The structure of the endogenous ESX-3 secretion system

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• ESX (Type VII) secretion systems are crucial for protein export in mycobacteria and Gram-positive bacteria.
• These systems are involved in essential functions like virulence, conjugation, and metabolic regulation.
• They translocate WXG100-superfamily protein substrates across the bacterial cytoplasmic membrane.

Purpose of the Study:

• To determine the high-resolution structure of the ESX-3 secretion system.
• To elucidate the architectural organization of the ESX-3 complex.
• To provide a structural basis for understanding the function of ESX-3 transporters.

Main Methods:

• Purification of the ESX-3 system from *Mycobacterium smegmatis* using recombineering and epitope tagging.
• Cryo-electron microscopy (cryo-EM) for high-resolution structural determination.
• Analysis of the protomer complex composition and assembly.

Main Results:

• The cryo-EM structure of the ESX-3 system revealed a stacked architecture extending across the inner membrane.
• The ESX-3 protomer complex comprises single copies of EccB3, EccC3, EccE3, and two copies of EccD3.
• Protomer complexes form stable dimers, suggesting assembly into larger oligomeric structures.

Conclusions:

• The determined ESX-3 structure provides unprecedented atomic-level detail of this essential bacterial secretion system.
• This structural framework facilitates further investigation into the mechanisms of ESX-3-mediated protein transport.
• Understanding ESX-3 structure is key to exploring its roles in bacterial physiology and pathogenesis.