Lecture 7: Actin - Exam 5

Question 1

When is there no lag phase?

Answer 1

If short filaments added at the start to act as nuclei

Question 2

Describe treadmilling and the slow hydrolysis of ATP in actin filament polymerization.

Answer 2

First, G-actin is in an ATP bound state, primarily. Upon polymerization, ATP hydrolysis increases about 40,000X. Although hydrolysis is fast, phosphate release is relatively slow.
This allows treadmilling.
(Draw this too)

Question 3

Describe the nucleation and growth of actin filaments.

Answer 3

Formins initiate and stimulate the elongation of actin filaments.
Formins associate with Profilin to increase local concentration of ATP bound G-actin.
Again, need to draw

Question 4

Describe the initiation and growth of actin branches with Arp 2/3.

Answer 4

Cdc42-GTP (a small GTPase like Rab or Ras) activates WASp (a nucleation promoting factor) which then activates Arp 2/3 branched actin filament networks.

Question 5

Describe the stabilization of actin filaments.

Answer 5

Actin filaments can be stabilized by capping proteins that bind to their plus or minus ends.
-There are filament-stabilizing proteins (e.g. tropomyosin) that bind along their length

Question 6

Actin filaments in bundles are cross-linked into parallel arrays by?

Answer 6

ABPs.
In contrast, networks are formed by large flexible proteins that cross-link orthogonal filaments.

Question 7

What does Cofilin do?

Answer 7

Binds to and severs actin filaments. The newly formed filament ends are then available for the polymerization or depolymerization of actin monomers.
So, some actin binding proteins modify existing rather than stabilize.

Question 8

Describe Arp 2/3-dependent actin assembly during endocytosis.

Answer 8

Endocytosis assembly factors recruit nucleation promoting factors (NPF). NPFs activate Arp2/3 to drive vesicle away from plasma membrane.

Question 9

Attachment of stress fibers to the plasma membrane at focal adhesions.

Answer 9

Talin is bound to integrin, integrin bound to extracellular matrix and bound to other proteins inside the cell.

Question 10

Describe actin filament remodeling at the leading edge.

Answer 10

Rho family members activate WASP and Arp2/3 as well as formin, leading to initiation and elongation of branched actin filaments.
-Profilin activates ADP-actin monomers through ADP/ATP exchange to promote filament extension.
-Cdc42-GTP activates WASp that activates Arp2/3 branches. cleavages of existing filaments by cofilin provides new plus ends for the initiation and growth of new filaments.

Question 11

Cell migration

Answer 11

Extension of leading edge
Attachment to substratum
Retraction of trailing edge

Question 12

Cell-cell contacts at adherens junctions are mediated by?

Answer 12

Cadherins, which serve as sites for attachment of actin filaments.

Question 13

In sheets of epithelial cells, adheren junctions form?

Answer 13

A continuous belt of actin filaments around each cell.