Actin in non-skeletal muscles

Question 1

Cytokinesis

Answer 1

• Actin/myosin functions to form the contractile ring that separates the daughter cells during mitosis
• Contractile ring is similar to a sarcomere but it is unstable
  
  • As it contracts, it depolymerises (gets smaller and smaller) until it disappears

Question 2

Smooth muscle

Answer 2

• Myosin phosphorylation is a major regulator of smooth muscle contraction
  
  • Smooth muscle contraction is involuntary (unlike skeleton; not triggered by nerves)
  • Triggered by extracellular signals
• Slower and more persistent contraction than skeletal muscles (no transient calcium)
• Phosphorylation of the regulatory light chain (LC) from the myosin thick filament allows conformational changes and contraction

Question 3

Vesicle transport

Answer 3

• Budding yeasts require the movement of DNA and organelles into the new bud when replicating
  
  • Uses myosin 5-bound vesicles that are carried along actin filaments
• Myosin 5 is inactive in the absence of cargo

Question 4

Cytoplasmic streaming

Answer 4

• Myosin generates cytoplasmic streaming: flow inside the cytoplasm, speeding up diffusion
• Key in plant cells
• Non-moving cortical actin goes all around the cell

Question 5

How do cells move?

Answer 5

• Cell movement depends on the actin-myosin cytoskeleton
• Stress fibres of actin and myosin are observed in the cell; similar to sarcomere
  
  • They contact transmembrane proteins (integrins) at both ends which connect it to the extracellular matrix
  • Integrins provide traction for movement

Question 6

What is chemotaxis?

Answer 6

Chemotaxis: movement of a cell towards a molecule or in response to something (requires chemotactic receptors)

The receptors are located on the whole surface of the cell

Question 7

Describe the 4 steps of cell movement

Answer 7

1. The cell is bound using focal adhesions and receives a chemotactic signal
   
   • The focal adhesions are created by the integrins
   • In between every focal adhesion are stress fibres
2. Extension occurs as actin polymerises
   
   • Forms filopodia (thin) and lamellipodia (large) structures
   • These structures have integrins on it which are looking for a place to stick and form a new focal adhesion and additional stress fibre
3. Adhesion and translocation occurs via actin-myosin contraction
4. De-adhesin and endocytic recycling occurs

Question 8

Describe the role of the proteins Cdc42, Rac, and Rho

Answer 8

Rho-GTP: regulates formin (unbranched actin regulation)

Cdc42/Rac: regulated Arp2/3 (branched actin regulation along with WASp and WAVE)

All of these are GTPase and must be in the GTP form

Question 9

What does ‘dominant’ mean?

Describe dominant Rho, Cdc42, and Rac

Answer 9

• Dominant-active Rho shows it is responsible for unbranched stress fibre formation
• Dominant-active Cdc42 shows it forms small filopodia
• Dominant-active Rac shows it forms lamellipodia

Note that all of these work together (if one or more is missing, a “wound” cannot be filled)

Question 10

Describe the coordination of cell migration

Answer 10

During this process, cell polarity causes microtubules to bring vesicles that are carrying G-actin, profilin, Arp2/3 and other resources to the front of the cell