David Chan Research Group

Molecular analysis of mitochondrial dynamics and membrane fusion

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Virus entry by HIV

In a second line of research, we are continuing our investigations of membrane fusion by viral envelope proteins. A key step in the life cycle of enveloped viruses is fusion of viral and host cell membranes.  A virally encoded glycoprotein, gp160, is responsible for mediating the entry process of HIV-1, the etiological agent of Acquired Immunodeficiency Syndrome (AIDS).   The envelope precursor, gp160, is cleaved to form the subunits gp120 and gp41.  gp120 directs target cell recognition, while gp41 mediates the merging of viral and cellular membranes.  gp41 is composed of several distinct domains including a hydrophobic fusion peptide, two coiled-coil domains (termed N- and C-terminal helices), a membrane-spanning region, and a cytoplasmic tail. 

Our current model for HIV membrane fusion invokes a series of conformational changes in the gp120/gp41 complex (Figure 1).  Interaction between gp120 and cellular receptors liberates the gp41 fusion peptide from its native conformation and allows its insertion into the host cell membrane.  A transient species termed the prehairpin intermediate is created, in which the N-terminal helices form a trimeric coiled-coil but do not interact with the C-terminal region.  Subsequently, a hairpin structure is generated, in which the C-terminal helices pack in an anti-parallel manner around the trimeric N core to form a fusion-active 6-helix bundle.  This N-C interaction brings the viral fusion peptide, inserted into the host-cell membrane, and the transmembrane segment, associated with the viral membrane, into close proximity.  In a process that remains poorly defined, fusion of the closely apposed viral and cellular membranes follows.   A mechanistic understanding of HIV membrane fusion may lead to new strategies to inhibit HIV entry into human cells.

To test this model of HIV entry, we are determining the mechanism through which peptide inhibitors prevent gp41-mediated membrane fusion.  Some of these peptide inhibitors bind to fusion intermediates of gp41, and our analysis has revealed sequential steps in the fusion pathway.  We are also characterizing a series of gp41 mutants that fail to fuse in order to better understand how formation of the 6-helix structure leads to membrane fusion.

Figure 1: Model of HIV entry into human cells. Adapted from Chan and Kim (Cell 93, 681-684).