Biomod/2011/MIT/Origami/Capsid
Introduction
Viruses have a protein shell called capsid which is essential for survival of the virus. It encloses the viral genome and is where the process of replication occurs. Without the capsid, the virus would not be able to infect other cells, which is why it could be a good target for treatment of HIV.
Structure
HIV has a conical capsid made up of viral 25.6kDa CA protein (p24) arranged into a mostly hexagonal lattice, similar to a honeycomb. It protects and carries the viral RNA genome inside. The capsid is also surrounded by a layer of matrix protein (p17), a lipid bilayer, that helps anchor gp120 and gp41. The CA proteins arrange into hexamers and pentamers. This assembly is controlled by an electrostatic switch. There are seven pentamers closing off the wider end of the cone, which in turn is made up of around 250 hexamers, and five pentamers closing off the other end. Pentamers are distributed asymmetrically among said regions. Close interactions between two adjacent CA domains stabilize this shell. At first, the immature capsid is spherical and then it undergoes a rearrangement to form the mature conical capsid. Little is known about this process, but studies have shown that the CA domains are usually cylindrical, and a single-point mutation allows them to fold into other shapes.
