8. You have two proteins that you suspect cap the ends of actin that end. To det
ID: 215845 • Letter: 8
Question
8. You have two proteins that you suspect cap the ends of actin that end. To determine which protein caps which end, you mix: - free actin monomers at varying concentrations (A) NO ADDED PROTEIN filaments, preventing actin monomers from either adding or leaving at filament an excess of ATP monomer - either Protein 1, Protein 2, or neither (control) then let the mixture come to steady state. At that point, you measure the mass of free actin monomers and assembled actin microfilaments 0 02 04 0.6 0.8 in the mixture. The plots on the right summarize the results. actin concentration (yM) (B) PROTEIN A. (4pts) From this evidence, please draw a conclusion as to whether Proteins 1 and 2 cap the plus and/or minus ends of actin microfilaments filament Protein 1 caps which end(s)? - 0 0.2 0.4 0.6 08 Protein 2 caps which end(s)?- actin concentration (uM) (C) PROTEIN 2 B. (8pts) Please provide the reasoning for your conclusion. filament 0 0.2 0,4 0.6 08 0 0.2 0.4 0.6 08 actin concentration 41M? Figure 16-13 MBoC6: The Problems Book (o Garland Science 2015)Explanation / Answer
A. Protein 1 caps minus end of the actin filament. Protein 2 caps positive end of the actin filament.
B. Actin filament has two ends - plus end and minus end. Both the ends are helpful in extension of the filament by addition of the monomers. But, extension is faster on the plus end and slower on the minus end.
In the plot (B), the monomer mass is low and filament mass is high. Also, the filament mass is increasing at the faster rate as compared to the plot (A) when no protein is added. This shows that plus end is open for extension and minus end is closed so that neither addition nor removal of actin monomers occurs. Hence protein 1 caps minus end of the actin filament.
In plot (C), the amount of monomer mass is very much higher than the filament mass. This shows that the filament extension is slow as compared to the other two plots. So, plus end is blocked by the protein 2. Furthermore, there is some amount of actin filament present, so the minus end is open for the extension of the filament. Hence, protein 2 only caps plus end.
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