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HIV envelope glycoprotein imaged at high resolution

Comparison of structures of trimeric Env in the closed, pre-fusion and open, activated conformations. Molecular models for the two conformations show how Env activation results in major rearrangements in the location of gp120 (white/gray) relative to the central gp41 bundle (cyan). The outward rotation of each gp120 section repositions its rapidly evolving V1V2 loop (base of loop shown in red) from the center to the periphery, exposing gp41 and altering the location of the V3 loop (base of loop shown in green).
Comparison of structures of trimeric Env in the closed, pre-fusion and open, activated conformations. Molecular models for the two conformations show how Env activation results in major rearrangements in the location of gp120 (white/gray) relative to the central gp41 bundle (cyan). The outward rotation of each gp120 section repositions its rapidly evolving V1V2 loop (base of loop shown in red) from the center to the periphery, exposing gp41 and altering the location of the V3 loop (base of loop shown in green).

The outer surface of the human immunodeficiency virus (HIV) is surrounded by an envelope studded with spike-shaped glycoproteins called Env that help the deadly virus identify, bind, and infect cells. When unbound, Env exists in a “closed” conformational state. Upon binding with target cells, such as CD4+ T cells, the protein transitions to an “open” configuration. Given that Env is the only viral protein expressed on HIV’s surface, knowing its detailed structure—especially in the unbound state—may be critical for designing antibodies and vaccines against HIV.

Most previous insights into Env’s structure have come from fragments of the protein complex; the intact trimeric (three-lobed) structure of Env has remained elusive. Now, Sriram Subramaniam, Ph.D., Head of the Biophysics Section in CCR’s Laboratory of Cell Biology, and his colleagues have derived sub-nanometer resolution images of the structure of Env in its closed conformation using cryo-electron microscopy (cryo-EM)—a type of transmission electron microscopy performed on samples that are quickly frozen to liquid nitrogen temperatures, which preserves the natural state of proteins. In 2012, the team was also able to capture Env in an intermediate conformation where HIV Env is activated and open, but not yet fused with the target cell membrane.  

The researchers took thousands of cryo-EM images of Env from different angles and digitally stitched them together to form 3D reconstructions of the protein’s structure. The reconstructed images boast an imaging resolution of 6 Angstroms for the inner region of Env—in complex with the antibody VRC03, which helps Env maintain a closed configuration. An imaging resolution under 10 Angstroms allows researchers to visualize proteins’ alpha helices. Generation of an atomic model requires a resolution of 3.5 Angstroms or better.

The 3D reconstructions reveal that the Env spike is a three-lobed structure where each lobe is comprised of an inner stalk-like section (the transmembrane glycoprotein gp41) and a bulbous outer section (the surface glycoprotein gp120). Comparing Env’s structure in the closed and open states suggests that the three gp41 helices at the glycoprotein’s core serve as an anchor around which the rest of the structure pivots outward during Env’s activation.

In the transition to an open configuration, Env’s gp120 sections relocate to the periphery of the structure. This shift exposes the gp41 helices, which are thought to insert themselves into the membrane of target cells like CD4+ T cells during the process of cell fusion and infection. Capturing the entire alpha helical structure of gp41 could lead to the creation of powerfully effective HIV antibodies, especially because gp41 is one of the most conserved portions of HIV—meaning it is less likely to mutate to evade antibodies.

Notably, Subramaniam’s team also reports that the three-lobed architecture of HIV-1 is similar to that of influenza and ebola, other viruses shrouded in a glycoprotein envelope. This observation allows the team to speculate that these very different viruses may share similarities in the mechanism by which the viruses gain entry into host cells.

Recent developments in cryoEM offer the potential to go to even higher resolutions than that obtained in this study. The advances that led to the progress with the HIV spike structure are also being extended by the team to analyze protein complexes relevant to cell signaling and cancer.

In summary, Subramaniam and colleagues have derived high-resolution reconstructions of the closed configuration of the envelope spike. This detailed view of Env may provide insight into the cell-entry mechanism of HIV-1, which now appears to bear a striking resemblance to that of the influenza virus. Researchers may use this knowledge of Env’s architecture to create new vaccines and antibodies against HIV.

Reviewed by Chris Palmer

Reference
Bartsaghi A, Merk A, Borgnia MJ, Milne JLS, Subramaniam S. Pre-fusion structure of trimeric HIV-1 envelope protein determined by cryo-electron microscopy. Nature Structural & Molecular Biology. October 23, 2013. PubMed Link