HIV-1 Envelope Glycoprotein Trimers Display Open Quaternary Conformation When Bound to the gp41 Membrane-Proximal External-Region-Directed Broadly Neutralizing Antibody Z13e1

Abstract

We describe cryo-electron microscopic studies of the interaction between the ectodomain of the trimeric HIV-1 envelope glycoprotein (Env) and Z13e1, a broadly neutralizing antibody that targets the membrane-proximal external region (MPER) of the gp41 subunit. We show that Z13e1-bound Env displays an open quaternary conformation similar to the CD4-bound conformation. Our results support the idea that MPER-directed antibodies, such as Z13e1, block viral entry by interacting with Env at a step after CD4 activation. The HIV-1 envelope glycoprotein (Env) is a key target for neutralizing antibodies that can bind to the virus and thus prevent entry into target cells (1-3). Functional Env is a trimer, with each protomer composed of a heterodimer of gp120 and gp41 (4). The three protomers associate to form a "spike" on the surface of the viral membrane (5). Cryo-electron tomographic studies have provided molecular structures of a variety of trimeric envelope glycoproteins, both as spikes displayed on intact viruses and as soluble ectodomains (5-9). These studies have shown that when trimeric HIV-1 Env is in an unliganded state or when bound to the neutralizing antibody VRC01, it exists in a "closed" conformation, with the V1/V2 loops located close to the apex of the spike. When trimeric HIV-1 Env is bound to the neutralizing antibody b12, the trimer displays a partially open conformation with only a slight rearrangement of each gp120 monomer. In contrast, when bound to soluble CD4 or to coreceptor binding site reagents, such as the monoclonal antibody 17b, both soluble and native forms of trimeric HIV-1 Env display a fully open quaternary conformation. In this open state, the three gp120 monomers display a major structural rearrangement relative to their conformation in the closed state, involving large rotations of each gp120 monomer (5, 7 Atomic-resolution structures are available for the complexes formed between the monomeric gp120 subunit of Env and a variety of antibodies that target the CD4 binding site region. Much less structural information is available for complexes formed between the gp41 subunit and gp41-targeted neutralizing antibodies. No atomic-resolution structural models are available for trimeric gp41 in the prefusion state. However, the region of the gp41 ectodomain that is closest to the viral membrane, the membraneproximal external region (MPER), has been identified as a key antigenic site that is the target of a number of neutralizing antibodies, such as 2F5, 4E10, 10E8, and Z13e1, with atomic structures available for the complexes formed between Fab fragments from each of these antibodies and the relevant peptide epitopes on gp41 (10-13). However, no structural information is available for the complex formed between MPER-binding antibodies and gp41 either as a protomer or in the context of intact trimeric HIV-1 Env.Here, we present cryo-electron microscopic studies of the complex formed between the Fab fragment of the MPER antibody Z13e1 and trimeric SOSIP gp140, which is a cleaved, solubilized ver...

Structural Characterization of Cleaved, Soluble HIV-1 Envelope Glycoprotein Trimers

“…This observation supports earlier biochemical studies suggesting that cross talk exists between the V1/V2 and V3 and MPER and that modification of either region could affect the binding of antibodies to the other (46,47). These findings are also consistent with a recent study illustrating that an MPER binding antibody can cause the trimer to adopt an open conformation, similar to the CD4-bound state (48).…”

Immunogenicity of Stabilized HIV-1 Envelope Trimers with Reduced Exposure of Non-neutralizing Epitopes

“…Additionally, the transition from the closed to the partially open conformation is at least partially reversible, and in a thermodynamic equilibrium (Munro et al, 2014). These rapidly interchanging configurations differ from the open conformation described by Scharf et al, which is more analogous to the CD4-induced fully open state that arises when the trimer undergoes a major reorganization and that is likely to be irreversible (Harris et al, 2013; Scharf et al, 2015). …”

“…The native Env trimer and its SOSIP.664 mimics are conformationally flexible; they can “breathe” such that they fluctuate between closed and more open forms in a dynamic equilibrium (Guttman et al, 2015; Harris et al, 2013; Munro et al, 2014; Pugach et al, 2015; Scharf et al, 2015). While unliganded BG505 SOSIP.664 trimers have a high propensity to remain in the closed, ground state conformation, the equilibrium for their B41, DU422 and ZM197M counterparts is shifted.…”

HIV-1 Envelope Trimer Design and Immunization Strategies To Induce Broadly Neutralizing Antibodies

“…Whether non-NAbs impede the NAb response to trimers, or are irrelevant, is under investigation. The induction and binding of NAbs might also be influenced by the conformational flexibility of the trimer, which fluctuates between closed and more-open conformations [28–31]. …”