Molecular Mechanism of HIV-1 Entry

Abstract

HIV-1 envelope glycoprotein [Env; trimeric (gp160)₃ cleaved to (gp120/gp41)₃] attaches the virion to a susceptible cell and induces fusion of viral and cell membranes to initiate infection. It interacts with the primary receptor CD4 and coreceptor (e.g., chemokine receptor CCR5 or CXCR4) to allow viral entry by triggering large structural rearrangements and unleashing the fusogenic potential of gp41 to induce membrane fusion. Recent advances in structural biology of HIV-1 Env and its complexes with the cellular receptors have revealed molecular details of HIV-1 entry and yielded new mechanistic insights. In this review, I summarize our latest understanding of the HIV-1 membrane fusion process and discuss possible pathways for productive viral entry.

Keywords: HIV-1; envelope glycoprotein; membrane fusion; viral entry.

Figure 1.

HIV-1 Env and Viral Entry. (A) The full-length HIV-1 Env, gp160. Segments of gp120 and gp41 include: C1–C5, conserved regions 1–5; V1–V5, variable regions 1–5; F, fusion peptide; HR1, heptad repeat 1; C–C loop, the immunodominant loop with a conserved disulfide; HR2, heptad repeat 2; MPER, membrane-proximal external region; TM, transmembrane anchor; CT, cytoplasmic tail; tree-like symbols, glycans. (B) Membrane fusion likely proceeds stepwise as follows. (i) Binding of gp120 to CD4, and a coreceptor allows viral attachment and triggers structural changes in Env. (ii) Dissociation of gp120 and insertion of the fusion peptide of gp41 into the target cell membrane leads to the prehairpin intermediate [116]. (iii) HR2 folds back onto the inner core of HR1 and brings the two membranes together. (iv) A hemifusion stalk forms and resolves into a fusion pore [117]. The number of Env trimers required for fusion pore formation remains speculative.

Figure 2.

Structures of the Key Players in HIV-1 Entry. (A) Structures of HIV-1 Env. The crystal structure of the unliganded HIV-1 BG505 SOSIP.664 Env trimer (pdb ID: 4ZMJ; [85]) that lacks the membrane-proximal external region (MPER), transmembrane domain (TMD), and cytoplasmic tail (CT) is shown in ribbon diagram with gp120 in cyan and gp41 in yellow. The EM density in gray is a 3D reconstruction of the unliganded HIV-1 BaL Env spike on the surface of the virion by cryo-electron tomography (EMDB ID: EMD-5019 (Env portion); EMDB ID: EMD-5022 (membrane portion)). The structure of the MPER-TMD reconstituted in bicelles that mimic a lipid bilayer was determined by nuclear magnetic resonance (NMR) (pdb ID: 6E8W; [36]). The MPER is in orange, and the TMD is in brown. The single particle cryo-EM structure of the detergent-solubilized clade B HIV-1 JR-FL EnvΔCT construct without the CT in complex with bnAb PGT151 (pdb ID: 5FUU; [29]) is shown with gp120 in cyan, gp41 in yellow, and PGT-151 Fab in gray. (B) Crystal structure of soluble 4 domain CD4 (pdb ID: 1WIO; [46]). D1–D4 and the location of the transmembrane segment (TM) are indicated. (C) Crystal structure of a modified CCR5 in complex with a modified chemokine [5P7]CCL5 (an antagonist; pdb ID: 5UIW; [66]). CCR5 is shown in ribbon diagram in red, the internally fused rubredoxin and the ligand in gray. N terminus (N), C terminus (C) and the second extracellular loop (ECL2) are indicated. Crystal structure of an engineered CXCR4 in complex with a viral chemokine antagonist vMIP-II (pdb ID: 4RWS; [65]). CXCR4 is shown in magenta, the fused T4 lysozyme in and the ligand in gray.

Figure 3.

Env–CD4 Interaction. (A) The crystal structure of the first gp120 core in complex with 2D CD4 and 17b Fab (pdb ID: 1G9M; [16]). the gp120 core is in cyan, CD4 in green, and 17b in gray. This structure led to a definition of the inner domain, outer domain, and the bridging sheet as indicated. (B) 3D reconstruction of the HIV-1 BaL Env spike on the surface of virion in complex with 2D CD4 and 17b Fab determined by cryo-electron tomography (EMDB ID: EMD-5020 (Env portion); EMDB ID: EMD-5023 (membrane portion)). (C) The single-particle cryo-EM structure of B41 SOSIP.664 Env trimer in complex with 2D CD4 and 17b (pdb ID: 5VN3; [82]) is shown with gp120 in cyan, gp41 in yellow, CD4 in green, and 17b Fab in gray. (D) The cryo-EM structure BG505 DS-SOSIP.664 Env trimer in complex with 4D CD4 and PGT145 Fab (pdb ID: 5U1F; [86]) is shown with gp120 in cyan, gp41 in yellow, CD4 in green, and PGT145 Fab in gray.

Figure 4.

Env–CCR5 Interaction. Left. Overall structure of the 4D CD4–gp120–CCR5 complex (pdb ID: 6MET; [89]) shown in ribbon diagram. N, N terminus; C, C terminus; ECL2, extracellular loop 2; I, II, III, IV, V, VI, VII for transmembrane helices (TM) 1–7. V3 loop and the bridging sheet of gp120 are also indicated. Right. Close-up views of the interfaces between gp120 and CCR5. First, the N terminus of CCR5 is attaching to the surface of the four-stranded bridging β sheet formed by the V1V2 stem and β21–β22 of gp120. Residues Ser7, Pro 8, sulfated Tyr 10, sulfated Tyr14, Tyr15, and Pro19, as well as the disulfide between Cys20 and Cys269 of CCR5, are highlighted in stick model. The O-linked glycan at Ser7 is also shown. Second, V3 is inserting into the CRS2. The conserved GPGR motif of V3 is highlighted in stick model, and ECL2 of CCR5 is indicated.

Figure 5.

Molecular Model of Early Steps of HIV-1 Entry. From left to right, the structures of unliganded Env trimer ectodomain and MPER–TMD in the prefusion conformation sitting in the viral membrane (light green) and the structures of CD4 and CCR5 in the target cell membrane (light orange). The virus attaches to the target cell by the binding of a single CD4 (green) to one gp120 (cyan) in the Env trimer. The stoichiometry among Env trimer, CD4, and coreceptor during the fusion process is unknown. If single CD4/coreceptor binding is sufficient to induce productive membrane fusion, immediate binding by CCR5 (red) prevents rapid dissociation between gp120 and CD4, stabilizes the CD4-induced conformational changes within the Env trimer, and also brings the trimer close to the cell membrane. The fusion peptide (magenta) flips out due to intrinsic conformational dynamics, allowing bending back of the N and C termini of gp120, which blocks the fusion peptide from resuming its original position in the trimer. The movements of the fusion peptide and gp120 termini effectively weaken the noncovalent association between the two subunits and may lead to partial or complete dissociation of gp120 and a series of refolding events in gp41 to adopt the prehairpin intermediate conformation with the fusion peptides inserting into the target cell membrane. On the right side, a close-up view of the gp120 N- and C-terminal region. Four helices (α6, α7, α8, and α9) of gp41 forming the 4-helix collar together with the fusion peptide that grips the N and C termini of gp120 in the CD4-bound trimer are indicated. The N terminus of the CCR5-bound gp120 overlaps with the fusion peptide in the CD4-bound trimer. If three CD4 molecules, and subsequently three coreceptors, are required to activate one Env trimer, a full occupancy of an Env trimer by three copies of each CD4 and CCR5 simultaneously is indeed possible, but such a stoichiometry may be difficult for the virus to fulfill.