When the gp120 protein binds to an immune cell receptor protein called CD4, it undergoes a change in shape that triggers a series of events culminating in the virus entering the cell. In 1998 researchers discovered the structure of the bound form of gp120. Now, working with the closely related simian immunodeficiency virus (SIV), Bing Chen of Harvard Medical School and his colleagues have obtained the first sharp images of the protein in its unbound form. Their results, detailed in the current issue of Nature, reveal that parts of the protein undergo a marked organizational change during the transition from unbound to bound. It is this shape-shifting that enables the virus to escape detection by the immune system while breaking and entering.
"Knowing how gp120 changes shape is a new route to inhibiting HIV--by using compounds that inhibit shape change," says team member Stephen Harrison of the Howard Hughes Medical Institute. "The findings will also help us understand why it's so hard to make an HIV vaccine, and will help us start strategizing about new approaches to vaccine development."