Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
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The part of the gp41 protein which protrudes into the virus particle (the part that isn't embedded in the membrane) can interact with other gp41 proteins, linking together.
When a HIV particle binds to a cell, the interaction of gp120 with CD4 changes gp120's shape. This in turn changes the shape of gp41, in such a way that the gp41 helps to initiate the process of membrane fusion between virus and cell.