Capsid proteins are the structural proteins that make up the protein coat, or capsid, of a virus. The capsid surrounds and protects the viral genome, which can be composed of DNA or RNA, depending on the virus. They play a critical role in the viral life cycle, including host cell recognition, attachment, entry, and assembly of new viral particles.
They spontaneously self-assemble to form the capsid structure, which can exhibit various shapes and symmetries. The two most common types of capsid structures are:
- Helical capsids: These capsids consist of capsid proteins arranged in a helical pattern around the viral genome, forming a rod-shaped or filamentous structure. Examples of viruses with helical capsids include the tobacco mosaic virus (TMV) and the rabies virus.
- Icosahedral capsids: These capsids have a spherical shape composed of 20 equilateral triangular faces, with capsid proteins arranged in a highly symmetrical pattern. Icosahedral capsids are common among many animal viruses, such as adenoviruses, poliovirus, and herpes simplex virus.
These proteins also play a crucial role in the host immune response to viral infections. They are often the primary targets for neutralizing antibodies, which bind to the capsid proteins and prevent the virus from entering host cells or assembling new viral particles. This is the basis for the development of many antiviral vaccines, which aim to elicit a strong immune response against specific capsid proteins, providing protection against future infections.
Understanding the structure and function of capsid proteins is essential for the development of new antiviral therapies and vaccines, as well as for gaining insight into the fundamental mechanisms of viral infection and replication. Structural studies, such as X-ray crystallography and cryo-electron microscopy, have been instrumental in revealing the detailed architecture of capsid proteins and their interactions with other viral components and host cells.
