Membrane fusion protein

Membrane fusion proteins (not to be confused with chimeric or fusion proteins) are proteins that cause fusion of biological membranes. Membrane fusion is critical for many biological processes, especially in eukaryotic development and viral entry. Fusion proteins can originate from genes encoded by infectious enveloped viruses, ancient retroviruses integrated into the host genome,[1] or solely by the host genome.[2] Post-transcriptional modifications made to the fusion proteins by the host can drastically affect fusogenicity, namely addition and modification of glycans and acetyl groups.[3]

Fusion in Eukaryotes

Eukaryotic genomes contain several gene families, of host and viral origin, which encode products involved in driving membrane fusion. While adult somatic cells do not typically undergo membrane fusion under normal conditions, gametes and embryonic cells follow developmental pathways to non-spontaneously drive membrane fusion, such as in placental formation, syncytiotrophoblast formation, and neurodevelopment. Fusion pathways are also involved in the development of musculoskeletal and nervous system tissues. Vesicle fusion events involved in neurotransmitter trafficking also relies on the catalytic activity of fusion proteins.

SNARE family

Syncytin family

Other

Pathogenic Viral Fusion

Enveloped viruses readily overcome the thermodynamic barrier of merging two plasma membranes by storing kinetic energy in fusion (F) proteins. F proteins can be independently expressed on host cell surfaces which can either (1) drive the infected cell to fuse with neighboring cells, forming a syncytium, or (2) be incorporated into a budding virion from the infected cell which leads to the full emancipation of plasma membrane from the host cell. Some F components solely drive fusion while a subset of F proteins can interact with host factors. There are four groups of F proteins categorized by their mechanism of fusion.

Class I

Class I fusion proteins resemble influenzavirus hemagluttinin in their structure. Post-fusion, the active site has a trimer of α-helical coiled-coils. The binding domain is rich in α-helices and hydrophobic fusion peptides located near the N-terminus. Fusion conformation change can be controlled by pH.[4][5]

Fusion Component Abbreviation Family Viruses
Hemagluttinin (H) H, HN Orthomyxoviridae, Paramyxoviridae Influenza, Measles, Mumps
Glycoprotein 41 Gp41 Retroviridae HIV

Class II

Class II proteins are dominant in β-sheets and the catalytic sites are localized in the core region. The peptide regions required to drive fusion are formed from the turns between the β-sheets.[4][5]

Fusion Component Abbreviation Family Viruses
Envelope protein E Flaviviridae Dengue, West Nile virus

Class III

Class III fusion proteins are distinct from I and II. They typically consist of 5 structural domains, where domain 1 and 2 localized to the C-terminal end often contain more β-sheets and domains 2-5 closer to the N-terminal side are richer in α-helices. In the pre-fusion state, the later domains nest and protect domain 1 (i.e. domain 1 is protected by domain 2, which is nested in domain 3, which is protected by domain 4). Domain 1 contains the catalytic site for membrane fusion.[4][5]

Prototypic Example Abbreviation Family Viruses
VSV G G Rhabdoviridae Vesicular Stomatitis Virus, Rabies lyssavirus
HSV-1 gB gB Herpesviridae HSV-1
Ebolavirus Glycoprotein GP Filoviridae Zaire-, Sudan- ebolaviruses, Marburgvirus

Class IV

Class IV are the smallest fusion proteins. They are also called fusion-associated small transmembrane (FAST) proteins and are most often associated with non-enveloped reoviruses.

See also

References
  1. Classification of viral fusion proteins in TCDB database
  2. Klapper, Robert; Stute, Christiana; Schomaker, Oliver; Strasser, Thomas; Janning, Wilfried; Renkawitz-Pohl, Renate; Holz, Anne (2002-01-01). "The formation of syncytia within the visceral musculature of the Drosophila midgut is dependent on duf, sns and mbc". Mechanisms of Development. 110 (1): 85–96. doi:10.1016/S0925-4773(01)00567-6. ISSN 0925-4773. PMID 11744371.
  3. Ortega, Victoria; Stone, Jacquelyn A; Contreras, Erik M; Iorio, Ronald M; Aguilar, Hector C (2018-06-06). "Addicted to sugar: roles of glycans in the order Mononegavirales". Glycobiology. 29 (1): 2–21. doi:10.1093/glycob/cwy053. ISSN 0959-6658. PMC 6291800. PMID 29878112.
  4. Backovic, Marija; Jardetzky, Theodore S. (April 2009). "Class III viral membrane fusion proteins". Current Opinion in Structural Biology. 19 (2): 189–196. doi:10.1016/j.sbi.2009.02.012. ISSN 0959-440X. PMC 3076093. PMID 19356922.
  5. White, Judith M.; Delos, Sue E.; Brecher, Matthew; Schornberg, Kathryn (2008). "Structures and Mechanisms of Viral Membrane Fusion Proteins". Critical Reviews in Biochemistry and Molecular Biology. 43 (3): 189–219. doi:10.1080/10409230802058320. ISSN 1040-9238. PMC 2649671. PMID 18568847.
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