Coarsening and accelerated equilibration in mass-conserving heterogeneous nucleation

We propose a model of mass-conserving heterogeneous nucleation to describe the dynamics of ligand-receptor binding in closed cellular compartments. When the ligand dissociation rate is small, competition among receptors for free ligands gives rise to two very different long-time ligand-receptor cluster size distributions. Cluster sizes first plateau to a long-lived, initial-condition dependent, "metastable" distribution, and coarsen only much later to a qualitatively different equilibrium one. Surprisingly, we also find parameters for which a very special subset of clusters have equal metastable and equilibrium sizes, appearing to equilibrate much faster than the rest. Our results provide a quantitative framework for ligand binding kinetics and suggest a mechanism by which different clusters can approach their equilibrium sizes in unexpected ways.