To infect a cell, a virus must first be able to enter it. HIV is an enveloped virus and accomplishes cell entry by fusing the viral membrane with the cellular plasma membrane. This process is carried out by the viral envelope proteins gp120 and gp41. These two proteins are synthesized as a single 160 kD protein which is then cleaved. The products of this cleavage remain associated until the process of viral entry into the cell begins. gp120 binds to CD4 on CD4+ T lymphocytes and cells of the monocyte/macrophage lineage. This binding event and further interaction between gp120 and cellular co-receptors lead to gp120 dissociation from gp41. The dissociation of gp120 occurs as part of a conformational change in gp41 that leaves it in a "fusion-active" form. This form of gp41 can then mediate fusion between the cellular and viral membranes.

gp41 "Gymnastics"

It is not known exactly how the process of HIV membrane fusion is accomplished. Insertion of a hydrophobic "fusion peptide" from gp41 into the target membrane is thought to bring the viral and cellular membranes into close proximity (Blacklow et al., 1995, by analogy from Yu et al., 1994). However, the process by which this apposition leads to fusion is unknown. Since a number of both viral and cellular proteins that carry out membrane fusion have similar structures, it is thought that the mechanisms of fusion may be similar in these different systems. The common structure is that of a helical coiled coil, in which three alpha helices wrap around each other. In the fusion-active state of many of these molecules, the helices undergo a reversal partway through the coiled coil and bend back on themselves, packing along the outside of the initial coil. (see image). This reversal places the transmembrane region and the fusogenic region near each other at one end of the coiled coil even though in the primary structure they are at opposite termini.

Images left to right: ectodomain of Ebola fusion protein, fragment of HIV gp41, portion of Moloney Murine Leukemia virus fusion protein (Weissenhorn et al., 1998; Weissenhorn et al., 1997; Fass et al., 1996). The HIV and MoMuLV proteins are found in the trimeric form seen with the Ebola protein. Images generated using Chemscape Chime from MDL.

Membrane Processes Leading to Fusion

Fusion is generally thought to occur via the formation of a pore between two closely apposed membranes. In its pre-fusion hairpin conformation, the fusion peptide is thought to be located in the target membrane, while the transmembrane region is in the viral membrane (as proposed in Weissenhorn et al., 1997 ; Chan et al., 1997). The coiled-coil structure forces these two membranes into close proximity. Subsequent to the formation of hairpin coiled-coil complexes, an unknown set of processes leads to fusion. In the post-fusion state, the hairpin conformation is retained, but both the fusogenic region and the transmembrane region are anchored in the same membrane (by analogy to models for influenza: Wharton et al., 1995; Bullough et al., 1994; Skehel et al., 1995). Based on analogy to the process of membrane fusion by influenza hemagglutinin, a process which has been extensively studied but is still not well-understood, it is likely that multiple hairpin structures interact in some way to form a narrow fusion pore in the apposed membranes, which then expands into a larger opening (Kanaseki et al., 1997; Danieli et al., 1996). Even if this analogy holds, the mechanism by which such a fusion pore is created is unknown and remains the subject of ongoing investigation.

Membrane Fusion as a Drug Target

One motivation behind the study of HIV fusion is the idea that an understanding of fusion events might allow one to block viral membrane fusion and thus prevent HIV from entering and infecting cells. One approach under investigation is the use of either peptides or small molecules to block formation of the fusion-active hairpin conformation by HIV gp41. Peptides derived from either the N-terminal or the C-terminal regions of gp41 can block viral entry (Wild et al., 1992; Wild et al., 1994; Jiang et al., 1993). gp41-derived peptides have also been used in clinical trials, and they appear to be effective in reducing viral load (Kilby et al., 1998). Approaches using small molecules to inhibit formation of the gp41 hairpin conformation are also under development. gp41 is a particularly attractive drug target because the amino acids comprising the area of gp41 to which hairpin inhibition drugs would bind is one of the most highly conserved sequences in HIV (Judice et al., 1997). A drug of this nature would be especially attractive because it stops HIV infection before cellular entry, reducing the likelihood that viral genetic material could be preserved inside the cell and lead to later reactivation.

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Unless otherwise noted, all contents are Copyright © 1999 by Peter Kasson.