The measles virus causes measles, a contagious disease transmitted through respiratory droplets. Understanding the virus’s structure is important for grasping how it infects cells and spreads. This knowledge informs the development of vaccines and antiviral therapies by revealing the mechanisms that facilitate its lifecycle.
Measles Virus: An Overview of its Form
The measles virus particle, or virion, belongs to the Morbillivirus genus. It has a spherical but irregular (pleomorphic) shape and measures between 100 and 300 nanometers in diameter. As an enveloped virus, its core is encased in a lipid membrane derived from the host cell it previously infected.
Within this envelope is the virus’s genetic material, a single, non-segmented strand of RNA. This negative-sense genome cannot be directly translated into proteins. It must first be transcribed into a complementary positive-sense strand that serves as the template for protein synthesis. The genome is approximately 15,900 nucleotides long.
The Viral Envelope and Its Components
The outermost layer of the virion is the viral envelope, a lipid bilayer acquired from the host cell membrane during budding. This active layer is embedded with viral proteins required for infection. While the envelope’s lipids are from the host, the proteins are encoded by the viral genome.
Lining the inner surface of the envelope is the matrix (M) protein. This protein forms a structural layer that maintains the virion’s shape and integrity. It also acts as a bridge connecting the outer envelope to the virus’s internal core. The M protein assists in the assembly of new virus particles, coordinating components at the budding site on the host cell membrane.
Surface Glycoproteins: H and F Proteins
Protruding from the viral envelope are spike-like glycoproteins, which are proteins decorated with sugar molecules. Two of these, the hemagglutinin (H) protein and the fusion (F) protein, are responsible for the virus’s ability to infect cells. These proteins work together to allow entry into a host cell, and their external location makes them primary targets for the immune system.
The H protein is the virus’s attachment mechanism, binding to specific receptor proteins on host cells. This binding is the first step in the infection process. These receptors include SLAMF1 on immune cells and Nectin-4 on epithelial cells in the respiratory tract.
After the H protein attaches to a host cell, the F protein initiates the next step. The F protein is responsible for merging the viral envelope with the host cell’s membrane. Following attachment, the F protein changes shape, inserting itself into the host membrane and pulling the two membranes together until they fuse. This fusion creates an opening for the virus’s internal contents to enter the cell and begin the infection.
The Nucleocapsid: Protecting the Viral Genome
At the center of the measles virus is the nucleocapsid, also called the ribonucleoprotein (RNP) core. This structure houses and protects the virus’s single-stranded RNA genome. The nucleocapsid has a helical, or spiral, organization, which allows the long RNA strand to be tightly packaged.
The primary structural component of the nucleocapsid is the nucleoprotein (N). Thousands of N protein copies bind directly to the viral RNA, coating its entire length. This protein shell shields the RNA from cellular enzymes that would degrade it and provides the scaffold for the helical shape.
Associated with the N protein-RNA complex are two other proteins that form the viral replication machinery: the phosphoprotein (P) and the large protein (L). The L protein is the main enzymatic component, acting as the RNA-dependent RNA polymerase. It transcribes the genome into messenger RNA and replicates the entire genome for new virus particles. The P protein assists the L protein by linking it to the N-RNA template to carry out its functions.