Lecture 8 - Microfilaments Flashcards Preview

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

Flashcards in Lecture 8 - Microfilaments Deck (19):
1

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

2

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

3

Describe the structure of actin

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

4

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

5

Describe various actin binding proteins and their functions

Monomer sequestering proteins - duh

Nucleating protein - sites for building chains

Side-binding protein (aka troponin)

6

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

7

What regulates actin filaments? 

Small g proteins regulate actin assembly

8

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

ARP Complex

9

What structures to all myosin family members share? 

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

10

Describe myosin 1 

Monomeric, stand-alone vesicle motor

Tail associates with membranes

"Haters gonna hate"

11

Describe myosin 2

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

12

What are the "thick" and "thin" filaments in muscle?

Thick = myosin, thin - actin

13

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

14

Describe the structure of skeletal muscle

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

15

Describe the structure of the sarcomere. 

16

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.

17

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)

18

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

19

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.