The Epstein-Barr virus, or EBV, is a member of the herpesvirus family and one of the most widespread viruses in human populations. The complete virus particle, or virion, is a sophisticated assembly of multiple layers, each with a distinct purpose. This architecture consists of a central genetic core, a protective protein shell, an intermediate protein layer, and an outer membrane derived from the host. These components protect the virus’s genetic material and execute the infection of specific human cells.
The Viral Genome
At the center of the Epstein-Barr virus lies its genetic blueprint, a large, linear, double-stranded DNA molecule. This genome is approximately 172,000 base pairs in length and carries the code for at least 85 distinct genes. The size of the EBV genome is substantial, reflecting its complexity and the large number of proteins it can produce to manipulate host cells and build new virions.
This DNA dictates the assembly of structural proteins that form the virion’s layers and encodes proteins used to take over the host cell’s machinery for replication. The genome is organized into unique short and long regions, separated by repetitive sequences of DNA. Once inside a host cell’s nucleus, this linear DNA can circularize, becoming a stable episome that can persist for the lifetime of the cell.
The Protective Capsid
Encasing the viral DNA is a protein shell known as the capsid. This structure’s primary role is to shield the genome from physical stress and enzymatic damage. The capsid of EBV has a symmetrical shape known as an icosahedron, which resembles a 20-sided die. This geometric arrangement provides maximum strength and volume with a minimum number of protein components.
The capsid is constructed from repeating protein subunits called capsomeres. The main component is the major capsid protein (VCA), which forms the hexagonal and pentagonal units of the icosahedral lattice. These capsomeres are linked together by smaller protein complexes called triplexes, which are composed of minor capsid proteins, resulting in a durable container for the viral DNA.
The Protein Tegument
Positioned between the capsid and the outer envelope is a less-defined layer called the tegument. Unlike the highly organized capsid, the tegument is a protein-filled space that appears more variable in structure. It contains a diverse collection of viral proteins for the initial stages of infection.
Upon entry into a cell, the tegument proteins are released into the cytoplasm. Some of these proteins immediately begin to counteract the cell’s innate immune defenses, shutting down antiviral alarm systems. Other tegument proteins help to prepare the cellular environment for viral DNA replication and gene expression.
The Envelope and Its Glycoproteins
The outermost boundary of the Epstein-Barr virus is a lipid membrane called the envelope. This membrane is not built from scratch by the virus; instead, it is acquired from the host cell during the process of budding, as new virus particles exit. Embedded within this lipid bilayer are numerous viral proteins, most prominently the glycoproteins, which appear as spikes on the virion’s surface.
The process of infection is highly specific and is mediated by these surface proteins, with the primary targets being B lymphocytes and epithelial cells. The most abundant glycoprotein, gp350, acts as the initial attachment key, binding specifically to a receptor called CD21 on the surface of B cells. This binding tethers the virus to the correct cell type, initiating the entry process.
Following attachment, a more complex series of interactions is required for the virus to fuse its envelope with the cell’s membrane. This fusion is managed by a core machinery of glycoproteins, including gB, gH, and gL. For B cell entry, an additional protein, gp42, is required; it interacts with MHC class II molecules on the B cell, which triggers the fusion event. In epithelial cells, the mechanism is slightly different, relying on the gH/gL complex binding to cellular integrins to start the fusion process.