Cryo-EM structure of the Shigella type III needle complex

Area of Science:

• Structural biology
• Microbiology
• Molecular mechanisms of bacterial pathogenesis

Background:

• The Type III Secretion System (T3SS) is a critical virulence factor in many Gram-negative pathogens.
• It functions as a molecular syringe, translocating effector proteins into host cells.
• Understanding the T3SS structure is essential for deciphering its mechanism of action.

Purpose of the Study:

• To determine the high-resolution structure of the isolated Shigella T3SS needle complex.
• To elucidate the molecular architecture and assembly of the T3SS nanomachine.
• To provide structural insights into the protein translocation process.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to image the isolated Shigella T3SS needle complex.
• Image processing and 3D reconstruction techniques were used to generate high-resolution structural maps.
• Analysis of the cryo-EM map revealed the arrangement of protein subunits and structural interfaces.

Main Results:

• The inner membrane (IM) basal body region exhibits 24-fold rotational symmetry, forming a channel.
• The secretin oligomer shows heterogeneous symmetry (16- and 15-fold) in its periplasmic and outer membrane components.
• Key interactions, including β-sheet augmentation between IM subunits and the secretin connector, were identified. The helical structure of the export apparatus core, inner rod, and needle was resolved.

Conclusions:

• The high-resolution structure of the Shigella T3SS needle complex reveals its intricate molecular organization.
• Structural details provide a basis for understanding the assembly and function of this essential bacterial nanomachine.
• This work facilitates the development of novel anti-virulence strategies targeting T3SS.