Abstract
The gp120 portion of the envelope spike on human immunodeficiency virus type 1 (HIV-1) plays a critical role in viral entry into host cells and is a key target for the humoral immune response, and yet many structural details remain elusive. We have used cryoelectron tomography to visualize the binding of the broadly neutralizing monoclonal antibody (MAb) 447-52D to intact envelope spikes on virions of HIV-1 MN strain. Antibody 447-52D has previously been shown to bind to the tip of the V3 loop. Our results show antibody arms radiating from the sides of the gp120 protomers at a range of angles and place the antibody-bound V3 loop in an orientation that differs from that predicted by most current models but consistent with the idea that antibody binding dislodges the V3 loop from its location in the Env spike, making it flexible and disordered. These data reveal information on the position of the V3 loop and its relative flexibility and suggest that 447-52D neutralizes HIV-1 MN by capturing the V3 loop, blocking its interaction with the coreceptor and altering the structure of the envelope spike. IMPORTANCE: Antibody neutralization is one of the primary ways that the body fights infection with HIV. Because HIV is a highly mutable virus, the body must constantly produce new antibodies to counter new strains of HIV that the body itself is producing. Consequently, antibodies capable of neutralizing multiple HIV strains are comparatively few. An improved understanding of the mechanism of antibody neutralization might advance the development of immunogens. Most neutralizing antibodies target the Env glycoprotein spikes found on the virus surface. The broadly neutralizing antibody 447-52D targets the highly conserved β-turn of variable loop 3 (V3) of gp120. The importance of V3 lies in its contribution to the coreceptor binding site on the target cell. We show here that 447-52D binding to V3 converts the Env conformation from closed to open and makes the V3 loop highly flexible, implying disruption of coreceptor binding and attachment to the target cell.