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Background:  We have previously used negative stain electron tomography to directly visualize tri-lobed Env structures on the surface of negatively stained HIV-1 and SIV virions. We demonstrated that the wild type HIV-1 and SIV particles had an average of 8 to 10 Env trimers per virion and mutant SIV, biochemical demonstrated to contain high levels of the viral Env proteins, had an average of 70 to 79 trimers per particle. The Env spikes on wild type virions appeared randomly distributed but some clustering was observed. The distribution of Env on the high Env SIV mutant appeared uniform but without obvious geometric pattern. 

Methods:  Because negative stain preparation techniques involve fixing, staining, and drying steps, all of which can result in structural distortions (including the flattening and lysis of virions), we are now exploring the use of stain-free cryoEM techniques for virion analysis. 

Results:  Our results show that Env trimers can be clearly visualized on the surface of 3-D images of undistorted virions suspended in vitreous ice. The core structures can also be readily visualized. The distribution patterns of Env on SIV virions with many Env spikes show a rather uniform spacing suggesting sequestration by the viral matrix. The distributions of Env on HIV-1 and SIV with few spikes are being analyzed and compared. 

Conclusions:  Cryo-EM can be used to visualize internal and external structural details on AIDS virions. The data suggest that there is an interaction between the cytoplasmic tail of TM and the viral matrix.

Keywords: envelope structure; HIV-1, SIV; electron microscopy