Secretory S complex of Bacillus subtilis forms a large, organized structure when released from ribosomes

Area of Science:

• Microbiology
• Molecular Biology
• Protein Secretion

Background:

• The S complex in Bacillus subtilis is a four-protein assembly implicated in protein secretion.
• Understanding the S complex's interaction with the cellular machinery is key to elucidating bacterial secretion pathways.

Purpose of the Study:

• To investigate the association of the Bacillus subtilis S complex with 70S ribosomes.
• To characterize the structure and properties of the S complex when released from ribosomes.

Main Methods:

• Immunological assays using antibodies against the S complex's 64-kDa component.
• Chemical crosslinking to identify interactions between the S complex and ribosomal proteins.
• Sucrose gradient centrifugation under low Mg2+ conditions.
• Electron microscopy for structural analysis of S complex aggregates.

Main Results:

• The S complex is demonstrably attached to 70S ribosomes in Bacillus subtilis.
• Antibodies to the 64-kDa component aggregate ribosomes, confirming complex association.
• The S complex can be chemically crosslinked to ribosomal proteins.
• Low Mg2+ or centrifugation releases the S complex, forming 76S aggregates.
• Electron microscopy reveals these aggregates possess a regular, cage-like structure (approx. 45 nm diameter).

Conclusions:

• The Bacillus subtilis S complex directly interacts with 70S ribosomes.
• Released S complex forms distinct, structured aggregates.
• The observed structure and aggregation suggest a function potentially distinct from eukaryotic signal recognition particles.