Chapter 17 the cytoskeleton

Cytoskeleton

gives a cell its shape and allows the cell to organize its internal components, a network of protein filaments including intermediate filaments, microtubules, and actin filaments, which extends throughout the cytoplasm

Two major cytoskeletal systems

Microtubules (green), Actin filaments (red)

Animals cells have no…

No cell walls

The cytoskeleton is both the

“skeleton” and “muscles” of the cell

The cytoskeleton is directly responsible for…

large-scale movements

The three types of protein filaments that form the cytoskeleton differ in their…

…composition, mechanical
properties, and roles inside the cell

Intermediate filaments

form a strong, durable network
in the cytoplasm of the cell

Desmosomes

How the filaments in each cell are
indirectly connected to those of
neighboring cells

The mechanical link of demosomes and intermediate filaments strengthen the…

epithelium

Intermediate filaments
have….

great tensile strength, and their main
function is to enable cells to withstand the mechanical stress

The intermediate filament
protein monomer (A) consists
of a…

…central rod domain with
globular regions at either
endPairs of monomers
associate to form

Pairs of monomers
associate to form…

A dimer

Two dimers then line up to
form a…

…staggered tetramer Tetramers
(C)

Tetramers

can pack together end-to-end as
shown in (D) and
assemble into a helical
arrayAn array contains

An array contains

eight strands of tetramers that
twist together to form the
final ropelike intermediate
filament.

The network of intermediate filaments and desmosomal junctions:

extends through the
sheet develops tension and limits the extent of stretchingN

Nuclear lamina

line the inner face of the
nuclear envelope and are thought to provide attachment sites for the DNA-
containing chromatin, constructed of lamina

Mutations in a nuclear lamin (lamin A) can cause a rare
class of premature aging disorders called…

progeriaP

Plectin:

aids in the bundling of intermediate filaments and links
these filaments to other cytoskeletal protein networks

Microtubules usually grow out of an

extend from an organizing center
such as (B) a centrosome, (C) a spindle pole, or (D) the basal body of a cilium
Microtubules

Microtubules

are long and
relatively stiff hollow tubes of
protein that can rapidly
disassemble in one location
and reassemble in anotherC

Cilia and flagella

permanent structures of microtubules

Microtubules are hollow tubes of…

tubulinTubulin polymerizes from

Tubulin polymerizes from…

nucleation sites on a centrosome

A centrosome consists of a…

matrix of protein containing the γ-tubulin rings that
nucleate microtubule growthThe

The centrosome contains a pair of…

centrioles, each made up of a
cylindrical array of short microtubules

The minus end of each microtubule is
embedded…

..embedded in the centrosome

The plus end of each microtubule is…

…is free in the cytoplasm

The location and orientation of these microtubule arrays are controlled by…

microtubule-organizing centers (red)

Each microtubule filament grows and shrinks…

…independently of its neighbors

GTP hydrolysis controls

the growth of microtubules

Selective stabilization of microtubules can

polarize a cell

Depolymerization

Protects newly formed microtubules and allows them to persist. Will lead to a rapid reorientation of the microtubule
arrays (C) and convert the cell to a strongly polarized form (D)

Minus ends of microtubules are protected by

organizing centers

The plus ends of microtubules can be stabilized by

other proteins

Microtubules in the axon of a nerve cell point in the…

…same direction

Kinesins

Motor proteins for outward traffic material. Move towards plus end of mictotubule

Dyneins

Motor proteins for inward traffic material. Move towards minus end of microtubule

Globular heads

Heads of dynein and kinesin with ATPase activity

Transition of 3 conformations in motor proteins is driven by…

…the hydrolysis of a bound ATP that allows it to “walk”

The tail of a motor protein…

…binds to some cell component and determines what cargo the protein transports

Cilia

hair-like structures, which are covered by plasma membrane

Repetitive cycle of cilium beats

consisting of a power stroke followed by a recovery stroke

Fast power stroke

cilium is fully extended
and fluid is driven over the
surface of the cell

Slower recovery stroke

cilium curls back
into position with minimal
disturbance to the
surrounding fluid

Flagella

propel sperm and are much like cilia but notable longer

How flagella generate movement

they propagate regular
waves along their length that drive
the cell through liquid

How microtubules in a cilium or flagellum are arranged

“9” doublet microtubules in a ring around “2” single dynein microtubules”
“9 + 2” array. Via a switch-inhibition mechanism A

Axoneme

The microtubule-based cytoskeleton of cilia and flagella

Without the outer doublet microtubules…

the doublets slide
against each other due to the repetitive action of dyneins

Actin filaments

allow eukaryotic cells to adopt a variety of shapes and perform a variety of functions

Lamellipodia

sheetlike protrusions caused by actin filamentsL

Filopodia

fingerlike protrusions caused by actin filaments

ATP hydrolysis decreases…

…the stability of the actin polymer

What do actin monomers in the cytosol carry?

ATP, which is hydrolyzed into ADP soon after assembly into a growing filament

Treadmilling

Regulates polymer length. Actin-binding proteinsccurs when ATP-actin adds to the plus end of an actin filament at the same time that ADP-actin is lost from the minus end

Actin-binding proteins

control the behavior of actin

Cells move forwards by

Forces generated in the actin-rich cortex

Protrusion

During cell movement, actin
polymerization at the leading
edge of the cell pushes the
plasma membrane forward

Attachment

New points of anchorage are
made between the actin
filaments and the surface on
which the cell is crawling

Traction

Contraction at the rear of the
cell then draws the body of the
cell forward

Order of actions in cell movement

Protrusion, attachment, traction, further protrusion

Nucleation of new actin filaments
(red) is mediated by…

…actin-related proteins (ARP) complexes (green)
attached to the sides of preexisting
filaments.

Myosin-I

has a single globular head and a tail that attaches to another molecule or organelle in the cell. Walks towards plus end of the actin filament it contacts, can bind to various components of the cell

Myosin-II

molecules can associate with one another to form myosin filaments, two globular heads and a coiled-coil tail.

Myosin-II mediates…

…the shortening of an actin filament bundle, they can slide actin filaments over each other

Bipolar myosin filament

Tails of myosin-ii associate with each other, heads project outwards in the middle in opposite directions

Myofibrils

Packed in skeletal muscle, have a repeating chain of sarcomeres

Multinucleate cells (also called muscle fibers)

They contain numerous myofibrils in which actin filaments and myosin filaments
are arranged in a highly organized structure with a striped appearance

Sarcomeres

the contractile units of microfibrils. Contain Z discs

Z discs

at either end of the sarcomere are
attachment points for actin filaments; the centrally located thick filaments
(green) are each composed of many
myosin-II molecules

Actin filaments are anchored at the plus end to the…

…Z disc

During contraction, the actin and myosin filaments slide past each other without…

…without shortening

Attached

A myosin head lacking a bound nucleotide is
locked tightly onto an actin filament in a rigor configuration

Released

ATP binds to the head of myosin and
immediately causes a conformation change of the actin-
binding site. This reduces the affinity of the head for actin
and allows it to move along the filament.

Cocked

The cleft closes around the ATP, triggering a large shape change that causes the head to be displaced
along the filament by a distance of about 5 nm. Hydrolysis of
ATP occurs, but the ADP and inorganic phosphate (Pi) produced remain tightly bound to the protein

Force-Generating

A weak binding of the myosin head
to a new site on the actin filament causes release of the
inorganic phosphate, along with the tight binding of the head
to actin. This release triggers the power stroke—the force-
generating change in shape during which the head regains
its original conformation and loses its bound ADP, thereby
returning to the start of a new cycle

Myofibrils are surrounded by

Transverse tubules and sarcoplasmic reticulum

The force-generating interaction between myosin and actin filaments takes place only…

…when skeletal muscle receives a signal from the nervous system

The electrical signal
is then relayed to the…

…sarcoplasmic reticulum

Voltage-gated Ca 2+ channel

Activates Ca2+ release in the sarcoplasmic reticulum

Troponin

Controls skeletal muscle contraction. When Ca2+ binds to troponin,
the troponin moves the tropomyosin that otherwise blocks the interaction of actin with the myosin heads