Comprehension L16-L17 Flashcards
Individual actin monomers move down the length of the microfilament (actin) from the plus end to the minus end in a process known as
Treadmilling
Stimulatory/excitatory neurotransmitters released from the terminal knobs of a presynaptic cell will have which effect?
increased sodium influx into the postsynaptic cell
What does Myosin Type 2 do?
Roles in muscle contraction, splitting cell during cell division, and cell motility
How is Myosin 2 oriented?
Tails point towards the centre and heads point outside
What are 2 characteristics of the Myosin filament?
Bipolar: Reversal of direction at the filament’s centre
Thick: Component of myosin
In order of largest to smallest, what are the levels of muscles?
- Muscle fibre, composed of hundreds of…
- Myofibrils, composed of repeating contractile units called…
- Sarcomeres, contractile unit with a characteristic banding pattern
What is the Z line?
contains proteins important for sarcomere structure stability
What is the M line?
Dark staining in the centre of the sarcomere, contains anchoring proteins.
What do I bands contain?
Only thin filaments
What is the H zone?
Contains only thick filament
What is the A band?
Dark staining overlap of thick and thin, also includes the H zone
Each thick filament is surrounded by what?
6 thin filaments
How far can a single myosin move an actin filament in a power stroke?
10nm
Describe the 5 steps of actin-myosin contraction
- ATP binds to myosin head and myosin dissociates from actin
- ATP hydrolysis, ADP and Pi remain bound
- Energized myosin binds actin
- Release of phosphate triggers conformational change: power stroke (actin moves to centre of sarcomere)
- ADP is released
Muscle fibres with a (__) are stimulated simultaneously by a single motor neuron
Motor unit
What is the neuromuscular junction?
Point of contact between motor neuron and muscle fibre; site of transmission of the nerve impulse
What are Transverse tubules?
Membrane folds that propagate an impulse to the interior of a muscle cell
What is the Sarcoplasmic Reticulum
Special smooth ER in muscle cells; stores Ca2+ in lumen (pumped in from cytosol
What does the arrival of an action potential at the SR do?
Opens Ca2+ channels, release Ca2+ into the cytoplasm
Thin filaments contain actin and:
Tropomyosin - rod shaped
Troponin - globular
What happens in the absence of Ca2+?
Troponin controls position of tropomyosin which blocks myosin-binding sites on actin
What happens in the presence of Ca2+?
Ca2+ binds troponin which moves tropomyosin, exposing the myosin binding site on actin
What blocks the myosin binding sites on actin thin filaments in a resting sarcomere?
Tropomyosin
What blocks the myosin binding sites on actin thin filaments in a stimulated sarcomere?
Nothing
What is the cell cortex and what is it’s function?
Actin network on the inner face of the plasma membrane, dynamic remodelling, enable cells to crawl, enable phagocytosis, cellular constriction
What do actin-binding proteins do?
regulate the assembly, disassembly, and rearrangements of actin networks
What are the 7 steps of actin binding proteins?
- Filament nucleating
- Monomer sequestering
- End blocking (Capping)
- Monomer polymerizing
- Depolymerizing
- Cross linking
- Filament severing
In filament nucleation, what is the Arp2/3 complex?
Binds to the side of an existing filament
Remains at the pointed end of new branch
Similar structure to actin monomers
In filament nucleation, what are formins?
Generate unbranched filaments
Stay associated with the barbed end
Promote rapid elongation of filaments
What happens in monomer sequestering?
Bind to actin-ATP monomers to prevent them from being added to the elongating filament
Able to modulate the available monomer pool in certain regions at certain times
What happens in end blocking (capping)?
Regulate the length of actin filaments and bind at either end
What happens in monomer polymerizing?
Binds to actin monomers to promote growth of actin filament
Promotes replacement of ADP w ATP on the actin monomers
What happens during depolymerization?
Bind to actin-ADP at the pointed end to promote depolymerization
What happens with cross linking and bundling?
Multiple actin binding sites, allowing them to alter the 3D organization
What is filament severing?
Break an existing filament in two
What happens in membrane-binding?
Actin filaments linked to the plasma membrane
Enabling the plasma membrane to protrude outward or inwards
What does Arp2/3 do?
branched filament nucleating
What is profilin?
Monomer polymerizing (ADP->ATP)
What is Cofilin?
Depolymerizing
A shift in the [ ] or activity of which types of actin binding proteins can cause a shift in the equilibrium between available actin monomers and filamentous actin?
- Monomer sequestering proteins
- Depolymerizing proteins
- Nucleating proteins
What is the lamellipodium?
The leading edge of a moving cell that extends over the surface, broad and flat
What promotes polymerization?
profilin
What are the steps (1-4) of dynamic actin network at the site of lamellipodium formation?
- Stimulus received at cell surface
- Arp2/3 complex at site of stimulation gets activated
- Arp2/3 binds the side of an existing filament
- ATP-actin monomers bind to the Arp2/3 complex, forming a new actin branch
What are the steps (5-6) of dynamic actin network at the site of lamellipodium formation?
- Additional Arp2/3 complexes can bind to the sites of new filaments. Older filaments are capped at barbed end so they remain short and stable
- Newer filaments continue to grow at the barbed end, pushing the membrane of the lamellipodium outward
What are traction forces?
When the cell grips the surface at adhesion points called focal adhesions
What are focal adhesions?
Structures in the cell membrane where integral proteins connect to actin
What are Integrin proteins?
Transmembrane proteins that mediate the interaction between actin and extracellular components
What are Contractile forces?
Pull the bulk of the cell forward
Myosin found near the rear of the lamellipodium
