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Published on: May 21, 2018
The cryo-EM structure of the acid activatable pore-forming immune effector Macrophage-expressed gene 1
Siew Siew Pang1,2, Charles Bayly-Jones1,2, Mazdak Radjainia2,3
1ARC Centre of Excellence in Advanced Molecular Imaging, Monash University, Melbourne, VIC, 3800, Australia.
Abstract:
Macrophage-expressed gene 1 (MPEG1/Perforin-2) is a perforin-like protein that functions within the phagolysosome to damage engulfed microbes. MPEG1 is thought to form pores in target membranes, however, its mode of action remains unknown. We use cryo-Electron Microscopy (cryo-EM) to determine the 2.4 Å structure of a hexadecameric assembly of MPEG1 that displays the expected features of a soluble prepore complex. We further discover that MPEG1 prepore-like assemblies can be induced to perforate membranes through acidification, such as would occur within maturing phagolysosomes. We next solve the 3.6 Å cryo-EM structure of MPEG1 in complex with liposomes. These data reveal that a multi-vesicular body of 12 kDa (MVB12)-associated β-prism (MABP) domain binds membranes such that the pore-forming machinery of MPEG1 is oriented away from the bound membrane. This unexpected mechanism of membrane interaction suggests that MPEG1 remains bound to the phagolysosome membrane while simultaneously forming pores in engulfed bacterial targets.
Insights
Macrophage-expressed gene 1 (MPEG1) forms pores to kill microbes. Structural studies reveal MPEG1 binds phagolysosome membranes via a unique domain, enabling pore formation in bacterial targets.
Area of Science:
- Structural biology
- Cellular microbiology
- Immunology
Background:
- Macrophage-expressed gene 1 (MPEG1), also known as Perforin-2, is a pore-forming protein crucial for innate immunity.
- MPEG1 operates within the phagolysosome to eliminate engulfed pathogens, but its precise mechanism of action has remained elusive.
Purpose of the Study:
- To elucidate the structural basis of MPEG1's pore-forming activity.
- To understand how MPEG1 interacts with membranes and mediates microbial killing.
Main Methods:
- Cryo-electron microscopy (cryo-EM) was employed to determine high-resolution structures of MPEG1 assemblies.
- Liposome binding assays and structural analysis were used to investigate MPEG1-membrane interactions.
Main Results:
- The 2.4 Å cryo-EM structure revealed a soluble hexadecameric MPEG1 prepore complex.
- Acidification, mimicking phagolysosome conditions, induced MPEG1 prepore-like assemblies to perforate membranes.
- A 3.6 Å cryo-EM structure of MPEG1 complexed with liposomes showed the MABP domain mediating membrane interaction, orienting the pore machinery away from the phagolysosome membrane.
Conclusions:
- MPEG1 utilizes an unexpected mechanism, binding the phagolysosome membrane via its MABP domain while forming pores in target bacteria.
- This dual function allows MPEG1 to remain anchored to the phagolysosome while effectively damaging engulfed microbes.
