Conformational Landscape of the HIV-1 Envelope Glycoproteins

点击翻译按钮获取中文摘要

PROJECT SUMMARY/ABSTRACT The entry of human immunodeficiency virus (HIV-1) into host cells is mediated by the envelope glycoprotein (Env) trimer. The Env trimer consists of three gp120 exterior glycoproteins non-covalently associated with three gp41 glycoproteins, which are anchored in the viral membrane and which possess long (~145-residue) cytoplasmic tails. As the only virus-specific molecule on the viral surface, Env is the major target for host neutralizing antibodies. Env conformational flexibility is essential for virus entry and also contributes to the ability of HIV-1 to evade the host antibody response. During virus entry, the binding of gp120 to the receptors, CD4 and CCR5/CXCR4, triggers conformational changes in the metastable Env that ultimately result in gp41- mediated fusion of the viral and target cell membranes. The HIV-1 Env on virions potentially samples at least three conformations: a “closed” pretriggered conformation (State 1), an “open” CD4-bound conformation (State 3), and an intermediate “partially open” conformation (State 2). Prior to receptor engagement, the State-1 Env conformation is energetically favored, rendering primary HIV-1 strains relatively resistant to the binding of potentially neutralizing antibodies. CD4 binding drives Env from State 1 to State 2 and then into State 3, the prehairpin intermediate. Binding of the State-3 Env to the CCR5 or CXCR4 coreceptor promotes the formation of the highly stable gp41 six-helix bundle, resulting in the fusion of the viral and target cell membranes. Multiple observations indicate that the State-1 Env conformation is metastable and easily disrupted by alteration of Env sequences or extraction of Env from a membrane environment. Indeed, stabilized soluble gp140 trimers or detergent-solubilized Env trimers for which detailed structures are available have been shown to adopt a State-2-like conformation! The proposed work will address our currently incomplete understanding of the State-1 Env conformation. How does HIV-1, despite its tremendous variability, maintain a State-1 Env conformation? We have previously shown that changes in multiple Env amino acid residues can disrupt the State-1 conformation. In the proposed studies, we will identify naturally polymorphic Env residues that, when changed, stabilize the functional State-1 conformation. We will evaluate how State-1-stabilizing and State-1- destabilizing changes in Env interact to determine viral phenotypes. One focus of these studies will be the relationship between the conformation of the gp41 membrane-proximal external region (MPER) and the conformation of the rest of the Env ectodomain. The metastability of the State-1 Env conformation impedes its presentation to the host immune system during natural HIV-1 infection and following vaccination. The proposed studies will inform efforts to produce purified HIV-1 Env trimers stabilized in a State-1 conformation, which serves as the major target of broadly neutralizing antibodies and potent small-molecule entry inhibitors. Understanding how the State-1 Env conformation is regulated should expedite the design of Env-directed therapies and vaccines.