  Membrane neck formation is essential for scission, which, as recent
experiments on tubules have demonstrated, can be location dependent. The
diversity of biological machinery that can constrict a neck such as dynamin,
actin, ESCRTs and BAR proteins, and the range of forces and deflection over
which they operate, suggest that the constriction process is functionally
mechanical and robust to changes in biological environment. In this study, we
used a mechanical model of the lipid bilayer to systematically investigate the
influence of location, symmetry constraints, and helical forces on membrane
neck constriction. Simulations from our model demonstrated that the energy
barriers associated with constriction of a membrane neck are
location-dependent. Importantly, if symmetry restrictions are relaxed, then the
energy barrier for constriction is dramatically lowered and the membrane
buckles at lower values of forcing parameters. Our simulations also show that
constriction due to helical proteins further reduces the energy barrier for
neck formation compared to cylindrical proteins. These studies establish that
despite different molecular mechanisms of neck formation in cells, the
mechanics of constriction naturally leads to a loss of symmetry that can lower
the energy barrier to constriction.
