Open and Closed Structures Reveal Allostery and Pliability in the HIV-1 Envelope Spike

For many enveloped viruses, binding to a receptor(s) on a host cell acts as a first step in a series of events culminating in fusion with the host cell membrane and transfer of genetic material for replication [for review see(1,2)]. The envelope glycoprotein (Env) trimer on the surface of HIV is responsible for receptor binding and fusion. While Env can tolerate a high degree of mutation in five variable regions (V1-V5), and also at N-linked glycosylation sites that contribute roughly half the mass of Env, the functional sites for recognition of receptor CD4 and co-receptor CXCR4/CCR5 are conserved and essential for viral fitness. Soluble SOSIP Env trimers are structural and antigenic mimics of the pre-fusion native, surface-presented Env(3,4), targets of broadly neutralizing antibodies (bnAbs). Thus, they are attractive immunogens for vaccine development [for review see(5–8)]. Here we present high-resolution cryo-electron microscopy (cryoEM) structures of subtype B B41 SOSIP Env trimers in complex with CD4 and antibody 17b, or with antibody b12, at resolutions of ~3.7 Å and ~3.6 Å, respectively, and compare them to cryoEM reconstructions of ligand-free B41 SOSIP Env trimers or in complex with either CD4 or CD4bs antibody PGV04, at ~5.6 Å, ~5.2 Å and ~7.4 Å, respectively. Consequently, we present the most complete description and understanding of the CD4/17b-induced intermediate and provide the molecular basis of the receptor-binding induced conformational change required for HIV-1 entry into host cells. Both CD4 and b12 induce large, previously uncharacterized conformational rearrangements in the gp41 subunits, and the fusion peptide becomes more buried in a newly formed pocket. These structures provide key details on the biological function of the type I viral fusion machine from HIV-1 as well as new templates for inhibitor design.