The decrease in the diameter of the interfacial line as the surface area of the bud increases for large interfacial line tension λ = 60 pN. The series of equilibrium membrane shapes is obtained by means of the variational method with the constraints that the mechanical forces applied to the membrane surface by the actin filaments is constant. The membrane parameters are as follows: bending rigidities κ_{1} = 50 *k*_{B}*T*, κ_{2} = 100 *k*_{B}*T*; the Gaussian bending rigidities κ_{G}^{(1)} = κ_{G}^{(2)}; the surface tensions σ_{1} = 5 × 10^{−5} N/m, σ_{2} = 1 × 10^{−4} N/m; the active force ƒ = 1.0 pN, α = 2π/3; the osmotic pressure *P*_{0} = 0. The surface areas of the buds are as follows: 1,668 nm^{2} (*a*); 2,980 nm^{2} (*b*); 4,760 nm^{2} (*c*); and 8,415 nm^{2} (*d*). The corresponding diameters of the interfacial line are as follows: 16.00 nm (*a*); 8.94 nm (*b*); 6.14 nm (*c*); and 4.71 nm (*d*). The natural length cutoff in this model is the width of the membrane, ≈5 nm. Therefore, the actual distance between the two inner leaflets at the interface for *d* is negative, i.e., the bud is already pinched off. The line is fit to the computed points (♦). Note that as the surface area of the bud region approaches zero, the diameter of the interface does as well. Because the surface area of the bud region is very small, the bending energy per area dominates the line tension; consequently, increasing the interfacial line dominates bending the membrane surface. Conversely, when the surface area of the bud is very large, the bending energy per area is dominated by the line tension, and the interface will shrink, making the membrane bend more. Therefore, the peak in the plot corresponds to the point where the bending energy per area is comparable to the line tension. We restrict ourselves to line tensions that dominate, corresponding to the case where the interfacial diameter decreases as the bud size increases.

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