Actin and Myosin

ID #1575

If it's true that when the end of the F actin is above critical concentration, then polymerization is favored and when it is below critical concentration, then depolymerization is favored. Then why does it say in the Cooper book that "the critical concentration of actin monomers is higher for addition to the pointed(-) end thanto the barbed (+) end of actin filaments" thus making this homework question true: Correct: The minus end of F actin has a higher critical concentration than the plus end, thus depolymerization is favored.

The explanation for this question lies in the understanding about the difference between what's constant and what's variable in this scenario. The critical concentration is constant -- it is the same for a given end of an actin filament at all times. What varies is the actual amount of available G-actin. Since the critical concentration is higher at the minus end than the plus end, and it is, that means it's going to take a lot more G-actin present to be at (or above) the critical concentration. So for a given amount of G-actin, you are more likely to be depolymerizing at the minus end.

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