A widespread proteinaceous sulfur storage compartment in bacteria

Intracellular compartmentalization is essential for all cells and enables the regulation and optimization of metabolism1. One of the main functions of subcellular compartments is the storage of nutrients2-4. As bacteria do generally not possess membrane-bound organelles, they often have to rely on functionally analogous protein-based compartments2,5-7. Encapsulin nanocompartments are one of the most prevalent protein-based compartmentalization strategies found in prokaryotes5,8. Here we show that desulfurase encapsulins represent a novel sulfur storage compartment in bacteria able to sequester large amounts of crystalline elemental sulfur. We determined the 1.78 [A] cryo-EM structure of a 24 nm desulfurase-loaded encapsulin highlighting the molecular details of the protein shell and desulfurase encapsulation. We found that elemental sulfur crystals can be formed inside encapsulin shells in a desulfurase-dependent manner with L-cysteine acting as the sulfur donor. Intracellular sulfur accumulation can be influenced by the concentration and type of sulfur source in growth media. The selectively permeable protein shell allows the long-term intracellular storage of redox-labile elemental sulfur by excluding cellular reducing agents from its interior. We found that encapsulation substantially improves desulfurase activity and stability while also preventing substrate inhibition. These findings represent the first example of a dedicated and widespread storage system for the essential element sulfur in bacteria and provide the basis for understanding how this novel protein-based storage compartment is integrated within bacterial metabolism.