Lecture 8 - Microfilaments Flashcards Preview

Unit 1 - Molecular and Cellular Principles of Medicine > Lecture 8 - Microfilaments > Flashcards

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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

1. Release - ATP binding to myosin lowers the affinity of myosin for actin, resulting in release

2. Cocked - ATP hydrolysis causes a 5nm translocation of the head to cock it in preparation for the power stroke - weak affinity for actin

3. Force generating - dissociation of the inorganic phosphate increase affinity of myosin for actin + power strooooke

4. 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.