You’ve been deceived: The outer surface of an HIV particle does not look like a spiky Koosh ball, as scientists and artists have long depicted it. Though the viral envelope has space for more than 100 glycoprotein spikes, it actually only keeps about 10 such proteins on its surface. Scientists speculate that this may be a defense against the immune system—fewer surface proteins means fewer places that the immune system cells can bind to identify the virus.
Virologist Hans-Georg Kräusslich of Heidelberg University and physicist Stefan Hell of the Max Planck Institute for Biophysical Chemistry in Germany joined forces to investigate the distribution of these rare proteins around the viral envelope. Traditional light microscopes only resolve to a distance of half the wavelength of visible light, which is about 500–600 nm, so anything less than 250 nm in size cannot be distinguished from nearby molecules. And since an HIV-1 virion is only 140 nm in diameter, researchers using traditional light microscopy cannot identify the positions of individual proteins on the viral envelope.
Instead, the duo employed a super-resolution microscopy technique pioneered by Hell in 1994 called stimulated emission depletion (STED) microscopy. STED utilizes two beams of light instead of one: a doughnut-shaped beam of light is shined down atop a regular beam of light, and where the two streams of light overlap they cancel one another out, resulting in a single, sharp beam of light that achieves resolutions as low as 35–40 nm, about a 7-fold improvement over traditional light microscopy. “It, thereby, reaches a resolution where you can identify structures on an individual virus particle which were not visible before,” said Kräusslich.
The team examined immature and mature HIV-1 particles using the tool. On immature particles, a precursor to the infectious virus, glycoproteins are dispersed around the viral envelope. But on mature viruses, a protein-cleaving process inside the virus rearranges the inner protein lattice and subsequently changes the outer surface of the virus, clustering the glycoproteins into a single patch on the viral surface. This patch of proteins attaches to receptors on a host cell, docking the virus to enter and infect the cell.
Overall, glycoprotein clustering proved to be necessary for efficient docking of the virus and subsequent infection. When a virus was unable to undergo the clustering of surface proteins, it was not adept at infecting a host cell, the researchers found.
“We’re not saying this process is the most essential step in infection,” said Kräusslich, “but it is helpful to understand the attachment and fusion process of the virus in a better way, and thus work to interfere with it.”
Kräusslich and Hell plan to continue to use STED microscopy to look at other molecules recruited to the site of HIV virus docking onto a host cell. “Now that we are able to look at individual events at this resolution, we’d like to identify and characterize other factors as well,” added Kräusslich.
1. Chojnacki, J., et al. 2012. Maturation-dependent HIV-1 surface protein redistribution revealed by fluorescence nanoscopy. Science, 338:524-8.