Actin Binding Protiens And Cell Motility

Question 1

What is the cell cortex/ actin cortex

Answer 1

A network of actin on the inner face of the plasma membrane that can enable cells to:

crawl/move

Undergo phagocytosis

And constrict during cell division

Question 2

What are actin binding protiens

Answer 2

They are protiens that bind to actin and regulate the assembly/disassembly and rearrangement of actin networks

This include more then 100 diff protiens

Question 3

What are the 8 types of actin binding proteins

Answer 3

Filament nucleating

Monomer-sequestering

End blocking (capping)

Monomer polymerizing

Depolymerizing

Cross linking and bundling

Filament severing

Membrane binding

Question 4

What are filament nucleating protiens

Answer 4

Filament nucleation (formation) of actin filament is slow so these protiens enhance the rate the they’re formed when a seed filament if present

Question 5

What are the two categories of filament nucleating protiens

Answer 5

The ARP 2/3 complex

Formins

Question 6

What does the ARP 2/3 complex do

Answer 6

It binds to the sides of an existing actin filament meaning it begins to create branches

It remains at the pointed end of the new branch and has a similar structure to actin monomers

Question 7

What do formins do

Answer 7

They make UNBRANCHED filaments by staying accosiated with the barbed end of the actin filimants

They promotes fast ELONGATION of the the actin filaments

Question 8

What are monomer sequestering protiens

Answer 8

They bind to actin atp monomers and prevent them from being added to the growing actin filament

Basically hide them away from the actin filament

This means they can alter the levels of available monomers in certain regions and keep them in separate actin pools for use later

Ex thymosins

Question 9

What are endblocking (capping) protiens

Answer 9

The regulate the length of the actin filaments by binding at either end (barbed or pointed, + or -)

If capped at the barbed end the pointed end gets degraded but if capping protien at the pointed end it doesn’t get degraded

Question 10

What are monomer polymerizing protiens

Answer 10

They bind to actin monomers to promote the growth of actin filaments

They do this by promoting the replacement of adp with atp. Since the monomers only fall off the filament when they have adp.

If we quickly change it to atp the monomers that fall off the pointed end can quickly get added back into the barbed end

Ex. Profilin

Question 11

What are depolymerizing protiens

Answer 11

They bind to actin adp at the pointed end to speed up their depolymerization (dissociation)

Ex. Cofilin

Question 12

What are cross linking and bundling protiens

Answer 12

The have multiple acting binding sites that let them change the 3D organization of filaments

They can cause filament to be either parallel (bundled) or perpendicular (cross linked) to each other

Ex. Bundling: vilin and fimbrin

Cross linking : filamin

Question 13

What are filament severing protiens

Answer 13

Break an existing actin filament in two

Ex. Gelsolin, cofilin

Question 14

What are membrane binding protiens

Answer 14

They link actin filaments to the plasma membrane

This allows the plasma membrane to protrude outward (cell locomotion) or inward (phagocytosis)

Ex. Spectrin which connects actin to the red blood cell membrane

Question 15

What does profilin do

Answer 15

Polymerizes actin monomers

Question 16

What does cofilin do

Answer 16

Depolymerizes actin monomers

Question 17

What does arp 2/3 do

Answer 17

Makes branched filament on the actin filament

Question 18

What is the first step in cell motility

Answer 18

1. The movement of the cell begins by a protrusion of the cell in the direction is wants to move

Called the lamellipodium

Question 19

What is the second step in cell motility

Answer 19

A part of the protrusion (lamellapodium) anchored to the surface that is moving on

Question 20

What is the 3rd step in cell motility

Answer 20

The bulk of the cell is pulled to the front while the parts connected to the surface stay in place

Question 21

What is the fourth step in cell motility

Answer 21

The adhesive contacts to the surface break and the trailing edge (tail) of the cell retracts

Question 22

What if the first few step in how the lamellipodium forms

Answer 22

First a stimulus is received at the cell surface

Then the arp 2/3 complex near the stimulation gets activated

The arp 2/3 binds to the side of an actin filament and the actin monomers bind to it to make a new branch

The polymerization of the new branch is promoted by profilin

Question 23

What are the last few step in how the lamellipodium forms

Answer 23

More new ARP 2/3 complexes bind to the side of actin filaments to make new branches

the old branches are capped at their barbed ends (where growth is happening) to make the filaments short and stable

The newer branches keep growing and pushed the membrane of the lamellipodium forward

The older capped filaments/ branches are disassembled on their pointed end which is enhanced by cofilin

Question 24

What are traction forces in relation to cell motility

Answer 24

When the cell grips the surface at adhesion pints in the membrane called focal adhesions

Question 25

What are focal adhesions in relation to cell motility

Answer 25

These are structures in the cell membrane where integrin protiens connect to the actin filaments

Question 26

What are integrin protiens in relation to cell motility

Answer 26

These are transmembrane protiens in the cell that mediate the interaction between the actin filaments in the cell and the surface outside the cell

They link the inside to the outside but are temporary, not always linked to one same spot

Question 27

What is at the rear of the lamellipodium and what does it do

Answer 27

Myosin

uses contraction forces to pull the bulk of the cell forward