You have recently been hired by a research team that studies cystic fibrosis (CF

Question

You have recently been hired by a research team that studies cystic fibrosis (CF), a fatal disorder that clogs the lungs and other organs with a viscous, sticky mucous that interferes with breathing and digestion. The leader of your research team has informed you that a major fraction of the mucous in CF patients consists of filamentous actin (F-actin). His hypothesis is that this actin may play a key role in interfering with breathing and digestion in CF patients. In order to test your laboratory skills and knowledge he decides to give you a quiz. Your first assignment is to assemble actin monomers into filaments. In a test tube you mix actin monomers (G-actin) with KCL MgCl, and ATP and then measure filament assembly with the following result: 1 001 %actin in filaments % actin time You immediately notice that there is a lag phase before actin filaments can be detected and that the "%actin in filaments" plateaus at less than 100%. 1. Explain the lag phase. (2 pts 2, Explain why the percent of actin in filaments peaks at a value less than 100% (2 pts)

Explanation / Answer

1. Actins initially form small aggregates consisting of three actin monomers. The filaments then grow my polymerization or reversible addition of monomers. The initial lag is the time taken to form the initial aggregates.

2. Actin polymerization is a reversible process and there exists an equilibrium between actin monomers and filaments. Complete polymerization of all of the monomers never occurs, and equal concentration of monomers exists as that of filaments. Thus, the peak value never reaches 100%, it is always less than 100% (100% polymerization does not occur)

3. There is an availability of short filaments of actin to which monomers are added. Since there is no requirement for formation of initial aggregates, there is an immediate log phase

4.Profilin. While cytochalasin D bind to the + ends of actin filaments and block their elongation, cofilins binds to ADP-actin. Cofilin remains bound to actin monomers following filament disassembly, thus sequesters the dissociated monomers away from reincorporation into filaments.

Profilin removes cofilin, profilin promotes exchange of ADP for ATP, and thus results in formation of ATP-actin monomers. ATP-actin monomers can be incorported faster and effectively into filaments.

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