Lecture 17: Contractile Protiens Flashcards
Actin
What does this cytoskeletal protein determine?
- Shape of the Cell
- Cell Locomotion
- Pinching of One Cell into two
- Mitosis
Myosin
What is this cytoskeletal protein functions?
- Use ATP to move organelles along filaments
- Use ATP to move filaments on proteins
On which end of actin do we typically see growth?
The plus end
In G Actin, where can you find ATP?
In the ATP cleft, which faces the minus end
How does actin assemble into a filament?
-
Stage 1: Nucleation (Lag Phase)
- Small acting oligomers must first form an aggregate of 3 actin monomers
- Each aggregate must be stabilized
-
Stage 2: Elongation (Growth Phase)
- Multiple subunits contract and elongate rapidly
- If actin is bound to ATP, polymerization happens much more quickly
-
Stage 3: Steady State (Equilibrium)
- Depends on concentration of free actin
- Rate of polymerization = Rate of depolymerization
What is the difference between the rate of subunit association and dissociation?
When is equilibrium reached between the two?
-
Subunit association: Proportional to concentration of free monomers
- C x kon
- Subunit dissociation: Independent of monomer concentration
-
Equilibrium: Reached at critical concentration of monomers
- koff = Cc x kon
Where can you find actin filaments?
- Beneath plasma membrane
- Mechanical Support
- Determining Cell Shape
- Movement of Cell Surface
What determines the nature of the association of actin filaments?
How is cross linking achieved in F-actin?
The size and shape of the actin binding proteins
Acessory Proteins
What is the basic organization of actin parallel bundles?
What monomer binds the parralel bundles?
- Actin fibers are crosslinked together with all of their plus and minus ends on the same side.
- Parallel fibers are very close and doesn’t allow Myosin II from entering the bundle
- Fimbrin binds actin
Where in the cell can you find actin parallel bundles?
What do they do?
Location
- Projections of Plasma Membranes
- Microvilli
- Cell Cortex
- Adherens belt
Functions
- Placement of receptors and channels
- Facilitate signaling, transport, uptake of nutrients
What is the basic organization of actin contractile bundles arrays?
What monomer binds the contractile bundles?
- Actin fibers are crosslinked together with all of their plus and minus ends on the same side.
- Parallel fibers are very loose and allows Myosin II from entering the bundle
- α-Actinin binds actin
What does actin contractile bundles do?
Functions
- Moving endocytic vesicles
- Squeezing of cytoplasm by contractile ring to cause cell divison
- Can also aid muscle contraction
What is a contractile ring made of?
Actin and Myosin II
What is the basic organization of actin bundling proteins?
What monomer binds this network?
- Actin fibers are held together in flexible 3-d meshowrks
- Filamin binds actin as a dimer
For Filamin:
Where are its actin binding domains located?
At the farthest ends of a dimerized structure
What is the organizational name for actin networks’ three dimensional meshworks?
Orthogonal Arrays
Where in the cell can you find actin bundle networks?
What do they do?
Location
- Filopodia: finger like prjoections
- Lamellopodia: sheet like extensions
- Pseudopodia: cross linking
Functions
- Phagocytosis
- Movement
What -podia are commonly associated with phagocytosis?
Pseudopodia
Erythrocytes contain spectrin:
What does it do?
What does it interact with and how?
- Forms the actin network that makes up the cortical cytoskeleton
- Interacts with membrane proteins via ankryin and protein 4.1
What is deficient in Hereditary Spherocytosis?
What happens to the RBC?
Cortical skeleton proteins of the red blood cells
- Spectrin
- Ankryin
- Protein 4.1
Loss of these proteins reduces RBC stbility and cannot move from large vessels to capillaries due to lack of flexibility and deformability
Briefly, how does myosin move?
Along actin filaments via ATP hydrolysis
How is myosin organized?
- Head Region
- Actin and ATP binding sites
- ATPase activity
- Neck Region
- Flexible
- Binds myosin light chain peptides
- Tail Region
- Can Bring myosin head in closer proximity
- Binds membrane/organelles
What are the freatures and function of Myosin I?
Myosin I
- 1 Light Chain in neck
- 1 Heavy Chain in tail
- Function: Membrane association and endocytosis
What myosin is most common in skeletal muscles?
What is its features and function?
Myosin II
- 2 Light Chains in neck
- 2 Heavy Chains in tail
- Function: Contraction
What are the freatures and function of Myosin V?
Myosin V
- 6 Light Chains in neck
- 2 Heavy Chains in tail
- Function: Organelle Transport
In more detail, how does a myosin move along F-actin?
What causes a myosin “power stroke?”
- If there is no ATP around, myosin is attached to actin.
- When ATP is bound, myosin releases actin
- Myosin now in “cocked” state
-
ATP Hydrolysis: Power Stroke
- The release of Pi gives off energy to move myosin on the actin filament
- Will remain bound to new actin until ATP is bound again
Which end of actin does myosin move towards?
The plus end
What will a myosin with a longer neck have over a myosin with a shorter neck?
A Myosin with a longer neck will have a greater power stroke velocity (increased rate of movement)
(Keep in mind this is not referring to the length of the tail, but the light chain neck portion)
What thin and thick filaments compose a skeletal muscle?
- Thick: Myosin Chains
- Thin: Actin, tropomyosin, toponin
In a sarcomere, which side of the actin would be the plus side?
The side attached to the Z Disc
In a sarcomere which side of the actin would be the minus side?
The side farthest from the Z Disk (in the A Band)
Briefly, how does a skeletal muscle contract?
- Activation of Voltage Gated Calcium allows calcium to enter the cell, prompting a release of Calcium from the Sarcoplasmic Reticulum to release its calcium
- The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction.
- It will bind to troponin to initiate contraction
Briefly, how does a smooth muscle contract?
- Calcium from the Sarcoplasmic Reticulum binds to calmodulin
- This complex binds to Myosin Light Chain Kinase
- Result: Active Mysoin Light Chain Kinase
- With ATP, there is phosophyrlation of myosin light chain
- Contraction occurs
What enzyme in smooth muscle activates myosin?
Myosin Light Chain Kinase
What contributes to the prevention of myosin binding to actin in smooth muscle?
What enzyme causes this inhibitory state?
Folding of the myosin by the regulatory light chains in the neck
Myosin Light Chain Phosphatase
What does the Dystrophen protein do?
- Connects Cytoskeleton to the basal lamina
- Stabilizes the membrane
- Helps to relieve the stress of contraction by spreading the forces out
- Shock absorber during contraction
What is the heredity of Duchene’s Muscular Dystrophy?
What is its symptoms?
- X Linked Recessive
- Muscle Wasting
- Normally will die in young adulthod
