Liquid-liquid Phase Separation Facilitates the Biogenesis of Secretory Storage Granules

The biogenesis of secretory granules (SGs) is poorly understood. Here, we show that chromogranin proteins form a liquid scaffold upon entry into the milieu of the trans-Golgi network (TGN). These scaffolds recruit clients such as proinsulin and facilitate their transport to SGs. Insulin is synthesized by pancreatic beta-cells and stored into secretory granules (SGs). SGs fuse with the plasma membrane in response to a stimulus and deliver insulin to the bloodstream. The mechanism of how proinsulin and its processing enzymes are sorted and targeted from the trans-Golgi network (TGN) to SGs remains mysterious. No cargo receptor for proinsulin has been identified. Here, we show that chromogranin (CG) proteins undergo liquid-liquid phase separation (LLPS) at a mildly acidic pH in the lumen of the TGN, and recruit clients like proinsulin to the condensates. Client selectivity is sequence-independent but based on the concentration of the client molecules in the TGN. We propose that the TGN provides the milieu for converting CGs into a "cargo sponge" leading to partitioning of client molecules, thus facilitating receptor-independent client sorting. These findings provide a new receptor-independent sorting model in beta-cells and many other cell types and therefore represent an innovation in the field of membrane trafficking.