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LASSI: A lattice model for simulating phase transitions of multivalent proteins

Fig 12

Slopes of tie lines within elliptic binodals are important for systems that undergo phase separation via obligate heterotypic interactions.

The ellipse is drawn to fit the locus of points based on the value that meets our criteria for a density transition (see main text). Data for constructing the ellipse were taken from simulations of the N130 + rpL5 system–see Fig 11(A). This ellipse is used to assess the impact of slopes of tie lines for a two-component system comprising of macromolecule A that undergoes phase separation via obligate heterotypic interactions with macromolecule B. (a) Ellipse with nearly horizontal tie lines. The vertical lines shown in green, grey, and blue correspond to fixed values for [B] along the abscissa. As [A] increases, the system traverses across the two-phase regime, delineated by the ellipse, starting outside the ellipse, crossing the ellipse, and exiting the ellipse at high concentrations of A. (b) For each fixed value of [A], the plot shows how [A]_Sol varies with [A]. The red points on each curve were extracted from within the two-phase regime, whereas the black points lie outside the two-phase regime. Clearly, [A]_Sol does not stay fixed as [A] increases. (c) Equivalent plot to that shown in panel (a) for the tie lines that we obtain for the N130 + rpL5 system. (d) Equivalent plot to that shown in panel (b). Note the non-linear variation of [A]_Sol as [A] increases. (e) Ellipse annotated with vertical tie lines. In this case, phase separation of [A] does not depend on obligate heterotypic interactions with B, but B can bind to A and has a choice of binding preferentially to A in either the dense or dilute phase. Here, A becomes the macromolecule and B the ligand. (f) Preferential binding of the ligand to the macromolecule in its dilute phase will shift the saturation concentration, assessed in terms of [A]_Sol, upward and this shift will depend on [B]. Accordingly, the plateau value of [A]_Sol in the two-phase regime shifts to higher values for higher values of [B].

doi: https://doi.org/10.1371/journal.pcbi.1007028.g012