The mumps virus (MuV) is the pathogen responsible for the illness known as mumps. This virus is a member of the Paramyxoviridae family and is classified within the Rubulavirus genus. Structurally, it is an enveloped virus, meaning its core genetic material is wrapped in a protective outer membrane. The genome consists of a single strand of ribonucleic acid (RNA). Individual viral particles (virions) are spherical or irregular in shape, with sizes ranging from 100 to 600 nanometers in diameter. Humans are the only natural host for this virus.
The Viral Envelope and Surface Proteins
The outermost layer of the mumps virus is the envelope, a lipid bilayer membrane that the virus acquires from the host cell it infects. This membrane is studded with spike-like protein projections that allow the virus to infect new cells. These projections are glycoproteins, meaning they are proteins with attached sugar molecules. Two primary types of glycoproteins are found on the mumps virus surface: the Hemagglutinin-Neuraminidase (HN) protein and the Fusion (F) protein.
The HN protein serves a dual-purpose role. Its first function is attachment, binding to sialic acid receptors on a host cell’s surface, particularly in the respiratory tract. This binding anchors the virus to a target. Later, after new viruses are assembled, the HN protein’s neuraminidase activity snips these sialic acid connections, allowing the new virions to detach and seek other cells to infect.
The second surface protein is the F protein, responsible for viral entry into the host cell. After the HN protein attaches the virus, the F protein activates and undergoes a change in its shape. This drives the merging of the viral envelope with the host cell’s membrane. This process, known as membrane fusion, creates an opening for the virus’s internal contents to be delivered into the cell.
Internal Viral Components
Just beneath the viral envelope lies the matrix (M) protein. The M protein forms a layer that lines the inner surface of the envelope, acting as a structural scaffold. It functions as a bridge, connecting the outer envelope and its surface glycoproteins to the innermost core of the virus. This protein helps stabilize the virion structure and is a principal organizer of viral assembly, helping gather all necessary components before a new virus particle is released.
At the center of the mumps virion is the ribonucleoprotein (RNP) core, also called the nucleocapsid. This core contains the virus’s genetic blueprint, a single-stranded RNA genome that is approximately 15,384 nucleotides long. The RNA is not bare; it is tightly wound and protected by the nucleoprotein (N), which coats the entire length of the genome and shields it from enzymes within the host cell. This helical complex of RNA and N protein is flexible and loosely coiled.
Attached to this RNP core is the viral polymerase complex, which is the machinery the virus uses to replicate itself. This complex consists of two additional proteins: the large polymerase (L) protein and the phosphoprotein (P). The L protein is the main RNA polymerase, the enzyme that reads the viral RNA to create new copies of the genome and transcribe it into messenger RNA (mRNA). The P protein assists the L protein, facilitating its RNA synthesis activities. Together, these components form a replication and transcription unit, ready to begin work upon entering a host cell.
How Structure Dictates Infection
The physical architecture of the mumps virus is directly linked to how it infects a cell. The process begins with attachment, mediated by the HN proteins on the viral envelope. These proteins bind to cells that display α2,3-linked sialic acid receptors. This specific targeting helps determine which cells the virus can infect, contributing to its tendency to infect glandular tissues and the central nervous system.
Once the HN protein anchors the virus, a signal is transmitted to the adjacent F proteins. This triggers the F protein to undergo a structural rearrangement. The F protein then extends a portion of its structure, the fusion peptide, into the host cell’s membrane. This action pulls the viral envelope and the cell membrane into close proximity.
This forced proximity culminates in membrane fusion, where the two lipid bilayers merge into one. This event creates a pore through which the internal contents of the virus are released into the cytoplasm. The primary component delivered is the RNP core, containing the viral genome and its polymerase complex.
With the RNP inside the host cell’s cytoplasm, the infection proceeds. The L and P proteins immediately begin their functions, using the viral RNA as a template to transcribe genes into mRNA. This mRNA is then used by the host’s ribosomes to produce viral proteins. They also replicate the viral genome to be packaged into new virus particles, hijacking the host’s resources.