Question

Question 4 (1 point) You have discovered a new type of protein in bacteria that polymerizes into filaments similar to actin and microtubules. Through careful experiments you determine that the critical concentration of the minus end of the filament is 10 uM and the critical concentration of the plus end is 1 uM. If the soluble concentration of free unassembled protein is 5 uM what happens? O Polymerization at plus end and depolymerization at minus end? O Polymerization at plus end and polymerization at minus end? O Depolymerization at plus end and depolymerization at minus end? O Depolymerization at plus end and polymerization at minus end?

Answer 1

Polymerization at plus end and depolymerization at minus end.

The general process of polymerization of filaments like actin and microtubules is as follows:

A few subunits aggregate into short oligomers and form what are called as nuclei, which are essentially the seeds around which the filament then grows. Following nucleus formation, more subunits start adding on to the nucleus at both ends - one is denoted as the plus end and the other as the minus end. Finally, there reaches an equilibrium point at which the concentration of the subunits is in equilibrium with the filament. This is called the critical concentration. Above the critical concentration, there are more subunits than would be at equilibrium, therefore polymerization i.e. addition of subunits occurs until equilibrium is reached. Below the critical concentration, there are fewer subunits than would be present at equilibrium, therefore the filament undergoes depolymerization, i.e. removal of subunits, to restore equilibrium.

In the given situation, we have the concentration of the subunit i.e. the free unassembled protein to be 5$\small \mu$M, which is lesser than the critical concentration of the minus end (10$\small \mu$M) and higher than the critical concentration of the plus end (1$\small \mu$M). Therefore, there is polymerization at the plus end and depolymerization at the minus end.