Structure of HIV-1 Env glycoprotein on virions reveals an alternative fusion subunit organization and native membrane coupling.

An effective vaccine for Human Immunodeficiency Virus type-1 (HIV-1) has yet to be developed, and detailed characterization of functional Env glycoprotein, the primary antigenic target on virions, has remained elusive. While engineered Env trimers recapitulate many aspects of functional Env, key differences in antigenicity and dynamic behavior have been reported. Here, cryo-electron tomography and subtomogram averaging of HIV-1 virus-like particles (VLPs) revealed conformational differences in critical membrane-proximal regions compared to soluble Envs. Hydrogen/Deuterium-Exchange Mass Spectrometry and Molecular Dynamics captured dynamic profiles of membrane-bound Env and identified critical interactions with membrane. We show that disruption of the viral membrane results in relaxation of Env to a form that resembles engineered, soluble trimers. Additionally, Env from mature and immature VLPs exhibit only minor conformational differences, while surface clustering on virions changes significantly. These studies provide new insights into the essential role the membrane plays in maintaining Env in its native conformational form.