Revealed: The Assembly of HIV

Researchers at the European Molecular Biology Laboratory (EMBL) and the University Clinic Heidelberg used cryoelectron tomography to peer at the structure of immature HIV, the precursor of the infectious variety. Turns out it has a fairly simple shape of a hexameric lattice that grows and seemingly randomly absorbs new proteins into the structure.
From the article abstract:

The major structural components of HIV are synthesized as a 55-kDa polyprotein, Gag. Particle formation is driven by the self-assembly of Gag into a curved hexameric lattice, the structure of which is poorly understood. We used cryoelectron tomography and contrast-transfer-function corrected subtomogram averaging to study the structure of the assembled immature Gag lattice to ≈17-Å resolution. Gag is arranged in the immature virus as a single, continuous, but incomplete hexameric lattice whose curvature is mediated without a requirement for pentameric defects. The resolution of the structure allows positioning of individual protein domains. High-resolution crystal structures were fitted into the reconstruction to locate protein–protein interfaces involved in Gag assembly, and to identify the structural transformations associated with virus maturation. The results of this study suggest a concept for the formation of nonsymmetrical enveloped viruses of variable sizes.

More: New electron microscopy images reveal the assembly of HIV…
Full article in PNAS: Structure and assembly of immature HIV
Image: Top: Sections through tomograms of (A) immature virus particles and (B) in-vitro-assembled Gag particles. Scale bar, 100 nm. Side: Global lattice maps of HIV particles. Positions of hexameric unit cells are marked with hexamers. Hexamers are colored according to cross-correlation on a scale from low (red) to high (green). Maps are shown in perspective such that hexamers on the rear surface of the particle appear smaller. (A) Lattice maps for immature HIV particles. The side of the particle toward the viewer lacks ordered Gag. (B) Close-up of defects in immature Gag lattice. (C) Lattice maps for in-vitro-assembled Gag particles. (D) Close-up of defects in in-vitro-assembled particle lattice.