Fusogen

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

This fusogenic activity of influenza hemagglutinin is frequently exploited in the laboratory. If, for example, the virus is bound to cells at a temperature too low for endocytosis and then the pH of the external medium is lowered, the hemagglutinin causes direct fusion of the viral envelope with the plasma membrane infection is achieved without endocytosis. Similarly, artificial vesicles with hemagglutinin in their membrane and other molecules in their lumen can be caused to fuse with cells by first allowing the vesicles to bind to the plasma membrane via the hemagglutinin and then lowering the pH of the medium. In this way the contents of the vesicles are delivered to the recipient cell s cytoplasm. 

Wyatt R, Sodroski J. The HIV-1 envelope glycoproteins fusogens, antigens, and immunogens. Science 1998 280(5371) 1884-1888. 

Platt EJ, Kuhmann SE, Rose PP, Rabat D. Adaptive mutations in the V3 loop of gpl20 enhance fusogenicity of human immunodeficiency virus type 1 and enable use of a CCR5 coreceptor that lacks the amino-terminal sulfated region. J Virol 2001 75(24) 12266-12278. 

Yuba E, Kojima C, Sakaguchi N, Harada A, Koiwai K, Kono K (2008) Gene delivery to dendritic cells mediated by complexes of lipoplexes and pH-sensitive fusogenic polymer-modified liposomes. J Control Release 130 77-83... 

Afonin S, Dur UHN, Glaser RW, Ulrich AS (2004) Boomerang -like insertion of a fusogenic peptide in a lipid membrane revealed by solid-state F-19 NMR. Magn Reson Chem 42 195-203... 

Guo X, Szoka E. Steric stabilization of Fusogenic Liposomes by a low-pH sensitive PEG-diortho ester-lipid conjugate. Bioconj Chem 2001 12 291. 

Fusogenic peptides, derived from viral sources, are particularly well characterized (107 111). The amphipathic peptide from the A-terminal region of the hemagglutinin (HA)-2 subunit of HA was one of the first such peptides described, and, subsequently, a range of influenza-derived peptides... 

Figure 3 Endosomal escape assisted by fusogenic peptides. These peptides assist the release of DNA from the endosome, avoiding degradative damage from the binding with the lysosome.

A synthetic peptide has been designed to mimic the effects of viral fusogenic properties (114,115). It consists of 30 amino acids with the major repeat of Glu-Ala-Leu-Ala so, it is referred to as a GALA peptide. It undergoes a conversion from an aperiodic conformation at neutral pH and becomes an amphipathic alpha helix at pH 5. In the more acidic environment, the peptide interacts with lipid bilayers (114,115). GALA has been incorporated into transferrin-targeted liposome, with the effect of significantly... 

Vaysse L, Burgelin I, Merlio JP, et al. Improved transfection using epithelial cell line-selected ligands and fusogenic peptides. Biochim Biophys Acta 2000 1475(3) 369-376. 

Subramanian A, Ma H, Dahl KN, et al. Adenovirus or HA-2 fusogenic pep-tide-assisted lipofection increases cytoplasmic levels of plasmid in nondividing endothelium with little enhancement of transgene expression. J Gene Med 2002 4(l) 75-83. 

Wagner E, Plank C, Zatloukal K, et al. Influenza virus hemagglutinin HA-2 N-terminal fusogenic peptides augment gene transfer by transferrin-polylysine-DNA complexes toward a synthetic virus-like gene-transfer vehicle. Proc Natl Acad Sci USA 1992 89(17) 7934-7938. 

Kakudo T, Chaki S, Futaki S, et al. Transferrin-modified liposomes equipped with a pH-sensitive fusogenic peptide an artificial viral-like delivery system. Biochemistry 2004 43(19) 5618-5628. 

THE M813 RETROVIRUS BELONGS TO A UNIQUE INTERFERENCE GROUP AND IS HIGHLY FUSOGENIC... 

Figure 7. M813 infection induces fusogenicity in vitro (A) PA317 cells incubated with M813 for 4 h. (B) Uninfected PA317 cells. Giemsa staining. Magnification 150. Reprinted from (Prassolov et al., 2001b) with permission from Elsevier.

Prassolov, V., Ivanov, D., Hein, S., Rutter, G., Miink, C., Lohler, J. and Stocking, C. (2001b) The Mus cervicolor MuLV isolate M813 is highly fusogenic and induces a T-cell lymphoma associated with large multinucleated cells. Virologyl Q, 39-49. 

Siess, D., Kozak, S. and Kabat, D. (1996) Exceptional fusogenicity of Chinese hamster ovary cells with murine retroviruses suggests roles for cellular factor(s) and receptor clusters in the membrane fusion process. J VirollO, 3432-3439. 

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