Filamentous structures in cells called microfilaments are comprised of a polymer

Question

Filamentous structures in cells called microfilaments are comprised of a polymer of actin protein molecules. Actin monomers (M) can bind to the growing ends of the filament, where the distinct ends (barbed and pointed) grow at different rates. The rate constants for filament growth at each end were measured as a function of diffusion constant of actin monomer which was varied by adjusting the viscosity of the solution. Examine the data, below and answer the questions. [Data from Drenckhahn & Pollard, JBC 261, 27, 12754-8] Under the conditions shown, which end of the actin filament grows more quickly? Is polymerization at the barbed end diffusion-limited? Is polymerization at the pointed end diffusion-limited? Justify your answer in each case. It is not required, but you may use the following information: D_F 10^-13 m^2/sec for the diffusion constant of the actin filament at all viscosities; a combined radius R_MF of 4 nm; and an electrostatic correction factor f = 1.

Explanation / Answer

A)The barbed end will grow more quickly as we can in the graph the sudden rise in size with increase in D(m).

B)The polymerization at pointed end is diffusion limited because it does not grow and reduces in size while the polymerization at barbed end is not diffusion limited as it grows in size .

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