Molecular architecture of native HIV1 gp120 trimers

1
Molecular architecture of native HIV-1 gp120
trimers

• Bushra Jahangir, Pranamee Das, Naomi Shipman,
  Fatehi Ahmed

2
Introduction

• Human Immunodeficiency Virus (HIV) is thought to
  have evolved from the Simian Immunodeficiency
  Virus (SIV).
• gt 33 million people worldwide are infected with
  HIV, making it a global epidemic.
• Has very high mutation rate hence immune system
  cannot keep up with it.
• Clear understanding of viral structure is
  fundamental for vaccine development.

3
Introduction

• Envelope glycoproteins (Env) are heterodimers of
• a transmembrane glycoprotein gp41.
• a surface glycoprotein gp120.
• Form trimers on surface of viral membrane.
• Facilitate viral binding to CD4 receptor on
  target lymphocytes to initiate infection.

4
Methods

• Incubated viral suspension with/without Fab
  fragment b12 or CD4 Fab fragment 17b.
• CD4 binding replicates in vivo HIV-1 entry.
• b12 used as a neutralising antibody.
• 17b used to stabilise interaction with CD4.

5
Methods

• X-ray crystallography Cryo-electron tomography
  (CET).
• colloidal gold used in tagging, prior to CET.
• both methods combined allowed visualisation of
  conformational changes.
• Mapping of co-ordinates based on CET spikes using
  Chimera software.
• density maps derived from CET, crystallography
  and Chimera.
• combination of methods provides a powerful
  approach to discern key elements of the structure
  e.g. V1/V2 and V3 loops.

6
Results

• 3D structure of Env in both liganded unliganded
  states has been elucidated.
• CD4 binding causes gp120 rotation and discernible
  changes to gp41.
• exposes V3 loop.

7
Results

• b12 binding locks gp120 in the Env trimer.
• averts further conformational change.
• prevents exposure of V3 loop.
• stops rearrangement of gp41.
• inhibits viral entry into host target cell.

8
Conclusion

• Knowing the 3D conformational change is important
  for understanding viral entry.
• Methods elucidated the 3D structure of the Env
  trimer in its different conformational states as
  dictated by specific ligand binding.
• unliganded.
• b12 - bound.
• CD4/17b - bound.

9
Conclusion

• CD4/17b binding induces dramatic structural
  changes to Env trimer in comparison to the b12
  antibody.
• gp120 rotation displacement.
• gp41 rearrangement.
• exposure of V3 loop and other antigenic
  determinants leads to contact between viral and
  target cell membranes.
• This is the first study to generate 3D images of
  the intact Env trimer in its different
  conformational states.
• HIV-1 stalk region (gp41) identified in this
  study resembles SIV stalk region determined in
  previous studies.

10
Further Study

• Investigate further the structural changes in an
  attempt to design drugs, which inhibit the
  conformational change thus preventing viral
  entry.
• use small angle X-ray scattering (SAXS) to detect
  structural changes in the viral proteins.
• generates X-ray scattering curves.
• Understand mechanisms of antibody binding to Env
  that lead to viral neutralisation.

11
References

• Fig 1 HIV prevalence map. http//www.contraboli.r
  o/stuff/p/epidemia-hiv.png. Accessed on
  25/02/2009.
• Liu, J et al. Molecular architecture of native
  HIV-1 gp120 trimers. Nature 455, 109-U76 (2008).