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Trimeric HIV Env provides epitope occlusion mediated by hypervariable loops.

Moscoso CG, Xing L, Hui J, Hu J, Kalkhoran MB, Yenigun OM, Sun Y, Paavolainen L, Martin L, Vahlne A, Zambonelli C, Barnett SW, Srivastava IK, Cheng RH - Sci Rep (2014)

Bottom Line: A prominent basal quaternary location of V2 and V3' that challenges previous reports would facilitate gp41-independent gp120-gp120 interactions and suggests a quaternary mechanism of epitope occlusion facilitated by hypervariable loops.Deletion of V2 resulted in dramatic exposure of basal, membrane-proximal gp41 epitopes, consistent with its predicted basal location.The structural features of HIV-1 Env characterized here provide grounds for a paradigm shift in loop exposure and epitope occlusion, while providing substantive rationale for epitope display required for elicitation of broadly neutralizing antibodies, as well as substantiating previous pertinent literature disregarded in recent reports.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Biology, University of California, Davis, CA 95616.

ABSTRACT
Hypervariable loops of HIV-1 Env protein gp120 are speculated to play roles in the conformational transition of Env to the receptor binding-induced metastable state. Structural analysis of full-length Env-based immunogens, containing the entire V2 loop, displayed tighter association between gp120 subunits, resulting in a smaller trimeric diameter than constructs lacking V2. A prominent basal quaternary location of V2 and V3' that challenges previous reports would facilitate gp41-independent gp120-gp120 interactions and suggests a quaternary mechanism of epitope occlusion facilitated by hypervariable loops. Deletion of V2 resulted in dramatic exposure of basal, membrane-proximal gp41 epitopes, consistent with its predicted basal location. The structural features of HIV-1 Env characterized here provide grounds for a paradigm shift in loop exposure and epitope occlusion, while providing substantive rationale for epitope display required for elicitation of broadly neutralizing antibodies, as well as substantiating previous pertinent literature disregarded in recent reports.

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Structure of full length gp140.(A) Raw cryoelectron micrograph of gp140. Arrows point to particles embedded in vitreous ice. (B) Sequence alignment of gp140 and gp140ΔV2, showing 30-residue deletion near tip of V234. (C) Top and side views of gp140 exhibit a clockwise handedness and protrusions from the basal trimer regions. Resolution is ~21 Å at 0.5 FSC. (D) SDS-PAGE of gp140 and gp140ΔV2, with molecular weights at approximately 140 kDa. (E) Detail in (C) bottom panel, accentuating protruding densities at base of gp140 assigned to V1 and V2 loops.
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f1: Structure of full length gp140.(A) Raw cryoelectron micrograph of gp140. Arrows point to particles embedded in vitreous ice. (B) Sequence alignment of gp140 and gp140ΔV2, showing 30-residue deletion near tip of V234. (C) Top and side views of gp140 exhibit a clockwise handedness and protrusions from the basal trimer regions. Resolution is ~21 Å at 0.5 FSC. (D) SDS-PAGE of gp140 and gp140ΔV2, with molecular weights at approximately 140 kDa. (E) Detail in (C) bottom panel, accentuating protruding densities at base of gp140 assigned to V1 and V2 loops.

Mentions: Isosurface rendering of the gp140TV1 (hereinafter gp140) density map showed that the trimer structure retained the propeller-shaped motif that gp140ΔV2 exhibited (Figs. 1A, 1C). The three subunits of gp120 displayed clockwise handedness, a pointed gp41 hub, and dimensions of 100 Å in diameter and 125 Å in height, taller than that of the gp140ΔV2 trimer (Figs. 1C). While retaining a congruent overall morphology, there are a few key differences between the constructs, aside from the slight difference in molecular weight (Fig. 1B, 1D). The gp140ΔV2 density map was approximately 90 Å in height, while the gp140 density map is about 125 Å in height. Another important difference is the smaller degree of tilt away from the threefold z axis; the gp140ΔV2 construct shows gp120 subunits at a tilt of approximately 25°, whereas the gp140 construct exhibits a tilt of about 15°–20°. A more relevant feature is the presence of a tail-like density at the putative location of the V2 loop (Fig. 1C, 1E). Two distinct densities at the location of the V2 loop can be seen, pointing in opposite directions, and likely displaying the branched, adjacent V1/V2 loops. Additionally, the center trimeric region was also similar to that of gp140ΔV2, suggesting that this region anchors the three subunits to the central stalk. If each gp120 subunit is assigned a long axis through the molecule at 90°, 210° and 330° (S1, S2 and S3, respectively), all normal to the threefold z axis, then each subunit would appear tilted away from the z axis about the Sx axis by ~25°, similar to the unliganded gp140ΔV2 structure. The three gp120 regions appear to be more closely associated near the trimer apex, with density at the outermost tip of each gp120 subunit oriented toward the threefold axis. The wedge-shaped trimer arm region is quite consistent when compared to the gp140ΔV2 map.


Trimeric HIV Env provides epitope occlusion mediated by hypervariable loops.

Moscoso CG, Xing L, Hui J, Hu J, Kalkhoran MB, Yenigun OM, Sun Y, Paavolainen L, Martin L, Vahlne A, Zambonelli C, Barnett SW, Srivastava IK, Cheng RH - Sci Rep (2014)

Structure of full length gp140.(A) Raw cryoelectron micrograph of gp140. Arrows point to particles embedded in vitreous ice. (B) Sequence alignment of gp140 and gp140ΔV2, showing 30-residue deletion near tip of V234. (C) Top and side views of gp140 exhibit a clockwise handedness and protrusions from the basal trimer regions. Resolution is ~21 Å at 0.5 FSC. (D) SDS-PAGE of gp140 and gp140ΔV2, with molecular weights at approximately 140 kDa. (E) Detail in (C) bottom panel, accentuating protruding densities at base of gp140 assigned to V1 and V2 loops.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4231788&req=5

f1: Structure of full length gp140.(A) Raw cryoelectron micrograph of gp140. Arrows point to particles embedded in vitreous ice. (B) Sequence alignment of gp140 and gp140ΔV2, showing 30-residue deletion near tip of V234. (C) Top and side views of gp140 exhibit a clockwise handedness and protrusions from the basal trimer regions. Resolution is ~21 Å at 0.5 FSC. (D) SDS-PAGE of gp140 and gp140ΔV2, with molecular weights at approximately 140 kDa. (E) Detail in (C) bottom panel, accentuating protruding densities at base of gp140 assigned to V1 and V2 loops.
Mentions: Isosurface rendering of the gp140TV1 (hereinafter gp140) density map showed that the trimer structure retained the propeller-shaped motif that gp140ΔV2 exhibited (Figs. 1A, 1C). The three subunits of gp120 displayed clockwise handedness, a pointed gp41 hub, and dimensions of 100 Å in diameter and 125 Å in height, taller than that of the gp140ΔV2 trimer (Figs. 1C). While retaining a congruent overall morphology, there are a few key differences between the constructs, aside from the slight difference in molecular weight (Fig. 1B, 1D). The gp140ΔV2 density map was approximately 90 Å in height, while the gp140 density map is about 125 Å in height. Another important difference is the smaller degree of tilt away from the threefold z axis; the gp140ΔV2 construct shows gp120 subunits at a tilt of approximately 25°, whereas the gp140 construct exhibits a tilt of about 15°–20°. A more relevant feature is the presence of a tail-like density at the putative location of the V2 loop (Fig. 1C, 1E). Two distinct densities at the location of the V2 loop can be seen, pointing in opposite directions, and likely displaying the branched, adjacent V1/V2 loops. Additionally, the center trimeric region was also similar to that of gp140ΔV2, suggesting that this region anchors the three subunits to the central stalk. If each gp120 subunit is assigned a long axis through the molecule at 90°, 210° and 330° (S1, S2 and S3, respectively), all normal to the threefold z axis, then each subunit would appear tilted away from the z axis about the Sx axis by ~25°, similar to the unliganded gp140ΔV2 structure. The three gp120 regions appear to be more closely associated near the trimer apex, with density at the outermost tip of each gp120 subunit oriented toward the threefold axis. The wedge-shaped trimer arm region is quite consistent when compared to the gp140ΔV2 map.

Bottom Line: A prominent basal quaternary location of V2 and V3' that challenges previous reports would facilitate gp41-independent gp120-gp120 interactions and suggests a quaternary mechanism of epitope occlusion facilitated by hypervariable loops.Deletion of V2 resulted in dramatic exposure of basal, membrane-proximal gp41 epitopes, consistent with its predicted basal location.The structural features of HIV-1 Env characterized here provide grounds for a paradigm shift in loop exposure and epitope occlusion, while providing substantive rationale for epitope display required for elicitation of broadly neutralizing antibodies, as well as substantiating previous pertinent literature disregarded in recent reports.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Biology, University of California, Davis, CA 95616.

ABSTRACT
Hypervariable loops of HIV-1 Env protein gp120 are speculated to play roles in the conformational transition of Env to the receptor binding-induced metastable state. Structural analysis of full-length Env-based immunogens, containing the entire V2 loop, displayed tighter association between gp120 subunits, resulting in a smaller trimeric diameter than constructs lacking V2. A prominent basal quaternary location of V2 and V3' that challenges previous reports would facilitate gp41-independent gp120-gp120 interactions and suggests a quaternary mechanism of epitope occlusion facilitated by hypervariable loops. Deletion of V2 resulted in dramatic exposure of basal, membrane-proximal gp41 epitopes, consistent with its predicted basal location. The structural features of HIV-1 Env characterized here provide grounds for a paradigm shift in loop exposure and epitope occlusion, while providing substantive rationale for epitope display required for elicitation of broadly neutralizing antibodies, as well as substantiating previous pertinent literature disregarded in recent reports.

Show MeSH
Related in: MedlinePlus