Lecture 6: 21/09

Question 1

How does myosin based internal stress differ from the rheometer based external stress?

Answer 1

The myosin based internal stress has a time-component due to the myosin cross-bridge cycle

Question 2

Compare NON-crosslinked actin filaments with and without myosin?

Answer 2

NON-crosslinked Actin is able to move and slide with active myosin in the system.
Myosin fluidizes the network. Myosin makes an entangled actin network have a lower apparent viscosity.

Question 3

How does ATP impact the actin flow?

Answer 3

ATP allows actin to behave in a more fluid-like manner (ATP activates myosin in the system, fluidizing the entangled network)

Question 4

How does active myosin impact the scaling of the reptation constant (i.e., time of relaxation)?

Answer 4

T ~ L^3 (thermal reptation) with no myosin
T ~ L (motor drive) with myosin

time of relaxation is faster with active myosin as it is more fluid (greater time component)

Question 5

Why does myosin fluidize an entangled actin network?

Answer 5

Myosin lowers the apparent viscosity of the entangled actin network because the stress is dissipated via heat faster than without myosin.

Question 6

In which situation does myosin fluidize or stiffen the actin network?

Answer 6

Fluidizes: no crosslinkers, entangled system
Stiffens: crosslinkers

Question 7

What are the types of crosslinking situations? What occurs when myosin applies movement?

Answer 7

• Permanent: myosin creates tension
• Transient: myosin creates movement
  The cell can change the fractional concentration of permanent to transient crosslinkers to control movement/tension.
• No crosslinkers: myosin creates greatest movement

Question 8

How does stronger crosslinking impact solid-like, stress, and movement?

Answer 8

Stronger cytoskeleton crosslinking makes a more ‘solid’ cell with larger stresses and less movement