Contractile Proteins Flashcards
The structural, spatial and mechanical functions of cells depend on what?
The cytoskeleton which is a remarkable system of filaments scaffolding the cell
What do actin and actin binding proteins determine?
The shape of the cell, its locomotion and ability to divide into two
What is myosin (motor protein)?
Molecular machine that converts biochemical energy from ATP hydrolysis to mechanical energy that can move filaments on proteins or move organelles along filaments
Describe actin
Major component of cytoskeleton
Subunits = globular actin or G actin which can rapidly diffuse in the cytosol
Associates with ATP or ADP
Actin monomer is divided by a central cleft into two equal sized lobes
Describe G-actin subunit assembly/polymerization
Assemble from head to tail to form a tight right handed helix called filamentous actin/microfilaments/F-actin
How are actin filaments polar and how is it important?
Have a slower growing minus end and a faster growing plus end
Important in both their assembly and in establishing a unique direction of myosin movement relative to actin
What are the three different phases of assembly of actin polymers?
Nucleation (lag) phase Elongation (growth phase) Steady stage (equilibrium phase)
Actin filaments become organized into what?
Higher order structures forming bundles of 3D networks within cells
May be associated with other cell structures such as the plasma membrane
Where are actin filaments abundant?
Beneath the plasma membrane where they form a network that provides mechanical support, determines cell shape and allows movement of the cell surface thereby enabling cells to migrate, engulf particles and divide
What is organization of actin into networks regulated by?
A variety of actin binding proteins which are critical components of the actin cytoskeleton
Cross linking of F actin is achieved by what?
Accessory proteins that typically have at least two actin binding domains (ABD)
What are bundling or cross linking proteins?
Typically small ridged proteins that force the actin filaments to align closely with each other
The nature of association of actin filaments is determined by what?
Size and shape of the cross linking proteins
What are the two general types of structures actin can assemble into?
Actin bundles or networks
What are actin bundles?
Cross linked into closely packed parallel arrays
Polarity of actin filaments same
What are actin networks?
Loosely cross linked in orthogonal arrays that form 3D meshwork with more flexible gel-like properties
Makes cell flexible
Polarity of actin filament different
What is an important mechanism by which cells control shape and movement?
Regulation of actin filament formation
What is the issue with small oligomers of actin?
Associate spontaneously but are unstable and disassemble readily
What is filament nucleation?
For a new actin filament to form subunits must assemble into an initial aggregate (nucleus made of 3 actin monomers) that is stabilized by multiple-subunit contacts This core (aggregate) then elongates rapidly by the addition of more subunits
Describe assembly of actin polymers
First go through filament nucleation
Then actin filaments grow by reversible addition of monomers to both ends
Plus end elongates 5-10 times faster than the minus end
Following filament assembly what occurs to the bound ATP on the actin monomers?
It is hydrolyzed to ADP and Pi which plays a key role in the assembly and dynamic behavior of actin filaments
How is actin polymerization reversible?
Filaments can depolymerize as needed by the dissociation of actin monomers
An equilibrium exists between which two structures of actin?
Actin monomers and filaments which is dependent on concentration of free monomers
Describe the critical concentration of actin monomers
The critical concentration of actin monomers at which the rate of their polymerization into filaments = the rate of dissociation
At this concentration the monomers and filaments are in apparent equilibrium
What are parallel bundles?
Made of closely spaced actin filaments in parallel arrangement
Fimbrin monomer binds to actin filaments and has 2 actin binding domains (ABD)
Holds two parallel filaments close together together
What are the two types of actin bundles?
Parallel and contractile bundles
What is the function of parallel bundles?
Increase cell surface and gives the structure stability
Allows for placement of additional receptors and channels which facilitate signaling, transport, uptake of nutrients, etc
What is an example of a structure that has parallel actin bundles?
Microvilli
What are contractile bundles?
Loosely bundled actin filaments
Loose structure is due to cross linking protein known as alpha actinin dimers
Actin filaments are separated by a greater distance
E.g. contractile ring in mitosis which allows squeezing of cytoplasm by contractile ring to cause cell division
Also allows myosin to interact during contraction
What are actin networks?
Actin filaments held together by large actin binding proteins such as filamin
Creates a 3D meshwork
Present in cells that need to withstand forces
What is spectrin?
Actin binding protein located in erythrocytes
Spectrin-actin network interacts with membrane proteins via interactions with ankyrin and protein 4.1
What is hereditary spherocytosis?
Caused by mutations in erythrocyte cortical cytoskeleton proteins such as spectrin, ankyrin or protein 4.1
Characteristics of the disease include impaired deformability which is required for movement from large vessels to capillaries, reduced stability of RBCs and spherical RBCs, anemia, splenomegaly, jaundice
What is myosin?
A superfamily of motor proteins
20 different types in eukaryotes
All move along actin filaments via ATP hydrolysis
Describe the structure of myosin II
Bipolar filaments
Heads exposed at both ends
Tails associate to form shaft of filament
Neck binds light chains
What are the three major domains of myosin?
Head, neck and tail
Describe the head of myosin
Contain actin binding and ATP binding sites
Has ATPase activity
Describe the neck of myosin
Flexible region
Binds myosin light chain peptides
Describe the tail of myosin
Intertwine to bring myosin head regions in close proximity
Bind membrane/organelles
What are the 3 different types of myosin?
Class I, II and V
All three move toward the + end of actin filaments
What is the function of class I myosin?
Membrane association and endocytosis
What is the function of class II myosin?
Muscle contraction
What is the function of class V myosin?
Organelle transport
Describe the structure of myosin class I
One heavy chain with a head domain and neck domain (only single headed myosin)
Describe myosin class II
Two heavy chains - each with head and neck domain that binds two different light chains Heavy chain long helical tail homodimerizes via coiled-coiled interaction Only class that can assemble into bipolar fragments through tail interactions
Describe myosin class V
Two heavy chains and six light chains per neck
Also homodimerizes
End of tails interact with specific receptors on organelles, which they transport along filament tracks
Describe the steps of ATP driven myosin movement along actin filaments
- ATP binding causes conformational change in the myosin head actin binding domain and head release from actin
- The head hydrolyzes ATP which induces rotation of the head causing a “cocked” state that stores the energy released by ATP hydrolysis as elastic energy
- Myosin remains in the cocked state until it binds to an actin filament
- Binding to actin causes myosin to release Pi which release the elastic energy to drive the power stroke involving a conformational change in the neck region which moves the actin filament with respect to the end of the myosin neck
- The head remains tightly bound to the actin filament unit ADP is released
What occurs in the absence of ATP?
Myosin head attaches firmly to the actin filament and is short lived in living muscle but cause rigor mortis stiffness in death when cells lack ATP
The power stroke mechanism is proportional to what?
Length of the myosin neck domain
Longer neck domain -> increases rate of movement
Describe the filaments found in skeletal muscle
Thick filaments made of 6 myosin polypeptide chains
Thin filaments made of 3 proteins including actin, tropomyosin and troponin
What is the sarcomere stabilized by?
Capping and scaffolding proteins
What are actin filaments stabilized by?
Stabilized on their + ends by CapZ and on their - ends by tropomodulin
What is nebulin?
A large protein that extends along the thin actin filaments all the way from the Z line to tropomodulin to which it binds
Has a number of actin binding repeats
Length of nebulin determines the length of thin filaments
What are thick filaments of a sarcomere made of?
Myosin II
What is the structure of titin?
A large protein that has its head associated with the Z line/disc and extends to the middle of the thick filament where another titin molecule extends to the subsequent thin filaments
What is the function of titin?
Holds the thick filaments in the middle of the sarcomere and prevents overstretching (thick filaments remain interdigitated b/w the thin filaments)
What do mutations in titin cause?
Cardiomyopathies
What are the cytosolic Ca concentrations in skeletal muscle under resting conditions?
Low due to the activity of SERCA that pumps them from the cytosol into the SR
What does SERCA establish in skeletal muscle?
Reservoir of Ca in the SR
Describe contraction of skeletal muscle
- Nerve impulse at a NMJ triggers an AP in the plasma membrane
- AP travels down T-tubules
- Depolarization stimulates opening of voltage gated Ca channels in the SR membrane
- Extracellular Ca is brought into the cell and causes the release of Ca from the SR and subsequent elevation of cytosolic Ca concentration occurs
- Elevated Ca induces conformational changes in tropomyosin and troponin which permit the myosin-actin interactions and hence contraction
What does the phosphorylation of myosin LC by MLCK cause?
Unfolds the myosin II which is now active and can assemble into bipolar fragments to participate in contraction
Describe non-muscle cell contraction
Non-muscle cells contain several types of actin-myosin structures similar to skeletal muscle fibers
Formed in a transient manner as needed by the cell
Ex. Cytokinesis
What is dystrophin?
Part of a protein complex that links the cytoskeleton of muscle fibers to the surrounding of CT (basal lamina)
Dystrophin is a long protein with numerous redundant coils that provides what?
A structural link between the cytoskeleton of the muscle cell and the ECM
Acts like a shock absorber during contraction and stabilizes the sarcolemma
What is Duchenne’s muscular dystrophy associated with?
Loss of dystrophin
What is Duchenne’s muscular dystrophy?
An x-linked recessive disorder caused by an abnormal dystrophin gene
Cause progressive muscle wasting
Pts are confined to wheelchairs by 12 and die by respiratory failure by age 22
What is Becker’s muscular dystrophy?
A milder form of DMD with a later onset
Defects in dystrophin weaken what?
Muscle cells and muscle fibers
In those with DMD, eventually the body cant keep up with repair which leads to what?
Death of skeletal muscle cells and muscle degeneration
What mutation do DMD pts have?
Out of frame mutations -> little to no expression of dystrophin
Pts with BMD have which mutation?
In frame mutation -> smaller protein with partial function of dystrophin
What are some treatments for DMD?
Gene therapy, dystrophin replacement, drugs to prevent exon skipping
