Conformational Dynamics of Single HIV Env Molecules on the Surface of Native Virions

Abstract

Entry of HIV is accomplished by the trimeric envelope spike (Env), a conformational machine that avoids recognition by antibodies through a 2-receptor mechanism of activation, involving CD4 and a coreceptor. We introduced fluorophores into distinct domains of gp120, the receptor-binding subunit of the spike, and monitored the conformational changes though single-molecule fluorescence resonance energy transfer (smFRET) measurements on the surface of infectious HIV virions. smFRET trajectories revealed transitions between three conformations intrinsically accessible to the unliganded Env. Binding of CD4 stabilized one of the pre-existing states. Binding of CD4 and the coreceptor-surrogate antibody 17b stabilized a second pre-existing state. Markov modeling of the smFRET trajectories indicated a preferred order to the conformational changes with the CD4-stabilized state an intermediate critical to achieving the likely coreceptor-stabilized state. Distances estimated from the observed FRET states were integrated with known structures of HIV Env to produce a temporally accurate simulation of gp120, as it activates the underlying gp41-fusion machinery. Our work provides a framework for understanding the stepwise activation of HIV Env through successive binding of the receptor and coreceptor.