Chapter 21 : Measles, Mumps, and Rubella Viruses

Abstract:

Measles virus is a single stranded, nonsegmented, negative sense RNA virus and the prototypic member of the Morbillivirus genus of the Paramyxovirnae subfamily of the Paramyxoviridae. The standard viral genome is 15,894 nucleotides in length and contains six genes and encodes eight proteins which include nucleoprotein (N), phosphoproteins (P) C and V, and matrix (M), fusion (F), hemagglutinin (H), and polymerase (L) proteins. The measles virion is spherical with a diameter ranging from 120 to 250 nm. The virus buds from the plasma membranes of infected cells and has an envelope composed of glycoproteins, the H and F proteins, and lipids. The H and F proteins appear as short surface projections and are responsible for receptor binding and virus entry into susceptible cells (1). Three cell surface receptors for wild-type measles virus have been identified and all interact with the H glycoprotein (2). The M protein is positioned under the virion envelope and anchors the nucleocapsids to the budding sites at the plasma membrane. Unlike the H and F surface proteins, the M is neither glycosylated nor transmembranous. The envelope encloses an elongated helical nucleocapsid in which protein units are spirally arranged around the nucleic acid. The nucleoprotein (N), phosphoprotein, and large polymerase protein, in conjunction with the virion negative strand RNA, comprise the ribonucleoprotein complex, the replicating, and transcriptional unit of measles virus (1, 3). Although measles virus has only a single serotype, it can be subdivided into 24 distinct genotypes based on the sequence variability of the last 450 nucleotides of the N gene. These sequences can vary by up to 12% among the genotypes and form the basis of the molecular epidemiology applied to tracking of transmission pathways, monitoring control measures, and distinguishing wild-type viruses from vaccine strains of measles (4, 5).