Actin Microfilaments

Front 1

Where is actin most present in the cell?
What are other places they are found in?

Back 1

They are present under the cell membrane--cell cortex
Also find in microvilli, stress fibers, lamellipodia.

Front 2

What are the two forms of Actin?

Back 2

1) G-Actin - unassembled protein. Has ATP binding site.

2)F-actin - assembled protein

In assembled form, you have several actin microfilaments coming together in a helix

Front 3

What are the 4 condtions for Actin polymerization?

Back 3

1. subunit concentrations
2. Ca+
3. ATP, can also be ATP analogoes like ADP (but not atp hydrolysis) Binding of ATP changes conformation of G-actin to make it assembly ready.
4. Salt concentration

Front 4

What does ATP hydrolysis do? is it delayed?

Back 4

There is delayed ATP hydrolysis. creates energy for tread milling

Front 5

What are the three stages of actin polymerization?

What helps the lag phase increase speed?

Back 5

1. nucleation - Lag -phase
-few monomers come together to form trimers.

2. Elongation - fast step
-once strands are formed, actin subunits are added to the end, rapidly growing in length

3. Steady state equilibrium
-plateu of microfilament growth.
-turnover=new growth
-steady state concentration

**addition of nucleating seeds helps to increase the speed of the lag phase. They are preformed trimers.

Front 6

Explain the polarity of Actin

-What are the two ends?
-What causes the polarity?

Back 6

There is a + end and - end.

+end: faster growing side

-end: slower growing side

Rate of additions depends on free Actin-G actin subunits.
The rate of disassociation is similar on both ends.

Front 7

What is tread milling? How is it achieved?

Back 7

It's when the rate of turnover and synthesis is at a steady state equibrium. On the +end, units are quickly added and ATP hydrolysis lags. On the -end, ATP hydrolysis catches up.

The energy that is stored from atp hydrolysis and contributes to critical concentration.

It only occurs during critical concentration. The monomer concentration has to be greater than Cc for +end and lesser than Cc for - end.

+end Cc< The critical concentration<-end Cc.

Front 8

What main drug affects Actin filaments? How?

What 2 things does the fungus Amanita Phalloides - Death cap mushroom do?

Back 8

Phalloidin binds and stabilizes filaments.

Has phalloidin. Also has AMANTIN that stops RNA polymerase.

Front 9

How can actin filaments be divided into two groups?
What parts of cells belong in those two groups?

Back 9

1. Motile - Contractile ring (cytokinesis), filopodia, lamellipodia

2. Non-motile: Microvilli, stress fibers

Front 10

There are several actin binding proteins that help out the function of actin?

What ABP is responsible for cutting/severing actin to control its length? What does it require?

Back 10

With Ca+, GELOSIN cuts F-actin into small pieces.

Front 11

Which ABP is resonsible for actin cross-linking?

Back 11

Filamin works as dimer to crosslink actin filaments into gel. Also important in scaffolding, links actin to membrane.

Front 12

What do bundling ABPs do? What are two examples?

Back 12

They form arrays of actin microfilaments.

1. Alpha actinin makes contractile bundling -loose packing

2. Fimbrin makes parallel bundling - tight packing (microvilli)

Front 13

What ABP is found in microvili? Where are the + ends located?

Back 13

Fimbrin

They are located at the tip of the microvilli.

Front 14

What intestinal disease is associated with Actin? how?

Back 14

Celiac disease is when the microvilli of hte intestines is messed up and you can't absorp nutrients in small intestines.

Front 15

What does actin need to bind to membrane?

Back 15

It always needs an adapter protein

Front 16

What is dystropin? mutations in this results in what disease?

Back 16

It is an ABP that attaches actin to integral glycoprotein complexes. Mutations in this result in Duchenne's muscular dystrophy.

Front 17

RBC has an elaborate cytoskeleton network. What ABP works in the red blood cell?

What disease is associated with it?

Back 17

Spectrin, Ankyrin, Band 3.1

hereditary spherocytic anemias - rbcs are rounder and anemia bc they rupture.

Front 18

What is Thymosin B and Prolifin?

Back 18

Thymosin B: It binds to actin monomers to stop from assembling. Cells have a high concentration of monomers and you need to have some control to prevent constant polymerization.

Prolifin: does the same thing as thymosin, but it can bind readily to G-protein -+end, but not -end.

Front 19

What are two types of actin nucleating proteins?

Back 19

Formin - that elongates actin by nucleating the +end.

Actin related proteins ARP 2 and 3 - They can't make own filaments or coassemble with actin. They bind to actin and when activited, they grow like the + end. They nucleate to make web of actin branches.

Front 20

Where is ARP mostly found in?

Back 20

migrating cells of lamellipodia to nucleate high actin assembly because its moving

Front 21

Explain the actin meshwork of a migrating cell
What does Colifin and Prolifin do???

Back 21

At the leading edge, ARP nucleates the actin. After actin is polymerized, ATP is hydrolyzed and COFILIN destabilizes old actin filaments. The broken parts of the old actin filaments are recharged by PROLIFIN which makes ATP-actin monomers for reassembly.

Front 22

What are the main events in cell movement

Back 22

1. extension/protrusion of leading process- actin polymerization at the + end. Make new focal contacts in front

2. attachment and contraction - actin myosin contractile forces move body up.

3. detachment and contraction of back end