Microstructure in biological phase transition

Many cellular structures are formed by the spontaneous condensation of biomolecules into liquid states. Biological condensed states may contain hundreds of different molecules, but only a few of them are necessary to form a condensates. These are called “Scaffolds” while other non-essential molecules, called “Clients”, provide the unique biochemical properties of droplets. We study a synthetic system of phase separating molecules designed by Rosen and co-workers. There are two types of scaffolds (poly-SUMO/poly-SIM) and two types of clients (SUMO/SIM). Mixture of multivalent scaffolds gives condensates which can recruit monovalent clients. Since the condensed phase has a low density, we propose that molecules are highly aligned to form zipper structures. These zippers have defects in the bonding structure that will allow for the subsequent formation of a network. We employ a transfer matrix formalism to compute the grand partition function of the zipper structures and to explain the curious non-monotonic client binding behavior observed in experiments. We find that the 1D nature of the scaffold assembly has important consequence for client recruitment.