Confinement-dependent localization of diffusing aggregates in cellular geometries

Confinement has a strong influence on diffusing nano-sized clusters. In particular, biomolecular aggregates within the shell-like confining space of a bacterial cell have been shown to display a variety of localization patterns, from being midcell to the poles. How does the confining space determine where the aggregate will localize? Here, using Monte Carlo simulations we have calculated the equilibrium spatial distribution of fixed-sized clusters diffusing in spherocylindrical shells. We find that localization to the poles depends strongly on shell thickness and the size of the cluster. Compared to being at midcell, polar clusters can be more bent and hence have higher energy, but they also can have a greater number of defects and hence have more entropy. Under certain conditions this can lead to polar clusters having a lower free energy than at midcell, favoring localization to the poles. Our findings suggest possible localization selection mechanisms within shell-like geometries that can arise purely from cluster confinement.