Lecture 14 Flashcards
Function of the cytoskeleton?
-Structure/support
-Shape, cell migration, division
-Organization/transport
Where is actin?
Right under the PM in the cell cortex
Actin fucntion?
-Makes it so the cell does not deform
-Important for cell migration
Function of microtubules?
-Important for transport
-Cell division (mitotic spindle)
Function of intermediate filaments?
-not as important as actin/microtubules
-Important for mechanical strength
G-Actin?
One actin subunit
Actin structure?
-Made up of globular actin monomers
-Monomers have polarity
How are actin filaments made?
-Monomers assemble in head to tail fashion (plus end to minus end)
-New monomers can be added to either end but are added more rapidly to the plus end.
ATP-binding cleft of G-actin?
Near the minus end of actin
What is nucleation?
When two monomers bump into one another the interaction is unstable(likely to fall apart) but if another monomer comes in prior to them falling apart they form a stable trimer that will be able to add more monomers before falling apart
What controls the rate at which monomers are added to the F-actin?
The concentration of monomers nearby
High concentration = faster rate of addition
Critical concentration?
When the rate of addition of monomers subunits is equal to the rate of disassembly of subunits
Lag phase of actin?
-Monomers floating around and eventually three trimerize to form nucleus
Growth Phase?
Once nucleation has occured subunits are rapidly added to the filament
-G-actin concentration decreases
Equilibrium Phase?
-Rate of addition of new subunit decrease to match the rate of subunit dissociation
G-actin is at its critical concentration
T/F: Actin can bind ATP/ADP
True
G-actin is usually bound to which nucleotide?
ATP
What happens when G-actin boudn to ATP is incorporated into F-actin?
- G-actin ATP has high affinity for F-actin so it will bind
- Once bound ATP hydrolysis will start to slowly occur
- ADP bound actin now longer has affinity for F-actin and if it is at the end of the filament the actin will fall off
Hydrolysis of ATP at the end of a filament vs middle ?
End : the monomer is likely to fall off
Middle: the monomer is less likely to fall off
How does the ATP cap form?
If an ATP bound monomer binds to the F-actin before the monomer already in the F-actin hydrolyzes ATP
-New ATP bound monomers are added faster than the rate of hydrolysis
Where does the ATP cap mainly occur?
Plus end
Function of the ATP cap?
Favours growth of the actin filament
Minus end of actin is typically?
All ADP actin
Treadmilling?
When growth occurs at the plus end and loss occurs at the minus end
-Occurs at equilibrium where the filament stays the same length but the actins are being cycled through the filament
Actin Nucleating Proteins?
Arp2/3
Formins
Monomer binding proteins
-Profillin
-Thymosin
How does Arp2/3 work?
NPF binds Arp2/3 and snaps them together which forms a structure that resembles the plus end of actin. Now minus ends of the monomer add and start growing the actin filament
When is Arp2/3 used?
When cell needs actin(cell crawling)
-Creates branches off already formed filaments
How do cells regulate Arp2/3?
Control the localization of NPF
ex. For cell migration NPF will be near the front of the cell
How do formins work?
-Sit at the plus end of filaments that have already started to form
-Have N-terminal tails that grab nearby monomers by binding profiling
-Accelerates growth of existing filament
Where does profillin bind?
Plus end of monomers
Purpose of Profillin?
-Binds plus end now growth can only occur at plus end of filaments
-Allows other proteins(formins + NPF) to bind and recruit actin monomers
-Speeds up growth of filaments
What does Thymosin do?
Binds both the plus/minus end of actin monomers
Function of Thymosin?
-Controls concentration of free actin(lowers it)
-Monomers released by thymosin are then bound by profilin
Where is tropomyosin found?
Muscle
Purpose of Tropomyosin?
-Binds lengthwise along actin filaments
- helps to stabalize and stiffen filaments
What does CapZ do?
-Binds to the plus ends of filaments and prevents the addition and substraction of subunits
-Minus end can still gain/lose subunits
-Lowers the overall rate of filament assembly/diassembly
Crosslinking proteins?
-Fimbrin
- Alpha-actinin
-Filamin
Bundled networks vs Gel-like networks?
Bundled: whole of bunch of filaments next to each other that are parallel and organized
Gel-like: contain filaments held at large angles from one another(disorganized, less uniform)
Structure of fimbrin?
-Monomer
-Two actin binding domains
-Short molecule
What does fimbrin do?
-Cross links actin into tight linked bundles
-No flexibility
-Important for filopodia
Structure of alpha-actinin?
-Dimer
-Two actin binding sites
What does alpha-actinin do ?
-Filaments are held further apart and in parallel
What are the proteins responsible for bundle networks?
- Fimbrin
- Alpha-actinin
Protein responsible for gel-like network?
- Filamin
Structure of Filamin?
Monomer with two actin binding domain
How does filamin work?
-Holds actin together in V-shape
-Looser actin structure which allows more space for other proteins to bind to actin
Gelsolin?
-Filament severing protein
-Cuts the actin filament
-Two binding domains (one for exposed actin and one for interior)
-Stays bound to severed filaments to prevent reformation
Coffilin
-Filament severing protein
-Binds along side of F-actin
-Twist filament tightly and causes it to fall apartH
Who does coffilin bind to?
-Preferentially binds to ADP bound actin
-Gets rid of old actin
