Question

Polymerization of actin in vitro illustrates several import properties of the polymerization process that occurs in cells. Which of the following is NOT true about G-actin monomer polymerization into F-actin filaments in vitro?

During the steady state phase, the total amount of F-actin does not change, but G-actins continue to add to and dissociate from both ends.

During the elongation phase, G-actin monomers add only to the (+) end of each actin filament.

During the steady state phase, actin is at a specific concentration, known as the critical concentration, and is always disassembled in the G-actin form despite continual addition and loss of actin monomers at both ends of each actin filament.

The critical concentration is the G-actin concentration at which the rate of addition is equal to the rate of loss of subunits from one end of the actin filament.

Answer 1

Answer

Option 2 is wrong.

During the elongation phase, the G- actin monomers add to both plus and minus ends. The Plus end has more affinity with G-actin, whereas,minus-end has less affinity towards G- actin. Option 1 says that during the steady-state phase, the third phase in the actin polymerization, the total amount of F- actin does not change, but G- actin continue to dissociate from the minus end and add to the plus end. This process is called as treadmilling. This results in the continuous movement of relative positions of actin subunits in the filament. The critical concentration can be explained as the least concentration of G-actin required to maintain the polymerization activity. As explained in the options 3 and 4, during the steady-state phase, the G- actin adds to plus end and dissociates from the minus end simultaneously to maintain the critical concentration of G- actin. As explained earlier, it does not cause the elongation of filament, but treadmilling occurs.