Fig. 1.

The trimeric Env spike of HIV-1/SIV. (a) Electron micrographs of SIV particles showing trimeric Env spikes on the surface (63). This micrograph (courtesy of Ken Roux, Florida State University, Tallahassee) shows trimers on the surface of an SIV particle expressing high levels of Env. HIV-1 Env appears to be less stable than Env of SIV, and there is likely heterogeneity in the number of Env spikes per virion. (b) Model of the Env spike based on the structure of core gp120 (11, 64), with three gp120 monomers shown in gray, pale green, and pale blue. gp41 is shown schematically as three pink tubes. Carbohydrate chains are shown in yellow, and the oligomannose cluster proposed to interact with mAb 2G12 is shown in cyan. The approximate locations of the epitopes for broadly neutralizing mAbs are indicated.