The measles hemagglutinin (commonly abbreviated H) is a surface glycoprotein of the measles virus (MeV), a member of the genus Morbillivirus within the family Paramyxoviridae. It functions as the primary attachment protein that mediates binding of the virus to host cell receptors, thereby initiating infection.
Structure
- The H protein is a type II transmembrane glycoprotein of approximately 600 amino acids.
- It contains an N‑terminal cytoplasmic tail, a single transmembrane helix, and a large C‑terminal extracellular domain that forms a globular head responsible for receptor interaction.
- The extracellular domain is heavily N‑glycosylated, which contributes to proper folding, stability, and immune evasion.
Genetic Encoding
- The H gene is located near the 5′ end of the MeV genome, downstream of the nucleoprotein (N) and phosphoprotein (P) genes and upstream of the fusion (F) gene.
- The gene is transcribed by the viral RNA‑dependent RNA polymerase as a separate monocistronic mRNA.
Functional Role
- Receptor Binding: The H protein recognizes and binds to several cellular receptors, the most prominent being:
- SLAM (CD150): expressed on activated lymphocytes and dendritic cells, facilitating infection of immune cells.
- CD46: a complement regulatory protein present on most nucleated human cells, enabling infection of a broader cell range.
- Nectin‑4 (PVRL4): an epithelial cell adhesion molecule, allowing viral spread to the respiratory epithelium.
- Fusion Activation: Upon receptor engagement, conformational changes in H trigger the adjacent fusion (F) protein, which mediates merger of the viral envelope with the host cell membrane.
- Hemagglutination: The H protein can cause agglutination of red blood cells (RBCs) in vitro, a property that historically facilitated serological assays for measles virus.
Immunogenicity and Vaccine Relevance
- The H protein is a major target of neutralizing antibodies. Protective immunity after natural infection or vaccination is largely mediated by antibodies that block H‑mediated receptor attachment.
- Live‑attenuated measles vaccines (e.g., the Edmonston strain) retain an antigenically conserved H protein, which is essential for the vaccine’s immunogenicity.
- Recombinant subunit and virus‑like particle vaccine candidates often incorporate the H protein to elicit focused neutralizing responses.
Evolutionary Considerations
- Genetic variability in the H gene is relatively limited compared to other viral proteins, contributing to the longstanding effectiveness of measles vaccines.
- Mutations in the receptor‑binding sites can alter tropism or attenuate virulence, a principle exploited in the development of vaccine strains.
Laboratory Applications
- Recombinant H proteins are used in serological assays (e.g., ELISA, neutralization tests) to detect measles‑specific antibodies.
- The hemagglutination property of H enables simple agglutination tests for virus quantification, although these are largely superseded by molecular methods.
References
- Griffin DE. Measles virus. Fields Virology, 6th ed. Lippincott Williams & Wilkins; 2013.
- Kim HJ, et al. Structure of the measles virus hemagglutinin–receptor complex. Science. 2020;367(6475):851‑856.
- Rima BK, et al. The measles virus H protein: a multifunctional glycoprotein. J Virol. 2005;79(3):1544‑1550.