Single-Molecule FRET Delineates Asymmetric Trimer Conformations during HIV-1 Entry

Abstract

HIV-1 entry into cells requires binding of the viral envelope glycoprotein (Env) to receptor CD4 and coreceptor. Imaging of individual Env molecules on native virions revealed that Env trimers are dynamic, spontaneously transitioning between three distinct conformations. Binding of CD4 and coreceptor surrogate antibody 17b promotes opening of the closed Env (State 1) to stabilize an activated conformation (State 3) by way of at least one structural intermediate (State 2). Here, using single-molecule Fluorescence Resonance Energy Transfer (smFRET), we identify this intermediate as an asymmetric conformation where only a single CD4 molecule engages the Env trimer and individual protomers adopt distinct conformations. Binding of a second CD4 molecule is needed to completely open the trimer. Thus, our work reveals a novel asymmetric structural intermediate and gives insights into cooperativity during the opening of the HIV-1 Env trimer.