Molecular & Cell Biology

S. Rahgozar,PhD

University of Isfahan

Faculty of Science

2. Cytoskeleton and Cell Movement

2.1. Actin Filaments

1389-90

Assembly and disassembly of actin filaments

Actin Filaments Actin monomer (G actin)

Barbed end

Pointed end

Assembly of actin monomers

Treadmilling cycle

ADP-bound actin, which is

primarily at the pointed

end of the filament,

dissociates more readily

from the filament than

ATP-bound actin.

In contrast, actin bound to

ATP associates more

readily with the rapidly

growing barbed ends.

ATP binding and

hydrolysis play a key role

in the dynamic behavior of

actin filaments.

A number of different types of actin binding proteins remodel

or modify existing filaments.

ADF/cofilin proteins bind to actin

filaments and enhance the rate of

dissociation of ADP-actin monomers

from the pointed end.

Profilin enhances the rate of

association of ATP-actin monomers

to the barbed ends.

1- ADF/cofilin and

Profilin

o Depolimerization/ polymerization

ADF/cofilin can sever actin

filaments.

ADF/cofilin binds to ADP-actin ,

severing the filaments and creating

new barbed and pointed ends.

o Filament severing

2- Formin

3- Arp2/3

Regulation of actin filament organization by actin-binding proteins

Organization of actin filaments

Actin – bundling proteins

Actin binding

domain

14nm

40 nm

Protrusions of the cell

surface

Association of actin filaments with the plasma

membrane

The network of actin

filaments and associated

actin-binding proteins is

called the cell cortex.

Cell cortex determines

cell shape and is

involved in a variety of

cell surface activities,

including movement.

(fodrin)

Association of the erythrocyte cortical cytoskeleton with the plasma

Attachment of stress fibers to the plasma membrane

at focal adhesions

Stress fibers and

focal adhesions in a

cultured human

fibroblast

Attachment of actin filaments to adherens junctions