Pset 2 Concepts

Question 1

How would free energy upon actin monomer binding and force exerted on the load change if the ATP was replaced by a non-hydrolyzable ATP analog,
and why?

Answer 1

These values would not change if the ATP was replaced by a non-hydrolyzable ATP analog,
because ATP hydrolysis is not required for actin polymerization. The reaction is energetically
favorable due to the excess of free actin monomers in solution.

Question 2

How would replacing the ATP with a non-hydrolyzable ATP analog affect treadmilling, and
why?

Answer 2

It would prevent treadmilling. All actin monomers within the filament would be bound to the non-
hydrolyzable ATP analog, abolishing the energetic difference associated with addition of monomer
to the barbed end versus the pointed end.

Question 3

If the monomer concentration was reduced so that it was below the critical concentration for the
barbed end too, would the filament disassemble faster at the barbed end or the pointed end?

Answer 3

It would disassemble faster at the barbed end.

Question 4

Actin network

Answer 4

The actin network determines the shape of an adherent cell. If the actin network is flowing inward
relative to the cell boundary, but the boundary is not moving, then the actin network must be
polymerizing at the same rate as the inward flow (50 nm/s).

Question 5

inhibits myosin contraction

Answer 5

Myosin contraction generates inward flow, so inhibition of myosin should reduce the inward flow
rate. Actin polymerization should push the cell membrane out, away from the center of the cell,
increased the cell area.

Question 6

inhibits actin polymerization

Answer 6

Inward flow should remain unchanged, but since it’s no longer balanced by actin polymerization,
the cell membrane should move inward and the cell area should decrease.

Question 7

inhibits adhesions

Answer 7

Inward flow should increase, because there’s less adhesive force to balance myosin contraction,
and the cell area should decrease.

Question 8

Local inhibition of adhesions

Answer 8

results in increased retrograde flow of the actin network on one side
of the cell. This results in retraction of the cell boundary, reducing the area of the cell and setting
the rear of the cell. This reduction in cell area is accompanied by a reduction in membrane tension,
which ultimately allows actin polymerization rates to increase at the front of the cell.