Actin

Question 0

What does actin do in a cell?

Answer 0

Cell migration, cell adhesion, shape, structure, mechanical strength

Question 1

What are the three major cytoskelatal systems in cells?

Answer 1

1. Actin microfilaments (dynamic)
2. MTs (biggest) (dynamic)
3. Intermediate filaments

Question 2

What are the components of actin cytoskeleton?

Answer 2

Lamellipodia/ruffles: sheet like membranes for mvt through environments.
Filopodia: spike extensions.
Stress fibers: intracellular fibers

Question 3

What are the subunit properties of actin? How abundant is it?

Answer 3

Monomer g actin binds to ATP.
Polymer f actin is a chain of noncovalent bonded actin monomers. With polarized ends….ATP binding cleft faces - end.

It’s very abundant 1-5% total protein. In all eukaryotes.

Question 4

What’s an experiment to see if actin polymerization is polar?

Answer 4

Take short “seed” and add g actin to polymerize. Faster polymerization at + end.

Question 5

Describe the steps of actin polymerization.

Answer 5

In absence of seed, nucleation must occur:

1. Nucleation. Slow. Trimer is stable. Rate limiting step.
2. Elongation. Fast.
3. Steady state: rate addition = rate of loss. This happens at Cc

Question 6

What Are the kinetics of actin polymerization?

Answer 6

Nucleation: lag phase; super slow
Elongation: fast (polymerization)
Steady state: treadmilling➡️ no change in mass over time. Plus end adds g actin, minus ends lose g actin

With seeds, don’t need nucleation.

Question 7

How is cell migration controlled by actin polymerization?

Answer 7

Extension protrusion in lamellipodium, adhesion, translocation, de adhesion.

Question 8

How is listeria used as a model for actin based mvt?

Answer 8

In finding which proteins are sufficient for polymerization, Act A in listeria acts as a nucleator by recruiting a host cell nucleator (arp2/3).

• Arp 2/3 organizes y branches
• Capping protein caps + ends if old filaments so newer ends can polymerize.
• cofilin= severs ADP filaments and triggers disassembly for monomer recycling. (Actin depolymerizjng factor= ADF)

Question 9

How does actin use myosin motors?

Answer 9

Myosin II: muscle contraction
-not processive, gliding rather than walking
Myosin V: vesicle mvt.
-processive and will stay on actin filament. Walk rather than glide.
- transfer vesicles to cell perifery

Question 10

Most myosin motors move toward the plus end of actin filaments. If you attach the myosin motor domain to glass and add actin filaments and ATP as depicted in the picture the actin filaments will move along the glass. Which end of the actin filament will be leading and which would be lagging?

Answer 10

Minus end will be leading, plus end will be lagging