  Living cells dynamically modulate the local morphologies of their actin
cytoskeletons to perform biological functions, including force transduction,
intracellular transport, and cell division. A major challenge is to understand
how diverse structures of the actin cytoskeleton are assembled from a limited
set of molecular building blocks. Here we study the spontaneous self-assembly
of a minimal model of cytoskeletal materials, consisting of semiflexible actin
filaments, crosslinkers, and molecular motors. Using coarse-grained
simulations, we demonstrate that by changing concentrations and kinetics of
crosslinkers and motors we can generate three distinct structural phases of
actomyosin assemblies: bundled, polarity-sorted, and contracted. We introduce
new metrics to distinguish these structural phases and demonstrate their
functional roles. We find that the binding kinetics of motors and crosslinkers
can be tuned to optimize contractile force generation, motor transport, and
mechanical response. By quantitatively characterizing the relationships between
modes of cytoskeletal self-assembly, the resulting structures, and their
functional consequences, our work suggests new principles for the design of
active materials.
