfilaments (8)
• Unconventional Myosins
– They have only a single head and are unable
to assembly into filaments in vitro.
– Myosin I’s precise role in cellular activities is
unclear.
– Myosin V is involved in organelle transport.
– Several of them are associated with
cytoplasmic vesicles and organelles.
Myosin V and organelle trasnport
Unconventional myosins in
intracellular transport