Wetting of junctional condensates along the apical interface promotes tight junction belt formation

ABSTRACT Biomolecular condensates enable cell compartmentalization by acting as membrane-less organelles 1 . How cells control the interactions of condensates with other cellular structures such as membranes to drive morphological transitions remains poorly understood. Here, we studied formation of tight junctions, which initially assemble as condensates that over time elongate around the membrane cell perimeter to form a closed junctional barrier 2 . We discovered that the elongation of junctional condensates is driven by a physical wetting process around the apical membrane interface. Using temporal proximity proteomics in combination with live and super-resolution imaging, we found that wetting is mediated by the apical protein PATJ, which promotes adhesion of condensates to the apical membrane resulting in an interface formation and linear spreading into a closed belt. Using PATJ mutations we show that apical adhesion of junctional condensates is necessary and sufficient for stable tight junction belt formation. Our results demonstrate how cells exploit the collective biophysical properties of protein condensates and membrane interfaces to shape mesoscale structures.