Viruses are infectious agents that require a host to replicate and come in many shapes and sizes. The rabies virus, a member of the Lyssavirus genus, is the causative agent of a severe neurological disease in mammals. Its particular form is a defining characteristic that is directly linked to how it functions and causes illness. Understanding this structure provides insight into its lifecycle and the mechanisms of the disease it produces.
The Distinctive Bullet Shape
The defining feature of the rabies virus is its cylindrical morphology, often described as bullet-shaped. This shape is a hallmark of the Rhabdoviridae family, to which rabies belongs. The virion, or single virus particle, is about 180 nanometers (nm) in length and has a diameter of 75 nm. This structure features one end that is rounded or conical, while the opposite end is planar or concave.
This bullet-like appearance sets it apart from many other viruses that infect humans, which are often spherical, like influenza, or have a more complex icosahedral (20-sided) shape, such as adenoviruses. This highly organized assembly contributes to the virus’s stability and its ability to infect its host.
Anatomy of the Rabies Virus
The rabies virus’s structure is a layered assembly of five distinct proteins. The outermost layer is a lipid envelope acquired from the host cell it previously infected. Studding this envelope are approximately 400 spike-like projections made of glycoprotein (G-protein). These spikes are trimers, formed from three identical G-protein units, and cover the entire surface of the virion except for the flat end.
Beneath the envelope lies a layer composed of the matrix (M) protein. The M-protein lines the inner surface of the lipid envelope, providing rigidity and acting as a bridge between the outer envelope and the virus’s core. This protein is largely responsible for the virus’s signature bullet shape.
At the center of the virion is the ribonucleoprotein (RNP) core. This core has a helical symmetry and consists of the virus’s genetic material—a single strand of negative-sense RNA—encased by the nucleoprotein (N). Associated with this RNP complex are two other proteins: the phosphoprotein (P) and the large protein (L), which is an RNA polymerase. This RNP structure protects the viral genome from being broken down.
How Viral Structure Aids Infection
The G-protein spikes on the exterior initiate infection, as they recognize and bind to specific receptors on the surface of host cells, particularly nerve cells. This specific binding allows the virus to attach to and enter its target cells, a first step in its lifecycle.
Once attached, the entire virus particle is taken into the host cell. Inside the cell’s cytoplasm, the viral envelope fuses with a cellular membrane, releasing the RNP core. The L-protein, which is part of the RNP complex, then begins the process of transcribing the viral RNA. This allows the virus to use the host cell’s own machinery to produce the five viral proteins needed to create new virions.
The assembly of new virus particles is a highly organized process. The M-protein congregates at the host cell’s membrane, forming a matrix that initiates the budding process. The newly synthesized RNP cores travel to these sites and bind with the M-protein, while G-proteins are inserted into the membrane above. The complete virus then buds from the cell surface, cloaked in a piece of the host’s membrane, ready to infect other cells and continue the cycle.