A widespread bacterial protein compartment sequesters and stores elemental sulfur

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Bacteria utilize protein-based compartments for storage and sequestration due to the absence of membrane-bound organelles.
• Encapsulins are widespread prokaryotic protein compartments involved in various cellular functions.
• Desulfurase encapsulins are known to interact with sulfur compounds.

Purpose of the Study:

• To investigate the ability of desulfurase encapsulins to sequester and store elemental sulfur.
• To elucidate the structural basis of sulfur storage within encapsulins.
• To understand the functional implications of sulfur encapsulation on desulfurase activity and stability.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of sulfur-loaded encapsulins at 1.78-angstrom resolution.
• In vitro experiments using l-cysteine as a sulfur donor to induce sulfur crystal formation within encapsulins.
• Culturing bacteria in media with varying sulfur source concentrations to study accumulation.

Main Results:

• Desulfurase encapsulins effectively sequester and store large quantities of crystalline elemental sulfur.
• The 1.78-angstrom cryo-EM structure reveals a 24-nanometer desulfurase-loaded encapsulin shell.
• Sulfur crystal formation is dependent on desulfurase activity and the presence of l-cysteine.
• Encapsulation enhances desulfurase activity and stability by excluding cellular reducing agents.

Conclusions:

• Desulfurase encapsulins provide a mechanism for prokaryotes to store redox-labile elemental sulfur.
• The protein shell acts as a selectively permeable barrier, protecting stored sulfur.
• Encapsulation offers a protective and stabilizing environment for desulfurase enzymes.