  Self-assembly and force generation are two central processes in biological
systems that usually are considered in separation. However, the signals that
activate non-muscle myosin II molecular motors simultaneously lead to
self-assembly into myosin II minifilaments as well as progression of the motor
heads through the crossbridge cycle. Here we investigate theoretically the
possible effects of coupling these two processes. Our assembly model, which
builds upon a consensus architecture of the minifilament, predicts a critical
aggregation concentration at which the assembly kinetics slows down
dramatically. The combined model predicts that increasing actin filament
concentration and force both lead to a decrease in the critical aggregation
concentration. We suggest that due to these effects, myosin II minifilaments in
a filamentous context might be in a critical state that reacts faster to
varying conditions than in solution. We finally compare our model to
experiments by simulating fluorescence recovery after photobleaching.
