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Actin is like a scaffold; it has other proteins and factors that assemble the actin at the right
time and place. Two of these proteins are:
Cofilin - an actin binding protein that regulates filament dynamics and depolymerization.
Profilin - small actin-binding proteins found in eukaryotes that are involved in cell
development and motility.
If you look inside cells the actin cytoskeleton is more dynamic than it is when placed in a test
tube because it is acted on by proteins; these are cofilin and profilin.
How can we make actin more dynamic?
Actin polymerization cycle – very important (Revise)
1) This represents a simple actin filament with a positive and negative end. ATP
hydrolyses filament to release the energy.
2) Cofilin cycle – a small protein that binds actin filaments in a monomeric form. It only
binds to actin in an unenergetic state (only binds to the – end). Actin filament
changes conformation and makes -end unstable, cofilin binds to the – end as it twists
making it more unstable.
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, Cofilin helps the filament to stabilize by breaking chunks off and creating a seed that
actin filaments can grow from. When it falls apart you have single actin (monomer)
filament (ADP) that falls into the profilin.
3) Profilin cycle – The profilin’s job is to reenergize the actin monomer and bind them
together. When it binds to the filament it opens a cleft, where the ADP pops out and
is replaced by ATP. Replenishing the pool of ATP actin molecules that can be added
to the + end.
Note: Actin should not be polymerized quickly- the energetic actin needs to be
controlled.
4) Thymosin cycle – This stores and controls the availability of the (+) energetic actin.
Thymosin binds to the energetic actin to stop it from adding to the + end. Instead, it
stores the actin for when it is needed, for example, when you get a cut you get a
release of actin b4 flow to the wound and build a scab. In platelets thymosin b4 holds
a large reservoir of G-actin.
Other proteins that manipulate actin filaments are capping proteins. Capping proteins bind to
the ends of filaments to regulate monomer addition, we get + and – end cappers. This keeps
the actin filament the same length, stopping it from growing or shrinking.
(-) End cappers = tropomodulin – these have an inhibiting addition or removal of actin
filaments.
(+) End cappers = Gelsolin – these have the ability to sever actin filaments to control and
reduce the network of actin filaments.
YouTube video – available to describe the cycle of actin dynamics
Actin at the leading edge of a motile cell.
(YouTube video available)
Electron micrograph of actin filament branch formation at the leading edge
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