Microfilaments and actin

Question 1

What are the 8 different arrangements of microfilaments?

Answer 1

Microvilli
Cell cortex
Adherens belt
Flipodia 
Lamellipodium 
Cell cortex
Stress fibres
Contractile ring

Question 2

Microvilli

Answer 2

Microfilaments which exist as a tight bundle of filaments

Question 3

Cell cortex

Answer 3

Microfilaments found less ordered beneath the plasma membrane where they provide support and organisation

Question 4

Adherens belt

Answer 4

In epithelial cells microfilaments form a contractile band around the cell to provide strength to the epithelium

Question 5

Lamellipodium

Answer 5

In migrating cells a network of microfilaments are at the front of the cell from which bundles of filaments may protrude

Question 6

When do contractile rings form?

Answer 6

In cytokinesis to separate the cell to two daughter cells

Question 7

What is actin required for?

Answer 7

Cell movement 
Cell division 
Vesicle transport 
Phagocytosis
Movement of organelles

Question 8

What 3 groups are actin isoforms classified into?

Answer 8

alpha-actin (associated with contractile structures)
beta-actin (found in cell cortex)
gamma-actin (filaments in stress fibres)

Question 9

Why is actin described as dynamic?

Answer 9

Length and organisation can change

Question 10

What is myosin?

Answer 10

molecular motor proteins that work with actin filaments

Question 11

G actin

Answer 11

Globular monomer that is the building block of actin

Question 12

G actin polypeptide folding

Answer 12

Actin polypeptide folds into 4 subdomains that generate two lobes separated by a cleft. At the base of the cleft there is an ATPase fold that binds ATP complexed with a Mg2+ ion.
The floor of the cleft acts as a hinge that allows the lobes to flex relative to each other. Binding of ATP induces a shape change.

Question 13

G actin and F actin

Answer 13

Actin exists as a glubular monomer (G actin) and as a filamentous polymer (F actin) that is a long chain of G-actin subunits

Question 14

Why is actin an ATPase?

Answer 14

It will hydrolyse ATP to ADP and Pi

Question 15

What happens when cations are added to a solution of G actin?

Answer 15

Polymerisation of G actin to F actin filaments.

Reversible process

Question 16

How are subunits arranged in actin filaments?

Answer 16

Helical structure and can be considered as two strands wound around each other

Question 17

F actin- polarity

Answer 17

Two strands of monomers where all subunits are oriented the same way so the whole filament exhibits polarity. One end of the element is favoured for the addition of actin subunits (+ end) and the other is favoured for subunit dissociation (-end)

Question 18

What is in contact with the + and - end of the F actin monomer strands?

Answer 18

The + end is in contact with neighbouring subunits

The - end is exposed to the surrounding solution

Question 19

F actin

arrow head

Answer 19

Arrow head points to the - end so the - end is called the pointed end
The + end is called the barbed end and is where the actin filaments grow

Question 20

What 3 steps are needed in actin polymerisation?

Answer 20

Nucleation phase
Elongation phase
Steady state phase

Question 21

Nucleation phase

Answer 21

Marked by a lag period in which G actin subunits combine into an oligomer of two or three subunits

Question 22

Elongation phase

Answer 22

Short oligomer quickly increases in length by addition of actin monomers to both ends. As F actin filaments grow the concentration of G actin monomers decrease until equilibrium is reached between the filament end and monomers.

Question 23

Steady state phase

Answer 23

G actin monomers exchange with subunits at the filament end, no net change in total length of filament

Question 24

Critical concentration

Answer 24

The concentration of free G-actin at which growth at one end is balanced by loss at the same end.