Actin part 2 Flashcards
How many bonds does an actin monomer have?
- Lateral
- Longitudinal
- Lateral: 2
- Longitudinal: 2
Do actin filaments have a a double helix or any helical structure?
No
When do actin filaments have barbed ends and pointed ends?
What is there significance?
When they have been decorated by myosin motor domains.
Barbs are the myosin tails sticking out towards the plus end.
Points are where the myosin heads attach to the actin fiber. These points point towards the minus end.
Actin added to the end (the plus end) of a growing actin filament are in what form.
ATP-actin
(ADP+Pi)-actin
ADP-actin
ATP-actin
ATP-actin will only be found where it has been freshly polymerized onto the end of actin.
Explain actin polymerization in terms of
ATP-actin
(ADP+Pi)-actin
ADP-actin
Actin monomers are added to the plus end in the ATP-actin form. They will then be converted to the (ADP+Pi)-actin form. Finally they will enter into the ADP-actin form.
What are the stages of actin polymerization.
1) Nucleation
2) Elongation
3) Steady-State
Adding additional monomers of actin to a polymer is disadvantageous until ____ monomers are present in the polymer, at which point it becomes advantageous because ______.
3
Any addition after this point will allow the middle polymers to have all four interaction surfaces, making all additions after three more favorable
How large is the nucleus for actin?
Is it hard to form?
it is a trimer.
It takes a lot of time to form, which causes a lag phase. This is bypassed by using pre-made nuclei.
What is the lag phase in terms of response of actin to polymerizing signals?
It is a gap, that occurs due to the difficulty of forming trimer nuclei.
Does actin treadmill as well? Is it in the same way?
Yes.
Yes.
C
k(on)C
k(off)
= concentration
= is equal to the rate at which monomers are added.
= this is constant. It is the rate at which monomers fall off the polymer.
What is the critical concentration?
Cc = k(off)/k(on)
The concentration at which the k(on)C=k(off).
Monomers are adding at the same rate they are falling off. Dynamic equilibrium.
What happens if the concentration is greater than the critical concentration?
What happens if the concentration is lower than the critical concentration?
The polymer grows
The polymer shrinks
The critical concentration is _____ for the plus end compared to the minus end.
lower.
Autumn Crocus is the source for what?
Colchicine
What does colchicine do?
Depolymerizes microtubules.
Forskolin does what? Comes from where?
It activates adenylyl cyclase increasing cAMP.
Indian Coleus
Which proteins are involved in actin regulation?
Nucleating proteins Sequestering proteins Bundling proteins Capping proteins Severing proteins Gelating proteins Motor proteins
Fimbrin and alpha actin are both?
Bundling proteins.
Fimbrin does what for the cell?
Dimer or monomer?
It bundles actin in close proximity. No space for myosin II.
Monomer.
Alpha-actinin does what for the cell?
Dimer or monomer?
It bundles actin but with a good amount of space, space, for myosin II.
Dimer.
Profilin does what?
Polymerizing protein (which adds monomers to the plus end of actin). Forms actin-profolin complex
Formin does what?
Formin is a nucleating protein which binds to the plus end. It elongates actin. (does NOT generate branching)
Formin whiskers are what?
Some formin have projections with profilin on the ends. These collect monomers and assist in building the actin chain.
Thymosin does what?
It binds to actin monomers and prevents them from being polymerized onto a polymer.
ARPs (Actin-Related Proteins) origin and similarity to actin monomers.
They arose via duplication of the actin gene. Followed by divergence and specialization.
ARPs are 45% identical to actin in primary amino acid structure
What do ARPs (Actin-Related Proteins) do?
Name two that are most important and seem to form a heterodimer.
They are nucleating proteins. They bind to the sides of actin filaments (at 70 degree angle), and produce a branching web-like tree struct structure.
ARP 2/3
What gives listeria motility?
Listeria is mobile because it propels itself using ARP 2/3 templated, actin branching polymers.
What proteins are necessary to recreate listeria motility?
1) Actin monomers
2) ARP 2/3
3) Capping protein
4) Cofilin
What protein does the bacteria use to activate the ARP 2/3 complex?
ActA
What does cofilin do?
It enhances depolymerization of the minus end. It increases the apparent twist of actin filaments, breaking down integrity of the four bonds.
What do capping proteins do?
1) Stabilize the actin filament
2) Inhibit elongation
3) Can bind to either plus or minus end
What does gelsolin do?
It is a capping protein which cuts actin then binds to the plus end, inhibiting growth of plus end.
What does Capping Protein do?
It binds the plus end inhibiting growth at plus end.
What is a cross-linking protein?
A protein which binds to two or more actin filaments.
What is the function of filamin?
Monomer or dimer?
It forms a dimer which binds actin filaments which cross in a roughly perpendicular manner in order to form a lattice-like network.
Dimer.
What happens to melanoma cells which don’t produce adequate levels of filamin?
The are unable to extend normal lamellipodia, thus they do not crawl well, and the do not adhere well to surfaces (they will be less likely to metastasize)
what are lamellipodia?
lamellipodia are actin based protrusions from the cell which bind to the surface and allow the cell to crawl (basic motility)
What is periventricular heteropia caused by?
What is periventricular heteropia?
A mutation in filamin A gene.
It is a disease which leads to seizures and normal or marginally compromised intelligence. It also causes epilepsy. It is caused by insufficient Filamen production leading to a lack of migration of inner brain cells to the outer cortex.
What is periventricular heteropia caused by?
What is periventricular heteropia?
A mutation in filamin A gene.
It is a disease which leads to seizures and normal or marginally compromised intelligence. It also causes epilepsy. It is caused by insufficient Filamen production leading to a lack of migration of inner brain cells to the outer cortex.
What is Fascin? What does it do?
It is a cross-linking protein which binds multiple actin polymers together in a to make a large filament.
What holds together microvilli?
Actin filaments run the length of the filament. They connect to amorphous dense region at the apex of the microvilli.
- microvilli have no movement capability
What holds the actin within the microvilli together?
What holds the actin to the sides of the microvilli?
Villin and Fimbrin cross-
linking.
Myosin-1, calmodulin (likely complexed together)
1) Actin bundles and networks are mechanically very
_______.
2)Actin cross-linking proteins have ____ binding sites
for actin filaments, so they can bind two ____
filaments.
3)Actin bundles help in forming ______, ______
and ______ (which are all cellular ______).
4) Orthogonal networks of actin filaments form ______
(lamellae) and ______.
1) Strong
2) two, different
3) microvilli, stereocilia
and filopodia, (protrusions)
4) sheets, gels
define orthogonal:
Orthogonal means involving right angles
Cell locomotion is also called?
Cell migration
How is cell locomotion (cell migration) achieved?
Cell migration is achieved via an extension of the plasma membrane using the force generated by elongating actin filaments.
What regulates cell locomotion?
cell signaling proteins which alter the activity of accessory proteins
Integrins
Are transmembrane proteins which mediate attachment to the outside environment
Is the Rho family monomeric or heterotrimeric?
Name three members of the Rho family.
Monomeric
Rho
Rac
Cdc42
Name the following monomeric G proteins of the Rho family induce formation of
Rho
Rac
Cdc42
Rho: Lamellipodia
Rac: contractile stress fibers
Cdc42: filopodia
Define Lamellipodia
A flattened extension of the membrane used to move the cell.
Define filopodia
They are thin extension of the membrane which stick out like mobile spikes. They form attachment points and are thought to be used in a sensory role in the lamellipodia of many migrating cells.
What is seen when overexpressing
Rho
Rac
Cdc42
Rho: Lamellipodia extending as far as possible in all/multiple directions
Rac: many, many stress fibers within the cell
Cdc42: Tons of filopodia sticking out in all directions
Erzin-Radixin-moesin (ERM) proteins do what?
ERM proteins bind to transmembrane proteins, linking them to actin filaments
How are Erzin-Radixin-moesin proteins activated?
They are either phosphorylated or bind PIP2
What does the plasma membrane do in response to epidermal growth factor
It forms a protrusion of the lamellipodium.
Name the steps of cell locomotion via lamellipodium.
1, protrusion
2, attachment and traction
3, contraction
Contraction and attachment can be occurring simultaneously (really all of these can be occurring simultaneously)
