Figure 4: Model captures the droplet scaffold composition observed in experiment.

(a) The ratio of poly-SUMO to poly-SIM (N_U/N_I) calculated from our theory agrees well with the droplet stoichiometry in experiments.⁶ At high stoichiometric mismatches the approximation of purely 1D filaments breaks down because the concentration of free scaffolds is high enough to allow binding in the gaps. A correction accounting for monovalent scaffold-gap binding (inset cartoon) resolves the discrepancy with experiment (dotted line). (b) Excess SUMO scaffold accumulates in the dilute phase and depletes the concentration of monomeric SIM scaffolds. (c) Small stoichiometric mismatches promote increased scaffold accumulation in the droplet phase, but the trend reverses as the average filament size drops. The discrepancy at 90 μM can be explained by a breakdown of the 1D approximation as depicted in the cartoons of panel (a). In all panels lines indicate theory and circles denote the experiments of⁶