Figure 7. Electron dense striations in tomograms of the scission regions of budding virions.

(A–E) Parallel electron dense striations (red arrowheads) circumscribing the necks of budding virions in positively-stained, plastic-embedded samples (A,B,D) and in negatively-stained samples (C,E). Parallel striations, suggested to be portions of the polymerized ESCRT-III complex, were observed only when the diameter of the neck was half or less than the diameter of the bud. (B) Higher magnification view of the bud in (A), rotated to optimize visualization of the striations. (F) Tomographic slice of a positively-stained budding profile displaying five dense spots (red arrows) in the neck region that may correspond to VPS4 complexes recruited to facilitate scission of the bud. Ribosomes in cytoplasm distal from the budding virion appeared slightly smaller and typically less electron dense than the presumptive VPS4 structures. (G) Galleries of HIV-1 budding profiles bearing presumptive VPS4 spots (row 1) and individual presumptive VPS4 spots and cytoplasmic ribosomes, extracted from tomograms and viewed at high magnification (rows 2 and 3, respectively). The VPS4 spots were pleomorphic and solidly dense, with an average width of 13.3±0.8 nm; n = 10). The spots appeared to be slightly larger than ribosomal densities (11.9±0.8 nm; n = 10), which were less dense and often showed a characteristic “groove” between the 30S and 50S subunits. Note that the ribosomes in this and previous ET studies involving positively-stained, plastic embedded samples [72] appear smaller than their 25–30 nm diameter.