Fundamentals of Cell Biology
Chapter 5: The Cytoskeleton and
Cellular Architecture
Chapter Summary: The Big Picture (1)
• Chapter foci:
– Cytoskeletal proteins form a skeleton inside
the cell
– Intermediate filaments provide the cell with
mechanical strength
– Microtubules are associated with cellular
trafficking
– Actin is responsible for large-scale movements
– Eukaryotic cytoskeletal proteins evolved from
early prokaryotes
Chapter Summary: The Big Picture (2)
• Section topics:
– The cytoskeleton is represented by three
functional classes of proteins
– Intermediate filaments are the strongest, most
stable elements of the cytoskeleton
– Microtubules organize movement inside a cell
– Actin filaments control the movement of cells
– Eukaryotic cytoskeletal proteins arose from
prokaryotic ancestors
The cytoskeleton is represented by three
functional classes of proteins
• Key Concepts:
– The cytoskeleton is a complex mixture of 3 different types of
proteins that are responsible for providing mechanical
strength to cells and supporting movement of cellular
contents.
– The most visible form of cytoskeletal proteins are long
filaments found in the cytosol, but these proteins also form
smaller shapes that are equally important for cellular
function.
– The structural differences between the 3 protein types
underscores their 4 different functions in cells.
Cytoskeleton
• occupies large portion
of cytosol and
appears to link
organelles to each
other and to plasma
membrane
• 3 elements:
IFs
MTs
Actin
• Elements do not form
mixed polymers
Figure 05.01: The cytoskeleton forms an
interconnected network of filaments in the
cytosol of animal cells.
IFs are the strongest, most stable
elements of the cytoskeleton
• Key Concepts (1):
– Intermediate filaments are highly stable polymers
that have great mechanical strength.
– Intermediate filament polymers are composed of
tetramers of individual intermediate filament
proteins.
– Several different genes encode intermediate
filament proteins, and their expression is often celland tissue-specific.
IFs are the strongest, most stable
elements of the cytoskeleton
• Key Concepts (2):
– Intermediate filament assembly and disassembly
are controlled by posttranslational modification of
individual intermediate filament proteins.
– Specialized intermediate-filament-containing
structures protect the nucleus, support strong
adhesion by epithelial cells, and provide muscle
cells with great mechanical strength.
IFs provide mechanical strength
to cells
Figure 05.02: Two types of intermediate
filaments.
Figure 05.03: Intermediate filaments have
the most mechanical strength of the
cytoskeletal proteins.
IFs are formed from a family of related
proteins
The primary building block of IFs is a
filamentous subunit
• α-helices in the
central rod domain
Figure 05.04: The central rod domain of
intermediate filament proteins forms an alpha helix.
The head (and tail) regions form globular shapes.
IF subunits form coiled-coil dimers
Figure 05.05: A model for intermediate
filament assembly. The coiled coil formed by
the dimer formed the structural basis for the
strength of intermediate filaments.
Heterodimers overlap to form
filamentous tetramers
• Coiled-coils align to
form antiparallel
staggered structures
Assembly of a mature IF from
tetramers occurs in 3 stages
Figure 05.05: A model for intermediate filament assembly. The coiled coil formed by the
dimer formed the structural basis for the strength of intermediate filaments.
Posttranslational modifications control
the shape of intermediate filaments
• Chemical modification of IF controls their shape
and function
– Phosphorylation-dephosphorylation
– Glycosylation
– Farnesylation
– Transglutamination of head and tail domains
IFs form specialized structures
Keratins in epithelium
Costameres
Figure 05.06: Keratin expression patterns
vary in different epithelial tissues.
Figure 05.07: Costameres link the contractile
apparatus of muscle cells to the plasma
membrane and extracellular matrix.
Microtubules (MT) organize movement
inside a cell
• Key Concepts (1):
– MTs are hollow, tube-shaped polymers comprised
of proteins called tubulins.
– MTs serve as “roads” or tracks that guide the
intracellular movement of cellular contents.
– MT formation is initiated at specific sties in the
cytosol called MT-organizing centers. The basic
building block of a MT is a dimer of two different
tubulin proteins.
MTs organize movement inside a cell
• Key Concepts (2):
– MTs have structural polarity, which determines the
direction of the molecular transport they support.
This polarity is caused by the binding orientation of
the proteins in the tubulin dimer.
– The stability of MTs is determined, at least in part,
by the type of guanine nucleotides bound by the
tubulin dimers within it.
– Dynamic instability is caused by the rapid growth
and shrinkage of MTs at one end, which permits
cells to rapidly reorganize their MTs.
MTs organize movement inside a cell
• Key Concepts (3):
– MT-binding proteins play numerous roles in
controlling the location, stability, and function of
microtubules.
– Dyneins and kinesins are the motor proteins that
use ATP energy to transport molecular “cargo”
along MTs.
– Cilia and flagella are specialized MT-based
structures responsible for motility in some cells.
MT cytoskeleton is a network of "roads"
for molecules "pass to and fro"
MT assembly begins at a
MT-organizing center (MTOC)
Figure 05.08: The distribution of microtubules
in a human epithelial cell. The microtubules
are stained green and the DNA is stained red.
The MTOC contains the gamma tubulin
ring complex (γTuRC) that nucleates
MT formation
• Centrioles
• Pericentriolar material
• gamma (γ ) tubulin
Figure 05.09:
The structure
and location of
the centrosome.
The primary building block of MTs is an
alpha-beta tubulin dimer
• α - and β -tubulin bind
together to form
stable dimer
• If purified α-β tubulin
dimers bound to GTP
are concentrated
enough (critical
concentration), they
spontaneously form
MTs
Figure 05.10: A three
dimensional model
of the dimer formed
by α- and β-tubulin.
Figure 05.11: In vitro assembly
of microtubules is spontaneous
and GTP-dependent. The graph
represents the turbidity of a
solution of α-β tubulin dimers
over time.
MTs are hollow "tubes" composed of 13
protofilaments
• Polymers of dimers  sheet composed of 13
protofilaments  folds into a tube
• GTP binding and hydrolysis regulate MT
polymerization and disassembly
Figure 05.12: A simple model of
microtubule assembly.
The growth and shrinkage of MTs is
called dynamic instability
• Some microtubules
rapidly grow and
shrink in cells =
dynamic instability
Figure 05.13: Growth and shrinkage of microtubules in a living
cell. The microtubules have been tagged with a fluorescent
molecule, and recorded by video over time.
• Elongation is at the
+ end by GTP-bound
dimers
Figure 05.14: The growth of microtubules begins at the gamma
tubulin ring and continues as long as the plus end contains
GTP-bound tubulin dimers.
Catastrophe?
• What happens when the
supply of GTP-bound
tubulin dimers runs out?
1) MT depolymerizes
at the + end
OR
2) Capping proteins
prevent
depolymerization
Figure 05.15: Two fates of the plus
ends of microtubules.
Some MTs exhibit treadmilling
• In cases where
neither end of MT is
stabilized, tubulin
dimers are added to
the + end and lost
from the - end
• Overall length of
these MTs remains
fairly constant, but the
dimers are always in
flux
Figure 05.16: Treadmilling in microtubules.
Benefits of dynamic instability
Allows cells to have
– flexibility with
trafficking during
cell movement
– ability to exert force
by bonding with
cargo molecules
Figure 05.17: Microtubules exert enough
force to move cargo by dynamic instability.
Figure 05.18: Longitudinal and lateral
bonds make microtubules strong.
MT-associated proteins regulate the
stability and function of MTs
• “MAPs” = capping proteins, rescue-associated
proteins, and proteins that govern the motion
• motor protein = special type of MAP that
transports organelles/vesicles
– Dyneins and kinesins
Motors
Figure 05.19: The structure of
dynein and kinesis, the two most
common motor proteins that bind
to microtubules.
Figure 05.20: How a microtubule motor protein
moves along a microtubule.
Cilia and Flagella
Axoneme
Sliding dynein = whip
movement
Figure 05.24: The coordinated motion of a
cilium and a flagellum.
Figure 05.23: The structure of an axoneme.
Actin filaments control the movement
of cells
• Key Concepts (1):
– Actin filaments are thin polymers of actin proteins.
– Actin filaments are responsible for large-scale
changes in cell shape, including most cell
movement.
– Actin filament polymerization is initiated at
numerous sites in the cytosol by actin-nucleating
proteins.
– Actin filaments have structural polarity, which
determines the direction that force is exerted on
them by myosin motor proteins.
Actin filaments control the movement
of cells
• Key Concepts (2):
– The stability of actin filaments is deteremined by
the type of adenine nucleotides bound by the
actin proteins within them.
– Actin-binding proteins play numerous roles in
controlling the location, stability, and function of
actin filaments.
– Cell migration is a complex process, requiring
assembly and disassembly of different types of
actin filament networks.
The building block of actin filaments is
the actin monomer
• Smallest diameter of
cytoskeletal filaments
– 7nm
“microfilament”
• Great tensile strength
• Structural polarity
 + end – barbed end
 - end – pointed end
• Often bound to
myosin
Figure 05.25: The
general structure of an
actin filament. The
lateral and longitudinal
bonds holding actin
monomers together
are indicated at right.
Figure 05.26: An electron
micrograph of an actin
filament partially coated
with mysoin proteins.
Actin found in wide variety of locations
and configurations
Figure 05.27: A number of different actin filament-based structures in cells.
ATP binding/hydrolysis regulate actin
filament polymerization and disassembly
• + ATP = polymerization
• ATPADP = depolymerization
Figure 05.28: The structure
of an actin monomer. A
ribbon model, derived from a
crystalized form of the
protein.
Actin polymerization occurs in 3 stages
Figure 05.29: The three stages of actin filament assembly in vitro.
Actin filaments have structural polarity
• Actin filaments undergo treadmilling
Figure 05.30: Treadmilling in actin
filaments. Note the similarity of this
treadmilling with that shown for
microtubules in Figure 5-16.
6 classes of proteins bind to actin to
control its polymerization/organization
1. Monomer-binding
proteins regulate
actin polymerization
Figure 05.31: The
structure and
function of
profilin, an actin
monomer-binding
protein.
2. Nucleating proteins
regulate actin
polymerization
Figure 05.32: ARP2/3 nucleates
the formation of a new actin
filament off the side of an
existing filament.
6 classes of proteins bind to actin to
control its polymerization/organization
3. Capping proteins affect
the length and stability
of actin filaments
4&5. Severing and
depolymerizing
proteins control actin
filament disassembly
6. Cross-linking proteins
organize actin
filaments into bundles
and networks
Figure 05.34: Three forms of crosslinked
actin filaments created by different
crosslinking proteins.
Cell Migration
• Actin-binding motor proteins exert force on actin
filaments to induce cell movement
• Cell migration is a complex, dynamic
reorganization of an entire cell
• Migrating cells produce three characteristic
forms of actin filaments: filopodia, lamellopodia,
and contractile filaments
Filopodia
Figure 05.35: Different forms of actin in stationary and migrating cells.
Myosins are a family of actin-binding
motor proteins
• myosins =
multisubunit proteins
organized into 3
structural domains
– Motor
– Regulatory
– Tail
Figure 05.36: Myosin proteins contain three
funtional domains
Contractile cycle
• Myosins move
towards one end of
the actin filaments
– myosin V crawls
towards the - end,
all other myosins
crawl towards the +
end
– Allows for
movement of cell
Figure 05.37: The contractile cycle of myosin.
Striated muscle contraction is a wellstudied example of cell movement
Figure 05.38: The anatomy of a skeletal muscle. The sarcomere contains actin and myosin
arranged in parallel bundles.
Eukkaryotic cytoskeletal proteins arose
from prokaryotic ancestors
• Modern prokaryotic cells express a number of
cytoskeletal proteins that are homologous to
eukaryotic cytoskeletal proteins and behave
similarly
– Vimentin (IF)
– FtsZ (MT)
– MreB and ParM (actin)
• Shared properties seem to include protection of
DNA, compartmentalization and motility.