Objective: To study the complex formed between Tat protein and Env soluble trimeric immunogen, and compare with previously determined structures of Env native trimers and Env–CD4m complexes.
Design: The soluble Env trimer was used to mimic the spike glycoprotein on the virus surface for the study. To overcome limitations of other structural determination methods, cryoelectron microscopy was employed to image the complex, and single particle reconstruction was utilized to reconstruct the structure of the complex from collected micrographs. Molecular modeling of gp120–Tat was performed to provide atomic coordinates for docking.
Methods: Images were preprocessed by multivariate statistical analysis to identify principal components of variation then submitted for reconstruction. Reconstructed structures were docked with modeled gp120–Tat atomic coordinates to study the positions of crucial epitopes.
Results: Analysis of the Env–Tat complex demonstrated an intermediate structure between Env native trimers and Env–CD4m structures. Docking results indicate that the CD4-binding site and the V3 loop are exposed in the Env–Tat complex. The integrin-binding sequence in Tat was also exposed in Env–Tat docking.
Conclusion: The intermediate structure induced by Tat-interaction with Env could potentially provide an explanation for increased virus infection in the presence of Tat protein. Consequently, exposure of CD4-binding sites and a putative integrin-binding sequence on Tat in the complex may provide a new avenue for rational design of an effective HIV vaccine.