You have discovered a new organism with a very simple actin cytoskeleton. In the

Question

You have discovered a new organism with a very simple actin cytoskeleton. In the resting state this cell has a lot of short actin filaments underneath the plasma membrane. These actin filaments are capped at their plus ends by the capping protein Gelsolin. The minus ends of these filaments are NOT capped and there are no other proteins bound to the actin filaments. The actin filaments in this resting cell are not growing or shrinking.

When the cell receives a stimulus to move it generates a lipid called PI(4,5)P2 which we will learn about later in the course. This lipid binds to Gelsolin and inactivates it releasing the actin filaments.

Based on this (fictional) story answer the following questions:

1) What is the concentration of G-actin in the resting cell? (I am not asking about a particular value but a description of such concentration).

2) In the resting cells why are the actin filaments not growing or shrinking?

3) What will happen to the actin filaments when Gelsolin is inactivated in the stimulated cell? Will they grow, shrink or stay the same? Why?

Explanation / Answer

1) The number will decrease below the critical concentrations.

2) In resting cells, actin filaments depend on the presence of calcium or magnesium and relative concentrations of ATP or ADP.

3) shrink

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