The cytoskeleton: microfilaments essential. Cell biology

1.

Lectures 21 and 22:
The Cytoskeleton:
Microfilaments
Essential
Cell Biology
Third Edition
Chapter 17

2.

The Cytoskeleton Includes Dynamic Networks
Of Microfilaments And Microfilaments

3.

Microfilaments composed of actin are often
found at the cortex of cells. They also form
stress fibers that give shape to the cell. They
are highly dynamic forming networks that
serve as the basis for cellular motility.

4.

Microfilaments Consist Of A Double Helix
Of Actin Monomers Bound To Each Other

5.

Microfilaments (MFs) consist of a double helix
Of actin monomers
Platinum coated microfilaments in a white blood cell
A single microfilament in vitro

6.

Actin Polymerization Can Occur In Vitro.
No Other Proteins Are Required.

7.

Microfilaments like microtubules have plus and
minus ends that are not identical. The plus end
favors polymerization. The minus end favors
depolymerization.
MINUS END
FAVORS
DISSOCIATION
Minus End = “pointed end”
ATP
HYDROLYSIS
PLUS END
FAVORS
ADDITION
Plus End = “barbed end”

8.

Animation: Regulation of Actin Polymerization and
Depolymerization inside cells using Profilin and Cofilin
Profilin binds to actin monomers (G-actin) to aid
polymerization; cofilin binds to microfilaments (f-actin) to
sever them and allow faster depolymerization

9.

3-D Microfilament Networks Are The Basis Of
Dynamic Cell Structures: Network construction
requires accessory proteins that interact with actin

10.

Microfilaments Interact With accessory proteins
(actin binding proteins) In Forming Networks
or rapid Disassembly of networks
Filamin dimer
Cofillin or
Gelsolin
Microfilament
bundling
And crosslinking
proteins

11.

Microfilaments Form Bundles using the
crosslinking proteins α-Actinin And Fimbrin
Stress Fibers
Microvilli
Can shorten
Can not shorten

12.

Microvilli Contain A Microfilament Bundle

13.

Microfilament
elongation in vivo
is aided by the
accessory proteins
profilin and formin.
Profilin binds to
ATP- actin and the
Formin
complex acts as a
building block. The
complex prevents
nucleation.
Formin binds P-A
complexes and guides
them to the growing
(barbed) end of the Microfilament
Profilin
Actin

14.

Animation of Actin Polymerization using Formin

15.

In cells, the ends of microfilaments are capped
with other proteins to control assembly. ARP 2
and ARP 3 cap the minus end of microfilaments.
CAPPING
PREVENTS
ACTIN
DISSOCIATION

16.

ARPs allow binding of minus ends to other
filaments. In this way microfilament networks
can be formed that are used as
superstructures for cell shape and motility.

17.

Dynamics of Actin Networks in a slime mold.

18.

MicroFilament
Networks
Are The
Basis Of
Amoeboid
Movement
THE LEADING EDGE
THE LEADING EDGE

19.

Steps In The Cycle Of Amoeboid Movement
1. Extension of leading edge due to actin
Polymerization.
2. Linking of cell cortex to substratum via
connection of cortical microfilaments to
Adhesion plaques/focal contacts.
3. Rear of cell contracts and moves forward
by interaction of microfilaments with myosin.
4. Microfilaments depolymerize at rear of cell.
Plasma membrane is retrieved by endocytosis.
Actin monomers and membrane vesicles move to
front of cell via microtubules tracks to be reutilized.

20.

Microfilament Networks Are Dynamic At The
Leading Edge

21.

Animation of ARP 2/3 induced branching of
microfilaments as facilitated by WASP proteins

22.

Gab1
Adaptor
Adaptor
Growth
Factor
Receptor
GEF
Auto-inhibited WASP
Cell Ruffling
Response
Actin
polymerization
and branching
ARP 2/3
Complex
Activated WASP G-Protein
“Switch”
Cell Signaling
Controls
The Actin
Cytoskeleton
Formin
Signal
Rho GTPase
Rac GTPase
Formin
polymerization
filopodia extend
WASP ARP 2/3 branching lamellapodia

23.

MicroFilament
Networks
Are the
Basis of
Ameoboid
Movement
THE LEADING EDGE

24.

Steps In The Cycle Of Amoeboid Movement
1. Extension of leading edge due to actin
Polymerization.
2. Linking of cell cortex to substratum via
connection of cortical microfilaments to
Adhesion plaques/focal contacts.
3. Rear of cell contracts and moves forward
by interaction of microfilaments with myosin.
4. Microfilaments depolymerize at rear of cell.
Plasma membrane is retrieved by endocytosis.
Actin monomers and membrane vesicles move to
front of cell via microtubules tracks to be reutilized.

25.

Focal adhesions use hundreds of transmembrane proteins
called integrins for linking the actin cytoskeleton inside
the cell to extracellular matrix fibers such as collagen
outsidethe cell. This linkage has a mechanical function.
Focal adhesions are
dynamically controlled
allowing them to bind,
and unbind from
extracellular matrix
fibers in a reversible
manner. This allows
them to serve as
temporary anchors
during cell movement.

26.

Steps In The Cycle Of Amoeboid Movement
1. Extension of leading edge due to actin
Polymerization.
2. Linking of cell cortex to substratum via
connection of cortical microfilaments to
Adhesion plaques/focal contacts.
3. Rear of cell contracts and moves forward
by interaction of microfilaments with myosin.
4. Microfilaments depolymerize at rear of cell.
Plasma membrane is retrieved by endocytosis.
Actin monomers and membrane vesicles move to
front of cell via microtubules tracks to be reutilized.

27.

Contraction of microfilament networks requires
Myosin II filaments to make microfilaments slide.
Myosin - II
Anchored Cortical
Microfilaments
Rear of Cell
Contracts

28.

The ability of myosin
to walk on actin filaments
is due to a cycle of force
producing conformational
changes that is powered
by ATP hydrolysis. This
cycle is the same as in
skeletal muscle.

29.

Steps In The Cycle Of Amoeboid Movement
1. Extension of leading edge due to actin
Polymerization.
2. Linking of cell cortex to substratum via
connection of cortical microfilaments to
Adhesion plaques/focal contacts.
3. Rear of cell contracts and moves forward
by interaction of microfilaments with myosin.
4. Microfilaments depolymerize at rear of cell.
Plasma membrane is retrieved by endocytosis.
Actin monomers and membrane vesicles move to
front of cell via microtubules tracks to be reutilized.

30.

Endocytosis and
vesicle transport
from back to front.
Depolymerization
And transport of
actin back to front.
Via microtubule
network and motor
proteins.
Movement of
nucleus, cytoplasm
and organelles
back to front via
Myosin-induced
movement along
sub-nuclear stress
fibers.

31.

LAMELLAPODIA = LEADING EDGE – powered by polymerization
FORMIN
WASP
CONTRACTION AT REAR
CORTICAL ACTIN LAYER (ANCHORED)
WHILE CELL ORGANELLES MOVE
FORWARD
ADHESION
PLAQUE/
FOCAL
CONTACT
ADHESION
PLAQUE/
FOCAL
CONTACT
Soon-Tuck Sit, and Ed Manser J Cell Sci 2011;124:679-683