Mussels Fabricate Porous Glues via Multiphase Liquid-Liquid Phase Separation of Condensates ARTICLE

Mussels (Mytilus edulis) adhere to hard surfaces in intertidal marine habitats with a porous underwater glue called the byssus plaque. The plaque is an established role model for bioinspired underwater glues and comprises at least six proteins, most of which are highly cationic and enriched in the post-translationally modified amino acid 3,4-dihydroxyphenylalanine (DOPA). While much is known about the chemistry of plaque adhesion, less is understood about the natural plaque formation process. Here, we investigated plaque structure and formation using 3D electron microscopic imaging, revealing that micro-and nanopores form spontaneously during secretion of protein-filled secretory vesicles. To better understand this process, we developed a method to purify intact secretory vesicles for in vitro assembly studies. We discovered that each vesicle contains a sulfate-associated fluid condensate consisting of similar to 9 histidine-and/or DOPA-rich proteins, which are presumably the required ingredients for building a plaque. Rupturing vesicles under specific buffering conditions relevant for natural assembly led to controlled multiphase liquid-liquid phase separation (LLPS) of different proteins, resulting in formation of a continuous phase with coexisting droplets. Rapid coarsening of the droplet phase was arrested through pH-dependent cross-linking of the continuous phase, producing native like solid porous microplaques with droplet proteins remaining as fluid condensates within the pores. Results indicate that histidine deprotonation and sulfates figure prominently in condensate cross-linking. Distilled concepts suggest that combining phase separation with tunable cross-linking kinetics could be effective for microfabricating hierarchically porous materials via self-assembly.

Automated summary

This study investigates the structure and formation process of mussel plaque, a bioinspired underwater glue composed of proteins, using 3D electron microscopic imaging. The study reveals that micro-and nanopores form spontaneously during the secretion of protein-filled secretory vesicles. Researchers also discovered that each vesicle contains a fluid condensate consisting of histidine and/or DOPA-rich proteins, which are likely the required ingredients for building the plaque. By rupturing vesicles under specific buffering conditions, controlled phase separation of different proteins occurs, resulting in the formation of a porous microplaque with fluid condensates within the pores. The results suggest that combining phase separation with tunable cross-linking kinetics could be effective for microfabricating hierarchically porous materials via self-assembly.