Molecule of the Month: HIV Capsid
At the center of HIV, an unusual cone-shaped capsid protects the viral genome and delivers it into infected cells
Exploring the Structure
HIV Capsid (PDB entries 3mge and 3p05)
The capsid protein is able to form hexamers and pentamers by shifting slightly in structure. This is an example of the principle of "quasiequivalence", first proposed by Caspar and Klug in 1962. Quasiequivalence is the way that many viruses build capsids that are much larger than is possible with perfect symmetry, but that still only use a single type of protein chain. In the HIV capsid, the interactions between the many subunits are similar, but are deformed slightly to accommodate the different shapes of the cone-shaped portion and the round caps. To take a closer look at these two structures, PDB entries 3mge and 3p05 , click on the image for an interactive Jmol.
Topics for Further Discussion
- You can explore the structure of capsid and other HIV proteins in the online animation: The Structural Biology of HIV.
- To explore the flexibility of capsid, you can use the Compare Structures tool at the RCSB PDB to overlap the different structures.
- In order to solve the structures of the hexagonal and pentagonal complexes, researchers engineered capsid with cysteines to lock the structure together--see if you can find them when you're viewing these structures.
July 2013, David Goodselldoi:10.2210/rcsb_pdb/mom_2013_7