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Lecture notes BIOS5030 Cell Biology (BIOS5030) on Actin Cytoskeleton (part 2)

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BIOS5030 Cell Biology (BIOS5030)

Institution
The University Of Kent (UKC)

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Uploaded on June 13, 2024
Number of pages 5
Written in 2023/2024
Type Class notes
Professor(s) Dr gourlay, mulvihill, shepherd, mulligan, goult
Contains All classes

bios5030
cell bio
cell biology
biochemistry
university of kent
bioscience
actin cytoskeleton
cytoskeleton
actin
cell movement

Institution
The University of Kent (UKC)
Education Unknown
Course
BIOS5030 Cell Biology (BIOS5030)

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Week Number: 9
Seminar Date: Wednesday 4th October
Time: 10am-11am
Module: Cell Biology BI503

Actin Cytoskeleton part 2

Actin Filament Treadmilling:

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.

1

, 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

2

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