  Filamentous actin, a species of dynamic protein polymers, is one of the main
components of the cytoskeleton of eukaryotic cells. We formulate a class of
models that predict the possibility of bistable steady states in populations of
dynamic actin filaments. They are built upon a basic model of actin dynamics
that includes severing and capping in the presence of a finite actin monomer
pool. The key additional ingredient is the presence of a single species of
effector molecules that is partially sequestered to an inactive state by
binding to free G-actin. In its unbound active state, this effector species can
\emph{enhance} the rate of nucleation of filamentous actin or its growth speed,
or \emph{inhibit} the activity of capping or severing proteins. Using an
explicit analytical solution of the basic actin dynamics model, we show that
bistability is predicted to occur in all of the proposed models. We verify
these predictions using particle-based stochastic simulations. In addition, we
show that switching between the two stable states can be achieved by transient
manipulation of the free G-actin pool size.
