Examining HIV-1 envelope conformational transitions at high temporal resolution.

Abstract

A series of structural transitions in HIV-1 Envelope glycoprotein (Env) mediate host cell fusion. The Env is a trimer of heterodimers composed of gp120 and gp41 domains responsible for receptor binding and membrane fusion, respectively. The host cell receptor CD4 induces rearrangements in gp120, leading from a closed to an open state and exposing the fusion machinery. The Env can sample these receptor-induced conformations independent of receptor engagement. While Env closed and open state structures are available, a step-by-step mechanism of structural motions connecting these states remains undetermined. Here, we use time resolved, temperature-jump small angle X-ray scattering (TR, T-Jump SAXS) to measure Env transitions on the microsecond timescale to reveal mechanistic steps during Env opening. Differential scattering intensity in static SAXS curves for a stabilized Env ectodomain at elevated temperatures correlated with differences predicted from known closed and open state structures. The TR, T-Jump SAXS data resolved two distinct structural transitions. The first transition occurs with a time constant of ∼5 μs with the second, slower transition, occurring with a time constant of ∼700 μs. These results are consistent with a model of Env opening in which a transient intermediate state forms rapidly during the initial stages of Env conformational changes followed by structural changes associated with opening and greater exposure of the gp41 fusion machinery. The early structural rearrangements captured here provide important mechanistic details regarding Env conformational dynamics with impacts on drug and vaccine design.