Lecture 8 - Microfilaments Flashcards
What are the general properties of the cytoskeleton?
Oragnize internal cell structure, segreagte organelles at cell division, maintain cell strength and shape, mediate shape changes, and generate force
What are the three types of filaments?
Actin filaments (microfilaments) - dynamic and important for cell shape determination, close to surface of cell
Int. filaments - for strength and not usally dynamic, spread throughout cell
Microtubules - organize cytoplasmic organelles, move chromosomes, dynamic - radiate from centriole
Describe the structure of actin
Helical chains, head-to-tail assembly (polarized filament), thin and flexible. + end is fast growing
Describe actin filament treadmilling
When the - end loses subunits while the + end gains subunits, looks like it’s “moving” - can be in a steady state
Describe various actin binding proteins and their functions
Monomer sequestering proteins - duh
Nucleating protein - sites for building chains
Side-binding protein (aka troponin)
Microfilament distribution and function in cell
Lamellipodia - flat protrusions of the cell formed by branched actin filaments
Filopodia - thin, spiky protrusions formed by unbranched, parallel filaments
What regulates actin filaments?
Small g proteins regulate actin assembly
What promotes the assembly of new filament branches in lamellipodia formation?
ARP Complex
What structures to all myosin family members share?
Plus-end directed, have globular catalytic heads, lever arm, actin binding site
Describe myosin 1
Monomeric, stand-alone vesicle motor
Tail associates with membranes
“Haters gonna hate”
Describe myosin 2
Heavy chains dimerize, and associated with light chains. Head contains ATPase motor domains
What are the “thick” and “thin” filaments in muscle?
Thick = myosin, thin - actin
Describe the action of Myosin 2 in non-muscle and smooth muscle cells.
Myosin 2 remains inactive as a monomer, phosphorylation of light chains is necessary for activation and assembly
Upon phosphorylation, light chain straightens and biopolar filament forms to contract actin - takes a while
Describe the structure of skeletal muscle
Long multi-nucleate cells that contain long myofibrils, surrounded by sarcoplasmic reticulum and divided into sarcomeres.
Describe the structure of the sarcomere.
What is dystrophin?
rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane.
Describe the mechanism of muscle contraction
- Release - ATP binding to myosin lowers the affinity of myosin for actin, resulting in release
- Cocked - ATP hydrolysis causes a 5nm translocation of the head to cock it in preparation for the power stroke - weak affinity for actin
- Force generating - dissociation of the inorganic phosphate increase affinity of myosin for actin + power strooooke
- Dissociation of the ADP is stimulated by translocation of myosin head back to original config
(5. Rigor - without ATP, myosin head becomes permanently fixed to actin)
What regulates skeletal muscle contraction?
Troponin-tropomyosin complex - (ex of thin filament based regulation
Troponin has 3 polypeptides
T- binds to tropomyosin covering myosin site
I - binds to actin
C - binds to Ca2+
When activated by Ca2+, troponin changes configuration, moving tropomyosin allowing myosin to bind
How does Ca2+ get into the myofibrils?
An action potential originating from a nerve cell is transmitted down the plasma membrane to
the transverses tubules (specialized invaginations of the plasma membrane).
• This action potential is relayed to the sarcoplasmic reticulum, a membranous network that
surrounds the myofibril and contains large stores of Ca++ ions.
• A voltage gated Ca++ channel imbedded in the transverse tubule senses the action potential and in
response sarcoplasmic reticulum membrane Ca++ release channels open to spill Ca++ ions into the
cytosol surrounding the myofibrils.
• When the nerve impulse stops, CaATPase pumps in the sarcoplasmic reticulum membrane pump
the Ca++ back out of the cytosol, and myosin is prevented from interacting with actin.
Contraction stops and the muscle relaxes.
