Abstract

Biomolecular condensates play a major role in cell compartmentalization, besides membrane-enclosed organelles. The multivalent SLP65 and CIN85 proteins are downstream B cell receptor (BCR)-signaling effectors, required for a proper immune response. Both proteins phase separate together with vesicles to form pre-signaling clusters. Within this tripartite system, six PRMs of SLP65 interact promiscuously with three SH3 domains of the CIN85 monomer, establishing 18 individual SH3-PRM interactions whose individual dissociation constants we determined. Based on these 18 dissociation constants, we measured the phase separation properties of the natural SLP65/CIN85 system as well as designer constructs that emphasize the strongest SH3/PRM interactions. By modelling these various SLP65/CIN85 constructs with the program LASSI (LAttice simulation engine for Sticker and Spacer Interactions) we reproduced the observed phase separation properties. In addition, LASSI revealed a deviation in the experimental measurement, which was independently identified as a previously unknown intramolecular interaction. Thus, thermodynamic properties of the individual PRM/SH3 interactions allow to model the phase separation behavior of the SLP65/CIN85 system faithfully.