Lecture 3: The cytoskeleton I - Introduction to Actin/Treadmilling Flashcards
What is the cytoskeleton?
The cytoskeleton provides structural framework for cells and is responsible for giving cells their shape and structure
What is the cytoskeleton made up of?
The cytoskeleton is composed of a network of filaments
What do filaments do?
They facilitate cellular movement and control the movement of organelles, proteins and nucleic acid within the cell
What are filaments composed of ?
Protein polymers
What are the three types of cytoskeletal filaments?
actin filaments, microtubules and intermediate filaments
What processes do the cytoskeletal filaments carry out ?
cell division, cell motility, segregation of chromosomes during mitosis, shape changes in developing embryo, muscle contraction, transport of organelles in cells, intracellular movements
What is the most abundant protein in many cell types?
actin filaments
What are the three classes of the actin filaments
alpha, beta and gamma
What form do actin filaments exist in?
- monomeric form (G-actin)
2. 7nm helical filaments of uniformly oriented globular actin molecules (F-action)
What do actin filaments look like?
They are thin and flexible, normally cross linked for extra strength (cortex)
Where is alpha actin expressed?
In muscles cells
Where is beta and gamma actin expressed ?
Non muscle/ most other cell types
What can cause actin filaments to assemble or disassemble ?
The concentration of actin monomers, also termed G-actin or globular actin
What makes up the cortex?
Networks of actin filaments and other contractile proteins
What type of structure does G-actin have ?
A globular structure with exposed protein domains creating a barbed and a pointed end, separated by a cleft
What is a globular protein ?
A spherical protein
What is the purpose of the cleft on the G-actin ?
This is where ATP is bound
What is polymerisation ?
When monomers combine chemically to form polymer chains
What happens to the G-actin during polymerisation ?
The G-actin assembles into the filament in the same orientation, resulting in a filament that has a plus or barbed end of G-actin exposed at one end and the minus or pointed end exposed at the other.
Do actin filaments have structural polarity ?
Yes
Why do actin filaments have structural polarity ?
G-actin assembles into the filament in the same orientation, resulting in a filaments that has the plus or barbed end of G-actin exposed at one end and the minus or pointed end exposed at the other.
Is the assembly of actin monomers faster to the barbed ends or the pointed ends ?
The actin monomers assemble more readily to the barbed ends which are the fast growing ends, than the pointed ends which are slower growing.
What do monomeric actin (globular) polymerise into?
Filamentous actin
What percentage of the cell type is monomeric and in filaments ?
Depending on the cell type 50% is monomeric and 50% is in filaments
What causes there to be actin in filaments and in monomers ?
The presence of actin monomer binding proteins
Name 3 actin monomer binding proteins
- Profilin
- Thymosin
- ADF
What does the actin binding protein ADF stand for ?
Actin depolymerizing factor
What is the purpose of actin binding proteins ?
To bind actin monomers in the cytosol and prevent them from being incorporated into actin filaments, resulting in a reserve of actin monomers
Why is is there a reserve of actin monomers retained in the cell at all times ?
So that rapid polymerisation can occur when needed
What is the basis for cell movement ?
The ability of the cytoskeleton, particularly actin filaments to rapidly disassemble in a specific direction, generating the force for cell movement.
What is the process known as ruffling ?
The formation of actin rich membrane protrusions
How do cells migrate ?
Using a crawling motion
What is the leading edge ?
The extension of protrusions in one direction
What are the two types of protrusions ?
- Lamellipodia
2. Filopodia
What is lamellipodia?
A protrusion of a meshwork of actin filaments with their plus ends close to the plasma membrane
What is a filopodia ?
Thin, stiff bundles of actin filaments with their plus ends close to the plasma membrane, which push the cell forward to form new regions of actin cortex
Where do the protrusions anchor themselves ?
To the surface on which the cell is crawling, making focal contacts.
What are focal contacts ?
Adhesions by which cells attach to the underlying substrates
How does the cell drag itself forward ?
It retraces its trailing edge by using the traction from the new anchorage points/focal contact.
What are the 3 stages of actin polymerisation ?
- Nucleation
- Elongation
- Steady state
What is the slowest stage of actin polymerisation ?
Nucleation stage
What does the nucleation stage involve ?
The association of three actin monomers to initiate the filament structure
What does the elongation stage involve ?
The rapid growth of the actin filament at both ends.
When does the growth of the filament slow ?
When the free monomer concentration diminishes.
What does the steady state involve ?
There is no net growth or shrinkage of the filament and there is a constant exchange of monomers between the filament and the free monomer pool
How can actin polymerisation be studied ?
In vitro using physiological salt concentrations
What is the critical concentration ?
The concentration of free actin monomers during the steady state stage or the concentration of actin monomers required to form filaments
When do filaments form ?
When the concentration of G-actin is above the critical concentration
When do G-actin remain as monomers?
When the G-actin concentration is below the critical concentration.
When can the critical concentration be influenced ?
By the presence of actin binding proteins
Where does the critical concentration differ ?
The critical concentration differs at the plus and minus ends of the filaments
When does the hydrolysis to ADP and the inorganic phosphate begin in treadmilling of actin filaments ?
When the monomer which is ATP bound is incorporated into the filaments
In actin treadmilling, what is ADP bound actins role?
It has an altered conformation and is less strongly bound within the filament.
What is actin treadmilling ?
Occurs when one end of the filament grows in length while the other shrinks.
What results in the difference of growth rates at the two ends of the filament when monomer concentrations become limiting ?
ATP and ADP being bound to the filaments.
Where does treadmilling occur ?
At monomer concentrations between the two critical concentration ranges
When does treadmilling occur ?
When ATP actin adds to the plus end of an actin filament at the same time that ADP is lost from the minus end.
Does unregulated treadmilling occur ?
No
What happens when the rate of addition and loss are equal ?
The filament stays the same length, although individual actin monomers move through the filament from the plus to the minus end.
What nucleates actin filaments ?
The leading edge of migrating cells
What is the purpose of nucleating proteins during cell migration ?
Serve to skip the rate limiting nucleation phase of actin filament formation, allowing intermediate filament elongation to occur.
What does ARP complex stand for ?
Actin related protein complex
What does the ARP complex consist of ?
ARP2, ARP3 and other accessory proteins
What do ARP2, ARP3 and the other accessory proteins do ?
Attach to the sides of pre-existing filaments and allow branched actin filaments to polymerise from the complex
What does the ARP complex do ?
It enhances the rate of actin filament assembly at the leading edge.
What does the resulting branching structure do after polymerisation ?
It pushes the plasma membrane forward, facilitating cell movement
What are the three nucleator systems ?
- ARP2/ARP3
- Spire
- Formin
What is the spire nucleator system ?
Slows the rate limiting step of polymerisation by binding 4 actin monomers, generating a tetramer which acts as the nucleating seed allowing rapid formation of new actin filaments
What is the formins nucleating system ?
A progressive elongation machine, associating with the barbed end of existing filaments to generate longer filaments
What are some of the multiple signals that lead to actin nucleation and cell motility ?
Growth factors, receptor tyrosine kinase, G coupled receptors, integrins and Rho family GTPases
What is the basis of muscle contraction ?
Myosins bind actin filaments and use hydrolysis to move along the filament
What is myosin II
Muscle myosin
What is myosin I and other myosins ?
Non muscle
What is the function of myosin filaments ?
Driving membrane furrowing during cell division and muscle contraction
What is the structure of myosin II
Two globular heads and a coiled-coil tail
What do the globular heads of myosin II have ?
ATPase and motor activity
What is the structure of the myosin II tail ?
Heavy chains form coiled-coil rod structure
What do the tails of myosin II form when they associate with each other ?
A bipolar myosin filament
