Question

1. Generally, how do the active filaments in the cell cortex attach to the plasma membrane?

2. What cellular components cause some actin filaments to form bundles and others to form networks?

3. What does gelsolin do to actin filaments? How does the cell control gelsolin activity and for what purpose?

4. Cells contain more G-actin that expected. What is the thought to account for the high concentration of G-actin?

Answer 1

1. The cell cortex abundant in F-actin filaments, actin binding proteins and myosin motors is a layer just beneath the cell membrane that gives shape to the cell. The cortex has membrane anchoring proteins called ERM (ezrin, radixin aand moesin)proteins that help in attachment with the cell membrane.

2.Actin binding proteins determine whether the actin will form bundles or filaments. formin helps in forming actin bundles and Arp2/3 complex helps to form actin network.

3.Gelsolin is an actin binding protein that helps is severing(cutting) actin filaments.

Gelsolin acts by elevated Ca2+ concentration so keeping Ca2+ at low doses its activity can be avoided.Gelsolin can also be inhibited by increasing concentration of  phosphatidylinositol (4,5)-bisphosphate . This binds to S1, S2, S3 sites on gelsolin preventing binding of gelsolin to actin and thus reducing its chances of getting severed .

4. Presence of more G actin can mean that the F-actin that is the filamentous actin is not being formed properly or are getting depolymerised at a faster rate than polymerisation.It could also mean the overexpression of the actin G-protein in excess of its utilization to form F- actins.