Microfilaments

Question 1

What does the bacterial cytoskeleton contain ?

Answer 1

Proteins that are homologous in structure to eukaryotic actin and tubulin and also other protein classes, possibly including intermediate filaments

Question 2

What does the hydrolysis of ATP destabilise ?

Answer 2

The actin polymer

Question 3

When does treadmilling occur ?

Answer 3

Treadmilling occurs with actin, where monomers are added at the plus end and lost at the minus end

Question 4

What does dynamic instability involve ?

Answer 4

Dynamic instability involves a switch from growth to shrinkage (or opposite) at the plus end

Question 5

What does alpha actinin help with ?

Answer 5

To create a scaffold

Question 6

Explain the function of Profilin ?

Answer 6

• Prevents globular actin from nucleating (ie forming new filaments)
• Binds to ATP bound globular actin
• Enhance formation of longer filaments
• Replenish the pool of ATP–actin monomers by increasing the rate of nucleotide exchange on the bound actin monomer 1000-fold compared with the rate of exchange based on simple diffusion

Question 7

Explain the function of Cofilin ?

Answer 7

• Severs microfilaments

- Helpful when filaments need to be broken up and monomers recycled

Question 8

Cell Migration via Lamellipodium

Answer 8

1. Polymerisation of actin at the leading edge PUSHES plasma membrane forward (PROTRUSION) forming new actin cortex regions
2. New anchor points are made between cell & substratum – via integrins
3. As new anchorage sites are made at the front, some at the back are released (using myosin & actin interactions)

Question 9

What is Lamellipodia ?

Answer 9

Based upon a thin sheet-like branched network of actin filaments

Question 10

What is Filopodia ?

Answer 10

They are highly organised and tightly cross-linked long bundles of unidirectional and parallel actin filaments

Question 11

What does the resulting branching structures push?

Answer 11

The plasma membrane forward. The plus ends of the actin filaments become protected from depolymerising by capping proteins (dark green), while the minus ends of actin filaments nearer the centre of the cell continually disassemble with the help of depolymerising proteins

Question 12

Because of this directional growth and disassembly, individual actin monomers?

Answer 12

Accumulate at the rear, while the web of actin as a whole undergoes a continual forward movement

Question 13

Nucleation of new actin filaments (pink ) is mediated by ?

Answer 13

ARP (Actin Related Protein) complexes (light green) attached to the sides of preexisting actin filaments

Question 14

Myosin-I has a ?

Answer 14

Single globular head that attaches to an actin filament and a tail that attaches to another molecule or organelle in the cell

Question 15

What does this arrangement allow ?

Answer 15

The head domain to move a vesicle relative to an actin filament, which in this case is anchored to the plasma membrane

Question 16

Myosin-I can also bind to ?

Answer 16

An actin filament in the cell cortex, ultimately pulling the plasma membrane into a new shape

Question 17

Activation of Rho family GTPases can have a dramatic effect on ?

Answer 17

The organisation of actin filaments in fibroblasts

Question 18

A small, bipolar myosin-II filament can slide two ?

Answer 18

Actin filaments of opposite orientation past each other

Question 19

Skeletal muscle contraction is triggered by?

Answer 19

The release of Ca2+ from the sarcoplasmic reticulum into the cytosol

Question 20

a Ca2+-release channel in the sarcoplasmic reticulum membrane is opened by ?

Answer 20

A physical linkage to a voltage-gated Ca2+ channel in the T-tubule membrane

Question 21

Skeletal muscle contraction is controlled by?

Answer 21

Tropomyosin and troponin complexes

Question 22

Cross section of the muscle actin filament reveals how ?

Answer 22

Ca2+ binding to the troponin complex (not shown) leads to movement of tropomyosin away from the myosin-binding site

Question 23

Actin exists in a ? and what is it called ?

Answer 23

Monomeric form (g) and polymer form (f) and the polymer is called a ‘microfilament’

Question 24

Actin interacts with partner proteins including

Answer 24

1. Myosin

2. And cofactors that stabilise or destabilise the polymers

Question 25

Actin is responsible for a lot of cellular movement including

Answer 25

1. Phagocytosis
2. Muscle movement
3. Cytokinesis
4. Cell migration