1-32 Cytoskeleton I Microfilaments Flashcards
describe the mechanism that regulates actin assembly into microfilamens
when an actin monomer is bound to ATP, it assembles into a filament (Adding to + end)
when the actin subunit has its ATP hydrolyzed to ADP, it leaves the filament (at minus end)
These processes occuring simultaneously leads to the process of treadmilling
What regulates the form and function of microfilaments?
what can they do?
Various actin binding proteins
they can use nucleate assembly (branching), block assembly (end caps), cross links, generate force, and regulate motility
Mysin does what? (general)
Generates Force
Myosin I
associations?
allows for?
standalone head-and-tail structured motor protein
Head associates with actin fibers
tail associates with membranes
allows for vesicular transport in the cell
Myosin II
two headed molecule, each head contains ATPase motor domains
they cause contraction of actin filament bipolar arrays (heads move towards the plus ends of the opposite actin filaments)
Called thick filament in muscle cells or “nonfilaments” in muscle cells
troponin
regulates motility
filamin
regulates crosslinking
alpha/beta capping protein
caps filaments
ARP complex
actin-related-protein complex
promotes assembly of new filament branches which push out membrane at the leading edge.
Allows for formation of microvilli, contractile rings, stress fibers, lamellipodia, and filopodia
phases of contraction
Attatched - myosin head attatched to thin filament in rigor conformation
released - binding of ATP causes myosin head to detach from the thin filament
cocked - when cleft region of myosin head clamps around ATP, myosin cocks back. ATP hydrolyzed, but ADP and Pi stay tightly bound to myosin
Force generating - myosin binds to the actin, causing Pi to be released. Triggering the power stroke causing the myosin to lose its ADP, returning it to the attatched phase.
describe the mechanism of muscle contraction regulation
- SR opens calcium channels to release Ca+2 ions into cytosol
- Ca+2 binds to troponin C
- conformational change causes movement of Troponin I and troponin T
- troponin T moves tropomyosin away from multiple mysoin binding sites along the actin
- during muscle relaxation, SR uses Ca-ATPase to pump Ca+2 back into SR lumen
alpha actinin and fimbrin
bundle
talin
anchors filaments to other structures such as the PM
ARP
actin related protein complex - promotes assembly of new filament branches, which push the membranes out at a leading edge.
a
ARP allow for
the formation of microvilli, contractile rings, stress fibers, lamellipodia, and filopodia
dissocation of the inorganic phosphate..
increases affinity of the myosin head for the actin filament
ATP binding to the myosin head
decreases the myosin heads affinity for actin
dissociation of ADP is stimulated by
translocation of the myosin head back to its original conformation
dystrophin
anchors the sarcoglycan (SG) and dystroglycan (DG) complexes to the plasma membrane.
this protein is defective in muscular dystrophy
where do branched filaments occur and what is the role of the ARP complex?
branched filaments are found in lamellapoida, and each branch point is formed by an ARP complex, which stimulates assembly at the branch.
why is actin filament structural polarity important?
polarity is important for directed assembly, which can generate directional changes in cell shape when the actin is associated with the cell membrane
polarity also important for directional movement of myosin
why does Myosin II form bipolar thick filaments but not myosin I
tails of myosin II molecules form coiled-coil rods that self associate.
myosin I lacks this tail domain, and instead has a tail that binds to membrane
what effect does the binding of calcium to the troponin/tropomyosin complex have on the interaction between Myosin II and actin in skeletal muscle?
when calcium binds, the complex moves out of the way and allows myosin to bind to actin
what is the difference between the relaxed state of muscle that is not contracting, and the rigor state of muscle post mortem
in relaxed muscle, ATP concentrations are high, calcium concentrations are low, myosin is detatched from actin
in rigor, ATP is depleted, calcium leaks out of SR and myosin binds to actin
how is ATP hydrolysis linked with actin filament assembly dynamics?
assembled actin hydrolyzes its bound ATP to ADP, and ADP actin dissociates easily from free minus ends of filaments.
exchange of ADP for a fresh ATP allows actin monomers to bind with high affinilty to filament plus ends, leading to treadmilling
The neurofilament class of intermediate filaments provides tracks for retrograde transport of neurotropic viruses.
False
The activation of contraction in non-muscle cells, smooth muscle cells, and striated (skeletal) muscle cells all involve regulation of myosin activity by ____________.
regulation of intracelluar calcium ion concentrations
Which of the following cellular motility events only depends on the regulated assembly of actin filaments?
A. Cytokinesis
B. Lamellipodium formation
C. Mitosis
D. Fibroblast migration
E. Transport of vesicles from the Golgi to lysosomes
B
A major difference between myosins that cause contraction of actin bundles in both muscle and non-muscle cells, and myosins that move vesicles along actin filaments, is that the contractile myosins can interact with _____.
themselves to form bipolar arrays
The normal distribution of endoplasmic reticulum and Golgi membrane systems in a typical mammalian cell is dependent upon ___________.
microtubules and microtubule associated motor proteins
Microvilli are protrusions of the plasma membrane whose shape is maintained by _____.
microfilaments
Motility supported by molecular motor proteins depends on motors attaching to and carrying specific cargoes along linear cytoskeletal elements. Eukaryotic cilia and flagella are powered by the motor protein dynein. What are the ‘cargoes’ carried by flagellar dyneins?
doublet microtubules
place these events in order
Which sequence of numbers lists the events that follow activation of skeletal muscle contraction, labeled 1-5 below, in their correct order?
- ATP binds to the myosin head
- Calcium binds to troponin C.
- Tropomyosin moves and exposes the myosin binding site on actin.
- Calcium is released from the sarcoplasmic reticulum.
- Myosin binds to actin and undergoes a conformational change.
4,2,3,5,1
