  We discuss a curious example for the collective mechanical behavior of
coupled non-linear monomer units entrapped in a circular filament. Within a
simple model we elucidate how multistability of monomer units and exponentially
large degeneracy of the filament's ground state emerge as a collective feature
of the closed filament. Surprisingly, increasing the monomer frustration, i.e.,
the bending prestrain within the circular filament, leads to a conformational
softening of the system. The phenomenon, that we term polymorphic crunching, is
discussed and applied to a possible scenario for membrane tube deformation by
switchable dynamin or FtsZ filaments. We find an important role of cooperative
inter-unit interaction for efficient ring induced membrane fission.
