  Understanding the role of non-equilibrium driving in self-organization is
crucial for developing a predictive description of biological systems, yet it
is impeded by their complexity. The actin cytoskeleton serves as a paradigm for
how equilibrium and non-equilibrium forces combine to give rise to
self-organization. Motivated by recent experiments that show that actin
filament growth rates can tune the morphology of a growing actin bundle
crosslinked by two competing types of actin binding proteins, we construct a
minimal model for such a system and show that the dynamics are subject to a set
of thermodynamic constraints that relate the non-equilibrium driving, bundle
morphology, and molecular fluxes. The thermodynamic constraints reveal the
importance of correlations between these molecular fluxes, and offer a route to
estimating microscopic driving forces from microscopy experiments.
