The hemolysin A secretion system is a multi-engine pump containing three ABC transporters.

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Type 1 secretion systems (T1SSs) are crucial for protein export in Gram-negative bacteria.
• The Escherichia coli hemolysin A secretion system is a well-studied T1SS prototype.
• T1SSs comprise an inner membrane ABC transporter, an adaptor protein, and an outer membrane pore.

Purpose of the Study:

• To elucidate the structure and stoichiometry of the T1SS inner membrane complex.
• To understand the mechanism of protein translocation via T1SSs.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures.
• Functional assays to assess the role of protein oligomerization in secretion.

Main Results:

• The inner membrane complex is a hetero-dodecamer of three HlyB homodimers and six HlyD subunits.
• Oligomerization of HlyB and HlyD is critical for efficient protein secretion.
• Polypeptide translocation occurs through the HlyB ABC transporter.

Conclusions:

• T1SSs utilize a multi-subunit complex involving ABC transporters for protein secretion.
• The structure reveals a novel mechanism involving multiple ABC transporters in substrate translocation.