The study investigates the phase transitions in the assembly of multi-valent signaling proteins, focusing on the interactions between synthetic multivalent macromolecules, including multi-domain proteins and RNA. These interactions lead to sharp liquid-liquid demixing phase separations, forming micron-sized liquid droplets in aqueous solutions. The macroscopic transition corresponds to a molecular transition from small complexes to large, dynamic supramolecular polymers. The concentration required for phase transition is directly related to the valency of the interacting species. For example, the phase transition in the actin regulatory protein N-WASP interacting with Nck and phosphorylated nephrin results in a sharp increase in activity toward the actin nucleation factor, Arp2/3 complex. The transition is governed by the degree of phosphorylation of nephrin, allowing kinases to control the system's regulatory effect. The widespread occurrence of multivalent systems suggests that phase transitions may be used to spatially organize and biochemically regulate information throughout biology. The study also examines the behavior of multivalent proteins in cells, where co-expression of SH3 and PRM proteins results in the formation of dynamic puncta. Additionally, the nephrin/Nck/N-WASP system is investigated, showing that the phase transition correlates with a biochemical activity transition, enhancing the activation of the Arp2/3 complex. These findings provide insights into how multivalent interactions can yield sharp transitions between physically and functionally distinct states, potentially contributing to the structure and function of cellular bodies and other compartments.The study investigates the phase transitions in the assembly of multi-valent signaling proteins, focusing on the interactions between synthetic multivalent macromolecules, including multi-domain proteins and RNA. These interactions lead to sharp liquid-liquid demixing phase separations, forming micron-sized liquid droplets in aqueous solutions. The macroscopic transition corresponds to a molecular transition from small complexes to large, dynamic supramolecular polymers. The concentration required for phase transition is directly related to the valency of the interacting species. For example, the phase transition in the actin regulatory protein N-WASP interacting with Nck and phosphorylated nephrin results in a sharp increase in activity toward the actin nucleation factor, Arp2/3 complex. The transition is governed by the degree of phosphorylation of nephrin, allowing kinases to control the system's regulatory effect. The widespread occurrence of multivalent systems suggests that phase transitions may be used to spatially organize and biochemically regulate information throughout biology. The study also examines the behavior of multivalent proteins in cells, where co-expression of SH3 and PRM proteins results in the formation of dynamic puncta. Additionally, the nephrin/Nck/N-WASP system is investigated, showing that the phase transition correlates with a biochemical activity transition, enhancing the activation of the Arp2/3 complex. These findings provide insights into how multivalent interactions can yield sharp transitions between physically and functionally distinct states, potentially contributing to the structure and function of cellular bodies and other compartments.