Download Chapter 12 Cytoskeleton

Chapter 12
Cytoskeleton
Microtubules
Microtubules usually grow out of an organizing structure
Microtubules are hollow tubes of tubuln
13 subunits


Tubulin polymerizes from nucleation site on a centrosome


Each microtubule filaments grows and shrinks
independently of its neighbors
shrink
growth
GTP hydrolysis controls the growth of microtubules
The selective stabilization of microtubules
can polarize a cell
Organizing center
polymerization
depolymerization
polymerization
depolymerization
Microtubules transport cargo along a nerve cell axon
Organelles move along microtubules at different speeds
0 ms
0.4 ms
0.8 ms
1.2 ms
1.6 ms
Motor proteins move along microtubules
using their globular heads
dimer
ATP-dependent “walking”
Different motor proteins transport cargo
along microtubules
Microtubules help to arrange the organelles
in a eucaryotic cell
ER
ER
Golgi apparatus
Centrosome
Microtubules
Microtubules
Golgi apparatus
Microtubule
Nucleus
Outward
Inward
Video-enhanced microscopy of cytoplasm squeezed
from a squid giant axon reveals the motion of organelles
Small vesicles
Mitochondrion
A motor protein causes microtubule gliding
Kinesin + Microtubule + ATP
Video microscopy can be used to track the movement
of a single kinesin molecule
A single molecule of kinesin moves along a microtubule
tail
Kinesin
head
Kinesin-GFP
microtubule
0.3 m/sec
Hairlike cilia coat the surface of many eucaryotic cells
The ciliated epithelium on the surface of the human respiratory tract
A cilium beats by performing a repetitive cycle
of movements consisting of a power stroke
followed by a recovery stroke
0.1-0.2 sec/cycle
Flagella propel a cell using a repetitive wavelike motion
Microtubules in a cilium or flagella
are arranged in a “9 + 2” array
“9+2” array
The movement of dynein causes the flagellum to bend
Sliding
Bending
Actin filaments
Actin filaments allow eucaryotic cells to adopt
a variety of shapes and perform a variety of functions
Microvilli
Contractile bundles
(stress fibers)
Lamellipodia (sheetlike)
Filopodia (fingerlike)
Contractile
ring
Actin filaments are thin, flexible protein threads
Actin filament
(two-stranded helix)
ATP hydrolysis decreases the stability
of the actin polymer
Actin-binding proteins control the behavior
of actin filaments in vertebrate cells
Forces generated in the actin-rich cortex
move a cell forward
Actin filaments allows animal cell to migrate

Toward the plus end
of the filaments
A web of actin filaments pushes the leading edges
of a lamellipodium forward
Actin filaments in lamellipodia
Actin filament dynamics
Formins help drive the elongation of actin filaments
The short tail of a myosin-I molecule contains sites
that bind to various components of the cell,
including membranes
1 tail
1 head
Plus end
Activation of GTP-binding proteins has a dramatic effect
on the organization of actin filaments in fibroblasts
cortex
lamellipodium
contractile bundles
(stress fibers)
filapodia
Myosin-II molecules can associate with one another
to form myosin filaments
A bipolar myosin filament
Even small bipolar filaments composed of myosin-II
molecules can slide actin filaments over each other, thus
mediating local shortening of an actin filament bundle
Each muscle cell has its
own contractile apparatus
Muscle
Several muscle fibers
Single muscle fiber
(cell)
Nuclei
Plasma membrane
Myofibril
Light
band
Dark
band
Light
band
Z line
Sarcomere
Thick
filaments
(myosin)
Thin
filaments
(actin)
Z line
Sarcomere
Z line
A skeletal muscle cell is packed with myofibrils,
each of which consists of a repeating chain of sarcomeres
The contractile apparatus of skeletal muscle
Sarcomeres are the contractile units of muscle
Myosin II
Muscles contract by a sliding-filament mechanism
A muscle contracts when thin filaments slide
along thick filaments
Sarcomere
Dark band
Z
Z
Relaxed muscle
Contracting
muscle
Fully contracted
muscle
35% shorten
Contracted sarcomere
The sliding-filament model of muscle contraction
A myosin molecule walks along an actin filament
through a cycle of structural changes
Power stroke
The mechanism of filament sliding
How a motor neuron stimulates muscle contraction
Motor neuron
axon
Mitochondrion
Action potential
Synaptic
terminal
T tubule
Endoplasmic
reticulum (ER)
Myofibril
Plasma membrane
Sarcomere
Ca2
released
from ER
T tubules and sarcoplasmic reticulum
surround the myofibrils
In skeletal muscle, contraction involve Ca2+ signaling
How a motor neurons stimulates muscle contraction
Thin filament, showing the interactions among actin,
regulatory proteins, and Ca2+
Myosin-binding sites blocked
Tropomyosin
Actin
Ca2-binding sites
Troponin complex
Ca2 floods the
cytoplasmic
fluid
Myosin-binding sites exposed
Myosin-binding site
Skeletal muscle contraction is controlled by troponin
The cytoskeleton gives a cell its shape and allows the cell
to organize its internal components
Microtubules
Actin filaments
Nucleus
Intermediate filaments
Intermediate filaments form a strong,
durable network in the cytoplasm of the cell
Intermediate keratin filaments
Intermediate filaments are like ropes made of long,
twisted strands of protein
Central rod domain
Globular region
Intermediate filaments strengthen animal cells
Desmosome
Intermediate filaments can be divided into
several categories
Plectin aids in the bunding of intermediate filaments and
links these filaments to other cytoskeletal protein network
Plectin
Intermediate filament
Microtubule
Intermediate filaments support and strengthen
the nuclear envelope
lamin