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Published April 18, 2007 | Supplemental Material
Journal Article Open

Electron cryotomography of immature HIV-1 virions reveals the structure of the CA and SP1 Gag shells


The major structural elements of retroviruses are contained in a single polyprotein, Gag, which in human immunodeficiency virus type 1 (HIV-1) comprises the MA, CA, spacer peptide 1 (SP1), NC, SP2, and p6 polypeptides. In the immature HIV-1 virion, the domains of Gag are arranged radially with the N-terminal MA domain at the membrane and C-terminal NC-SP2-p6 region nearest to the center. Here, we report the three-dimensional structures of individual immature HIV-1 virions, as obtained by electron cryotomography. The concentric shells of the Gag polyprotein are clearly visible, and radial projections of the different Gag layers reveal patches of hexagonal order within the CA and SP1 shells. Averaging well-ordered unit cells leads to a model in which each CA hexamer is stabilized by a bundle of six SP1 helices. This model suggests why the SP1 spacer is essential for assembly of the Gag lattice and how cleavage between SP1 and CA acts as a structural switch controlling maturation.

Additional Information

© 2007 European Molecular Biology Organization. Received: 3 January 2007; accepted: 5 March 2007; published online: 29 March 2007. This work was supported in part by NIH Grant PO1 GM66521 (to WIS and GJJ), NIH Grant A145405 (to WIS), NIH Grant F32 GM075543 (to ERW), the Beckman Institute at Caltech, and gifts to Caltech from the Ralph M Parsons Foundation, the Agouron Institute, and the Gordon and Betty Moore Foundation. Supplementary data are available at The EMBO Journal Online (http://www.embojournal.org).

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