Adenoviruses are common viruses that cause a range of illnesses, from respiratory infections to gastroenteritis, with over 100 identified types. The function of these viruses is linked to their physical structure, which is not merely a container but an active participant in the infection process.
The Icosahedral Capsid
The defining feature of an adenovirus is its outer shell, the capsid, which has the shape of an icosahedron. An icosahedron is a geometric solid with 20 faces, 30 edges, and 12 vertices, similar to a 20-sided die. This structure provides a strong and stable container for the virus’s genetic material and is a highly efficient way to enclose a maximum volume with minimal material.
This geometry is common among many viruses because it allows for self-assembly from repeating protein subunits. The adenovirus capsid is a durable structure, approximately 90-100 nanometers in diameter, that protects the viral DNA from environmental damage. The arrangement of proteins creates a robust yet dynamic shell.
Proteins That Form the Structure
The icosahedral capsid is constructed from several distinct proteins. The majority of the capsid is made of proteins called hexons. These proteins assemble into groups of three and form the 20 triangular faces of the icosahedron. There are 240 hexon units in total, creating the main body of the viral shell.
Positioned at each of the 12 vertices of the icosahedron are the pentons. Each penton consists of a base integrated into the capsid and a long, protruding fiber. These fibers are projections that extend outward from the main viral particle. Minor proteins are also present that help stabilize the structure, holding the hexons and pentons together.
How Shape Influences Infection
The shape and protein composition of the adenovirus are directly related to its ability to infect host cells. The process begins with the fibers that extend from the penton bases. These fibers act as the initial point of contact, binding to specific receptor proteins on the surface of a host cell, which determines which cell types the virus can infect.
Following the initial attachment, the penton base interacts with a different set of cellular proteins, called integrins. This interaction triggers the cell to engulf the virus particle through a process called endocytosis. Once inside the cell, the stable capsid protects the viral DNA. The capsid is eventually disassembled, and the genetic material is released and transported to the cell’s nucleus to be replicated.
Using the Adenovirus Shape in Medicine
Scientists have leveraged their understanding of the adenovirus’s structure for medical applications. By modifying the virus, its infectious properties can be harnessed for therapeutic purposes. The most prominent use is in gene therapy, where the virus is used as a vector to deliver genetic material into cells. The viral genes that cause illness are removed and replaced with a healthy copy of a gene a patient may be missing.
The adenovirus’s ability to efficiently enter cells makes it an effective delivery vehicle. This same principle is applied in the development of vaccines. For some COVID-19 vaccines, an adenovirus was engineered to carry the genetic instructions for the SARS-CoV-2 spike protein. When the modified adenovirus enters human cells, it causes them to produce the spike protein, triggering an immune response without causing disease.