b. You are characterizing a novel actin filament end-binding protein that you cl

Question

b. You are characterizing a novel actin filament end-binding protein that you cleverly name "cappin." You wish to determine if cappin caps actin filaments at the (+) end or at the (-) end. You incubate an excess of cappin with various concentrations of G- actin under conditions that induce polymerization (bottom graph). Control actin samples are incubated in the absence of cappin (top graph). The results of in vitro actin polymerization assays are shown below. The mass contributed by filaments and by monomeric actin was determined over a range of increasing actin concentrations. Based on these results, does cappin most likely bind to the plus end of the filament or the minus end? Design an experiment, using myosin S1 fragments and electron microscopy, to corroborate your conclusion. Include the result that you would expect to see if you are correct. 10 pts 100 Control (-protein X) Filament - Monomer 100 Experimental (+protein X) Monomer Filament 100 Actin concentration

Explanation / Answer

first graph shows that filament formation occurs at a very low concentration of monomer.

after a specific concentration of monomers, increase in actin concentration led to saturation of monomers. however, filament formation started.

in the second graph, when cappin protein was added, filament formation started at a very high concentration of actin monomers. till then, filament formation was not started, resulting in a high accumulation of monomers.

so, i assume that cappin binds to + end and delays the filament nucleation.

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