57. The concentration of unpolymerized actin in cells is a couple orders of magn

Question

57. The concentration of unpolymerized actin in cells is a couple orders of magnitude higher than critical concentration for actin assembly that is observed in vitro.
A. In the absence of proteins such as __________________________ that nucleate the assembly of microfilaments, there is a long “lag phase” in assembly of microfilaments. Why do proteins that nucleate assembly eliminate the “lag phase” and why is this important in vivo?

B. Define “critical concentration” and briefly speculate why it would make sense for cells to maintain high concentrations of unassembled actin.

Explanation / Answer

A. Actin Nucleation: Two actin molecules bind weakly, but addition of a third stabilizes the complex. This trimer then adds additional molecules and forms a "nucleation site". This is the slow, or lag phase of the polymerization process. One could add fragments of actin filaments to speed this up, in vitro. Or, key actin binding proteins may help to speed this process. Elongation: Addition of actin molecules to form a long helical polymer. After a period of growth, an equilibrium phase is reached in which depolymerization controls the length as new monomers are added. This is important in VITRO (not vivo!) because it allows us to isolate actin as the protein that nucleates the assembly of microfilaments. To test the simulation, an arbitrary in vitro environment is chosen for which good experimental evidence is available. B. critical concentration - concentration of surfactants above which micelles form and almost all additional surfactants added to the system go to micelles. High concentrations are good because - just in case something "breaks" or a wound occurs - the microfilaments can quickly assembly and repair the wound.

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