Biogenesis and structure of a type VI secretion membrane core complex

Area of Science:

• Microbiology
• Structural Biology
• Bacterial Pathogenesis

Background:

• The type VI secretion system (TSS) is crucial for bacterial competition and virulence.
• It functions like a nano-crossbow, delivering toxic effectors via Hcp tubes and VgrG spikes.
• A membrane core complex anchors this system to the bacterial cell envelope.

Purpose of the Study:

• To elucidate the assembly and structure of the bacterial type VI secretion system's membrane core complex.
• To understand the mechanism of pore formation in the outer membrane for effector delivery.

Main Methods:

• Negative-stain electron microscopy to determine the structure of the assembled complex.
• Crystal structure analysis of key subunits (TssMct-TssJ).
• Whole-cell accessibility studies to investigate outer membrane pore formation.

Main Results:

• The membrane core complex assembles sequentially from TssJ, TssM, and TssL subunits.
• The 1.7-megadalton complex exhibits C5 symmetry with a cytoplasmic base and a periplasmic double-ring structure.
• Structural data suggest conformational changes leading to transient outer membrane pores.

Conclusions:

• The TssJ-TssM-TssL complex forms the anchor for the type VI secretion system.
• Conformational changes in the complex mediate outer membrane permeabilization.
• This mechanism allows the passage of Hcp/VgrG effectors for interbacterial warfare and pathogenesis.