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Ancient but Not Forgotten: New Insights Into MPEG1, a Macrophage Perforin-Like Immune Effector
Charles Bayly-Jones1,2, Siew Siew Pang1,2, Bradley A Spicer1,2
1ARC Centre of Excellence in Advanced Molecular Imaging, Monash University, Melbourne, VIC, Australia.
Abstract:
Macrophage-expressed gene 1 [MPEG1/Perforin-2 (PRF2)] is an ancient metazoan protein belonging to the Membrane Attack Complex/Perforin (MACPF) branch of the MACPF/Cholesterol Dependent Cytolysin (CDC) superfamily of pore-forming proteins (PFPs). MACPF/CDC proteins are a large and extremely diverse superfamily that forms large transmembrane aqueous channels in target membranes. In humans, MACPFs have known roles in immunity and development. Like perforin (PRF) and the membrane attack complex (MAC), MPEG1 is also postulated to perform a role in immunity. Indeed, bioinformatic studies suggest that gene duplications of MPEG1 likely gave rise to PRF and MAC components. Studies reveal partial or complete loss of MPEG1 causes an increased susceptibility to microbial infection in both cells and animals. To this end, MPEG1 expression is upregulated in response to proinflammatory signals such as tumor necrosis factor α (TNFα) and lipopolysaccharides (LPS). Furthermore, germline mutations in MPEG1 have been identified in connection with recurrent pulmonary mycobacterial infections in humans. Structural studies on MPEG1 revealed that it can form oligomeric pre-pores and pores. Strikingly, the unusual domain arrangement within the MPEG1 architecture suggests a novel mechanism of pore formation that may have evolved to guard against unwanted lysis of the host cell. Collectively, the available data suggest that MPEG1 likely functions as an intracellular pore-forming immune effector. Herein, we review the current understanding of MPEG1 evolution, regulation, and function. Furthermore, recent structural studies of MPEG1 are discussed, including the proposed mechanisms of action for MPEG1 bactericidal activity. Lastly limitations, outstanding questions, and implications of MPEG1 models are explored in the context of the broader literature and in light of newly available structural data.
Insights
Macrophage-expressed gene 1 (MPEG1) is an immune effector that forms pores to fight microbial infections. Its unique structure may protect host cells from lysis, suggesting a novel role in innate immunity.
Area of Science:
- Immunology and Cell Biology
- Molecular and Structural Biology
- Evolutionary Biology
Background:
- Macrophage-expressed gene 1 (MPEG1), also known as Perforin-2 (PRF2), is a pore-forming protein (PFP) within the MACPF/CDC superfamily.
- MPEG1 is structurally related to immune proteins like perforin (PRF) and the membrane attack complex (MAC), suggesting a conserved role in host defense.
- Loss-of-function studies indicate MPEG1 is crucial for combating microbial infections, with mutations linked to recurrent pulmonary mycobacterial infections.
Purpose of the Study:
- To review the current understanding of MPEG1's evolution, regulation, and function as an immune effector.
- To discuss recent structural studies of MPEG1 and proposed mechanisms for its pore formation and bactericidal activity.
- To explore limitations, outstanding questions, and implications of MPEG1 models in the context of broader literature and structural data.
Main Methods:
- Bioinformatic analysis to infer evolutionary relationships and gene duplication events.
- Review of existing literature on MPEG1 expression, regulation, and functional studies.
- Analysis of recent structural studies elucidating MPEG1's oligomerization and pore formation capabilities.
Main Results:
- MPEG1 expression is upregulated by pro-inflammatory signals (TNFα, LPS), highlighting its role in the immune response.
- Structural data reveals MPEG1 forms oligomeric pre-pores and pores, with an architecture suggesting a mechanism to prevent host cell lysis.
- Germline mutations in MPEG1 are associated with increased susceptibility to microbial infections, particularly in the lungs.
Conclusions:
- MPEG1 functions as an intracellular pore-forming immune effector, critical for host defense against microbial pathogens.
- Its unique structural features suggest a specialized mechanism for pore formation that balances antimicrobial activity with host cell integrity.
- Further research into MPEG1's structure-function relationship is essential for understanding its precise role in innate immunity and potential therapeutic applications.
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