Question

Actin treadmilling is a phenomenon observed both in vitro and in cells.

(a) Describe how actin filaments are able to “treadmill;” what drives this phenomenon?

(b) What is a cellular actin binding protein that would increase actin treadmilling, and describe how.

Answer 1

*******************************************************

(A) Threadmilling is a phenomenon mainly observed in actin filaments.It occurs becuase of change in filament length.One end grows while the other end shirnks resulting in section of a filament seemingly moving across the cytosol.

The polymerization of actin filaments proceeds in three sequential phases:-

• The first phase is marked by a lag period in which G-actin aggregates into short, unstable oligomers. Once the oligomer reaches a certain length ,

• second stage begins by promoting rapid elongation into a filament by the addition of actin monomers to both of its ends. As F-actin filaments grow, the concentration of G-actin monomers decreases until it is in equilibrium with the filament.

•This third phase is called steady state because there is no net change in the total mass of filaments.

Once the steady-state phase is reached, the equilibrium concentration of the pool of unassembled subunits is called the critical concentration (Cc).

One end of the filament, the (+) end, elongates five to ten times faster than does the opposite, or (−), end.It is due to difference in Cc values in 2 ends.

(B)

Cellular actin binding proteins are the proteins that bind to actin. Eg alpha actinin ,beta spectrin etc

One of the cellular actin binding protein is profilin . The main function of profilin probably is to promote assembly of actin filaments in cells. It appears to do so by several mechanisms.

• First, as a complex with G-actin, profilin is postulated to assist in addition of monomers to the (+) end of an actin filament.

• profilin also interacts with membrane components involved in cell-cell signaling, suggesting that it may be particularly important in controlling actin assembly at the plasma membrane

*******************************************************