  In many organisms, cell division is driven by the constriction of a
cytokinetic ring, which consists of actin filaments and crosslinking proteins.
While it has long been believed that the constriction is driven by motor
proteins, it has recently been discovered that passive crosslinkers that do not
turn over fuel are able to generate enough force to constrict actin filament
rings. To study the ring constriction dynamics, we develop a model that
includes the driving force of crosslinker condensation and the opposing forces
of friction and filament bending. We analyze the constriction force as a
function of ring topology and crosslinker concentration, and predict forces
that are sufficient to constrict an unadorned plasma membrane. Our model also
predicts that actin-filament sliding arises from an interplay between filament
rotation and crosslinker hopping, producing frictional forces that are low
compared to those of crosslinker-mediated microtubule sliding.
