Assembly of tight junction belts by surface condensation and actin elongation

Formation of biomolecular condensates via phase separation enables compartmentation of many cellular processes. However, how cells can control condensation at specific locations to create complex cellular structures remains poorly understood. Here, we investigated the mechanism of tight junction formation, which involves condensation of scaffold proteins at cell-cell contacts and elongation of the condensates into a belt around the cellular perimeter. Using cell biology, reconstitution, and thermodynamic theory, we discovered that cells use surface phase transitions to control local condensation at the membrane far below bulk saturation. Surface condensation of junctional ZO-scaffold proteins is mediated by receptor binding and regulated by the receptor’s oligomerization state. Functionally, ZO surface condensation is directly coupled to actin polymerization and bundling, which drives elongation of receptor-ZO-actin condensates similar to tight junction belt formation in cells. We conclude that surface phase transitions provide a robust mechanism to control the position and shape of protein condensates. One-Sentence Summary Local surface binding of cytosolic scaffold proteins provides spatial control of protein condensation to assemble adhesion junctions. ### Competing Interest Statement The authors have declared no competing interest.