Lecture 8 - Microfilaments

Question 1

What are the general properties of the cytoskeleton?

Answer 1

Oragnize internal cell structure, segreagte organelles at cell division, maintain cell strength and shape, mediate shape changes, and generate force

Question 2

What are the three types of filaments?

Answer 2

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

Question 3

Describe the structure of actin

Answer 3

Helical chains, head-to-tail assembly (polarized filament), thin and flexible. + end is fast growing

Question 4

Describe actin filament treadmilling

Answer 4

When the - end loses subunits while the + end gains subunits, looks like it’s “moving” - can be in a steady state

Question 5

Describe various actin binding proteins and their functions

Answer 5

Monomer sequestering proteins - duh

Nucleating protein - sites for building chains

Side-binding protein (aka troponin)

Question 6

Microfilament distribution and function in cell

Answer 6

Lamellipodia - flat protrusions of the cell formed by branched actin filaments

Filopodia - thin, spiky protrusions formed by unbranched, parallel filaments

Question 7

What regulates actin filaments?

Answer 7

Small g proteins regulate actin assembly

Question 8

What promotes the assembly of new filament branches in lamellipodia formation?

Answer 8

ARP Complex

Question 9

What structures to all myosin family members share?

Answer 9

Plus-end directed, have globular catalytic heads, lever arm, actin binding site

Question 10

Describe myosin 1

Answer 10

Monomeric, stand-alone vesicle motor

Tail associates with membranes

“Haters gonna hate”

Question 11

Describe myosin 2

Answer 11

Heavy chains dimerize, and associated with light chains. Head contains ATPase motor domains

Question 12

What are the “thick” and “thin” filaments in muscle?

Answer 12

Thick = myosin, thin - actin

Question 13

Describe the action of Myosin 2 in non-muscle and smooth muscle cells.

Answer 13

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

Question 14

Describe the structure of skeletal muscle

Answer 14

Long multi-nucleate cells that contain long myofibrils, surrounded by sarcoplasmic reticulum and divided into sarcomeres.

Question 15

Describe the structure of the sarcomere.

Answer 15

Question 16

What is dystrophin?

Answer 16

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.

Question 17

Describe the mechanism of muscle contraction

Answer 17

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)

Question 18

What regulates skeletal muscle contraction?

Answer 18

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

Question 19

How does Ca2+ get into the myofibrils?

Answer 19

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.