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 end- and 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 functionof ciliary dynein

A model of structure and functionof 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 micro-

filaments 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 force-generating 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 inintracellular transport