Cytochalasin D is a fungal alkaloid that binds specifically to the (+) end of F-
ID: 58455 • Letter: C
Question
Cytochalasin D is a fungal alkaloid that binds specifically to the (+) end of F-Actin and prevents further polymerization at this end. The Cc of ATP-Actin for the pointed end of F-actin is 0.6 µM and the Cc of ATP-Actin for the barbed end of F-actin is 0.12 µM. At what concentration of G-Actin would only the pointed end of a MF elongate if it was incubated in the presence of excess cytochalasin D?
A. 0.15 µM ATP-Actin
B. 0.4 µM ATP-Actin
C. 0.7 µM ATP-Actin
D. Pointed end would not elongate at any of these concentrations.
Explanation / Answer
ans. 0.7 µM ATP-Actin
The association rate at the plus end of ~12 µM-1s-1 and ~ 1.3 s-1 at the minus end means, that 1 µM of free ATP-actin leads to the addition of on average 12 monomers per second to the plus end of already existing filaments.
this is why at 0.7µM G-Actin would only the pointed end of a MF elongate if it was incubated in the presence of excess cytochalasin D
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