Capturing pore-forming intermediates of MACPF and binary toxin assemblies by cryoEM.

Area of Science:

• Structural Biology
• Molecular Biology
• Biochemistry

Background:

• Pore-forming proteins are critical virulence factors for bacteria and key components of host immune responses.
• These proteins assemble into pores in target cell membranes, facilitating the translocation of effector molecules.
• Understanding the dynamic assembly process of these protein complexes is essential for deciphering their mechanisms of action.

Purpose of the Study:

• To elucidate the structural mechanisms underlying pore formation by bacterial binary toxins and immune complexes.
• To capture and analyze intermediate assembly states of pore-forming proteins using advanced cryo-electron microscopy techniques.
• To provide a mechanistic understanding of effector protein translocation mediated by bacterial toxins.

Main Methods:

• Cryo-electron microscopy (cryoEM) was employed to determine high-resolution structures of protein assemblies.
• In silico purification techniques were utilized to analyze heterogeneous conformational states.
• Structural analysis focused on membrane attack complex perforin (MACPF) proteins and bacterial binary toxins.

Main Results:

• The structure of the soluble membrane attack complex (sMAC) revealed sequential templating and β-hairpin insertion during pore formation.
• CryoEM structures of bacterial binary toxins captured sequential transitions along the pore formation pathway.
• A general mechanism for effector protein translocation mediated by these toxins was elucidated.

Conclusions:

• Cryo-electron microscopy has provided unprecedented insights into the assembly pathways of pore-forming proteins.
• The findings illuminate distinct yet related mechanisms of pore formation in immune and bacterial systems.
• Future time-resolved cryoEM studies hold promise for capturing transient states in protein pore assembly.