Figure 3. Time evolution of ${\displaystyle R_{\mathrm{g}}}$.

The neck’s gyration radius is shown in the end stages of a pure constriction protocol. The time series starts when the scaffold radius is ${\displaystyle R=R_{\mathrm{c}}=10.5\sigma }$, which leads to ${\displaystyle R_{\mathrm{g}}/\sigma =4.187\pm 0.02}$ (as measured over the subsequent $\mathrm{600}\mathrm{}\tau$, with an error determined via blocking (Flyvbjerg and Petersen, 1989)). After that, a further constriction of the filament to $R\mathrm{=}\mathrm{10}\mathrm{}\sigma$ reduces the neck’s gyration radius to ${\displaystyle R_{\mathrm{g}}/\sigma =3.890\pm 0.04}$ but does not trigger hemifission (which corresponds to the much smaller value ${\displaystyle R_{\mathrm{g}}\approx 2\sigma }$—see Figure 2f). Turning off the adhesion between scaffold and membrane only reduces the gyration radius by a very minor amount, ${\displaystyle R_{\mathrm{g}}/\sigma =3.841\pm 0.035}$, a change that is not statistically significant ($p\mathrm{=}\mathrm{0.36}$). Once we additionally let the scaffold disassemble into (non-adhesive) dimers (see also Video 4), the neck very rapidly doubles its radius within about ${\displaystyle 200\tau }$, after which the definition of its location becomes ambiguous.