Secretion-Catalyzed Assembly of Protein Biomaterials on a Bacterial Membrane Surface

Protein-based biomaterials have historically played a key role in tissue engineering, and additional exciting applications as self-healing materials and sustainable polymers are emerging. Their fibrous forms are often the most applicable. Over the past few decades, recombinant expression and production of various fibrous proteins from microbes have been demonstrated; however, the resulting proteins typically must then be purified and processed by humans to form usable fibers and materials. Here, we show that the Gram-positive bacterium Bacillus subtilis can be programmed to secrete silk through its translocon. Surprisingly, we discover that this translocation mechanism drives the silk proteins to assemble into fibers spontaneously on the cell surface, in a process we call secretion-catalyzed assembly (SCA). Furthermore, these fibers form self-healing hydrogels with minimal processing. This work provides a blueprint to achieve autonomous assembly of protein biomaterials in useful morphologies directly from microbial factories.