Actin Microfilaments

Question 1

Actin structure

Answer 1

Made of actin monomers.
In presence of ATP/cations, G-actin assembles to form 2-stranded helical filaments (F-actin). Bound ATP is stable.
Polarized.
on/off rates determine which end grows, grow by addition of ATP.

Question 2

Actin fx.

Answer 2

Critical for cell shape, movement, and polarity.

Question 3

Accessory proteins

Answer 3

60 accessory proteins regulated 60 accessory proteins.

• Profilin: binds actin –> polymerization
• Capping: cap + or - end to inhibit pol/depol. STOPS growth.
• Cofilin: severs actin filaments, induces depol.

Question 4

Regulated at 2 major steps

Answer 4

1. ) Nucleation: not sponatneous

2. ) Extension (after nucleation)

Question 5

Nucleation

Answer 5

Making new actin filaments. Nucleating factors stimulate actin polymerization.

Question 6

Arp2/3

Answer 6

mimics G-actin dimer to stimulate nucleation of monomeric G-actin.
Binds single actin monomer. nucleation site.
Branched filaments. starts at angle, branching.

Question 7

Formin

Answer 7

Parallel bundles.
Typically inactive, results in actin filament formation.
Long w/ multiple filaments, cable-like
Divison

Question 8

Make actin from scratch…

Answer 8

1. ) Create nucleation center: binding of 3 monomer, this is inefficient, will not be spontaneous
2. ) Building filament is easy: spontaneous.

Question 9

Profilin

Answer 9

Sequesters G-actin and ADP-ATP exchange. Growth regulator.

Question 10

Actin in Epithelial junctions

Answer 10

Anchors proteins involved in TJ’s and adherens junctions that hold together epithelial cells
Proteins anchored are: cadherins and catenins
Decreases association of actin, decreased adhesion… epithelial to mesenchymal transition (cancer)

Question 11

Actin in microvilli

Answer 11

On apical surface of epithelial cells.
Microvilli are made of actin bundles, held together by cross-linking proteins (villin & fimbrin)
Attached to PM by myosin-1 and calmodulin
Microvilli increase SA of epithelial cells for housing cell transporters.
Their loss: microvilli inclusion disease (diarrhea, dehydration)

Question 12

Myosin structure

Answer 12

Actin associated.
Coiled coil dimers that polymerize into filaments
2 heads that bind to actin & ATP, tail binds cargo
Towards + end using ATP

Question 13

3 classes of myosin

Answer 13

Cargo transport (+)
Muscle contraction
Unconventional myosins

Question 14

Myosin II

Answer 14

Coiled coil complex of myosin filaments, forms thick filament that walks along actin thin filament in muscle contraction.
Thick filament has many actin binding heads, only some bound at a time. (vs 50:50 ratio in cargo myosins)

Question 15

Myosin I and V

Answer 15

Membrane associated, tails bind organells, move organelles around.

Question 16

Amoeboid protrustion

Answer 16

Rac and Wasp signaling. F-actin polymerize lamillipodia ARP2/3
Fillipodia form via formins. Sense enviro.

Question 17

Attachment

Answer 17

lamillipodia attach

Question 18

Traction

Answer 18

cell pulled by tension from actin protrustions and sliding myosin motors

Question 19

Detachment

Answer 19

Attachments released via depolymerization by COFILIN

Question 20

Wiskott Aldrich Syndrome (WAS)

Answer 20

Wasp mutations (protrustions). Thrombocytopenia.

Question 21

Lisenchephaly

Answer 21

Gyri are smooth. No cofilin.

Question 22

Actin and cell division

Answer 22

Contracts through action of mysoin heads and ATP hydrolysis. Timing regulated.

Question 23

Asymmetric cell division

Answer 23

Enucleation of RBC
Platelet formation
Can be left connected to canals with persistent ring
Epithelial cell division, vertical perfect axis, ensure monolayer