Figure 2.

Tube dynamics of cortex and bulk liposomes. (A) Stepwise tube extrusion from liposomes with an actin cortex attached to the bilayer membrane (red) and with actin in bulk (blue). The numbers above each step indicate the hydrodynamic flow force, F_h = 6πηUR, in pN. At F_h = 6.7 pN (red curve), the tube initiated from the cortex liposome retracts itself, showing that 6.7 < F₀ < 7.9 pN, whereas for bulk liposome (blue curve), F₀ < 6.9 pN. The solid curves in the inset and the dotted lines in the main figure are adjustments to the extrusion curves with a modified KV body schematized in (B). The dashed line in the inset of (A) shows the case where μ₂ = ∞. The R² for the successive fits for the bulk liposomes are 0.97 for the first curve and 0.99 for the remaining, and for the cortex liposome, 0.97 for the first two, and 0.99 for the next three. (B) Modified KV body: a dashpot with a damping coefficient μ₂ is added in series to a KV body comprising a spring constant k and a dashpot with a damping coefficient μ₁ assembled in parallel and pulled by a force f. (C) L(t) curve corresponding to the mechanical model represented in (B). At short times, the tube elongates rapidly to a length L_cross, after which the elongation occurs at a constant speed ${\dot{L}}_v$.