PMC

(A) Clear density for a Man₅ glycan at N262 can be seen in the EM map.
(B) While the last glycan in the D1 am has not been built, we see a small amount of density in this region (orange) connecting the N262 D1 arm glycan to the base and D2 arm of the N301 glycan.
(C) The N332 glycan from 3TYG structure fits tightly into the EM density. In blue and lavender are PGT128 Fab HC and LC, respectively.
(D) The N301 glycan and its density.
(E) The D3 arm of N301 does not interact with P74 (yellow) in PGT128 (HC shown inblue) as was suggested by the X-ray structure.

(A) Densities of some of the prominent α-helices in the Env trimer.
(B) Densities of some of the prominent β-sheets in the Env trimer.
(C) Stereo view of the N-terminal end of the α7 helix of gp41 following the unresolved FPPR region. L568 just prior to the start of the a7 helix of HR1 forms a hydrophobic interaction at the start of the trimeric coiled-coil at the center of the trimer, and K567 makes interactions with the adjacent HR1 helix. The two residues are shown in yellow sticks
(D) A stereo image of a different view of the K567 inter-HR1 interaction. The K567 side chain nitrogen could potentially interact with the backbone carbonyl oxygen of T569 (residues shown as yellow sticks, and interaction shown by the dashed line). We can be confident in the relative conformation due to the clear side chain density of W571 near by anchoring the peptide registration (orange).

(A) Top (left) and side (right) views of the cryoEM reconstruction is segmented into gp120, gp41 in the trimer, and variable and constant regions in the Fab.
(B) Local resolution analysis of the 4.36 Å resolution reconstruction. Local resolution in Å is indicated by colors shown in the key at the bottom. The reconstruction shown at a higher threshold level (right) shows that the resolution is largely isotropic, especially at the core of the trimer.
(C) The refined PGT128 bound trimer model. gp120 and gp41 are shown in pink and yellow, respectively, and glycans are shown as green sticks. Only the Fab variable region was built. The heavy chain (HC) is shown in blue, and light chain (LC) in lavender. (D) Superposition of the PGT128 bound trimer (pink and yellow) with the PGT121 and 35O22 bound trimer solved by X-ray crystallography (gray) ().

(A) Stereo view of the α0 region (yellow) of published trimer structures, 4TVP (top), 4NCO (bottom), and our model (blue mesh). The density of this region in our map does not fit neither 4NCO nor 4TVP well.
(B) The α4 helix of gp120 (residues T386-N392, yellow) is well resolved despite it being a small helix. The N-GlcNAc at N392 is shown in green.
(C) A close-up of the C-terminal region of HR2. Unlike HR1, there is a slight downward shift of ∼3.3 Å (measured between Q653 Cα) in the HR2 region of the PGT128 bound trimer (yellow) in comparison to the X-ray structure (gray).
(D) The density for the entire V2 loop is visible, although the lack of confidence in side-chain densities makes it difficult to build. The dotted line indicates the loop path of the 10 missing residues (E185a-S187).
(E) Density for the V4 loop is unresolved. The end and start of the V4 loop in the atomic model is shown in red, and the predicted path of missing regions (residues T399-S410) is shown by the dotted line.
(F) Three gp41 protomer densities are shown as shades of orange. As in (D), the entire FPPR density was visible but could not be built unambiguously. Residues Q551-L565 are missing from this atomic model.

(A) The N295 glycan (green), which can act as a compensatory glycan when the N332 glycan is missing, must move ∼60 degrees (orange) in order to compensate for the loss of N332.
(B) Even though the glycan density cannot be seen, the position of N137 (yellow) indicates that a glycan projecting from this site (orange shading) could sterically affect PGT128 binding. The lower image is an orthogonal view of the upper image to show how the glycan may project out towards the Fab.
(C) Additional density branching off from N156 (green) can be seen, indicating the path of the higher-branch glycan residues (dotted line). Residues from both the HC (orange) and LC (yellow) of PGT128 may be contributing to interaction with atoms in this density.
(D) R327 in the GDIR motif (red) seems to be able to make electrostatic interactions with D100d in CDRH3, as opposed to the backbone interactions indicated in the X-ray structure. The two interacting residues are shown as yellow sticks.
(E) The density indicates an interaction between Y52e of CDRH2 and the hydrocarbon portion of R444 side chain in C5. An interaction between R444 and the base of N295 glycan (green) may also be present.