Viral entry relies on a fine interplay between the virion and the host cell. The spike protein is also the principal player in determining host range. Whereas the M and E proteins are involved in virus assembly, the spike protein is the leading mediator of viral entry. In addition, some coronaviruses also contain a hemagglutinin esterase (HE). The virion envelope contains at least three viral proteins, the spike protein (S), the membrane protein (M) and the envelope protein (E) ( Figure 1B). The genome is packaged into a helical nucleocapsid surrounded by a host-derived lipid bilayer. Variable numbers of ORF encoding accessory proteins are present between these genes. The first ORF comprises two-thirds of the genome and encodes the replicase proteins, whereas the last third contains the structural protein genes in a fixed order: (HE)-S-E-M-N ( Figure 1A). The genome is composed of six to ten open reading frames (ORFs). They possess a 5' capped, single-strand positive sense RNA genome, with a length between 26.2 and 31.7 kb, the longest amongst all RNA viruses. Coronaviruses belong to the Coronavirinae subfamily that together with Torovirinae form the Coronaviridae family in the Nidovirales order.Ĭoronaviruses are enveloped, spherical or pleiomorphic viruses, with typical sizes ranging from 80 to 120 nm. Ĭoronaviruses are classified in four different genera, historically based on serological analysis and now on genetic studies: alpha-, beta-, gamma-, and delta-CoV ( Table 1). Coronaviruses infect a wide variety of mammals and birds, causing respiratory and enteric diseases and, in some rarer cases, hepatitis and neurologic disease. Before gaining importance for public health in 2003, the diseases associated with coronaviruses were mainly of veterinary interest. This episode also shed light on the capabilities of coronaviruses to jump across species. Interest in this family of viruses grew in the aftermath of this epidemic, leading to the identification of many new family members. We also highlight commonalities between coronavirus S proteins and other class I viral fusion proteins, as well as distinctive features that confer distinct tropism, pathogenicity and host interspecies transmission characteristics to coronaviruses.Īlthough the first member of the coronavirus family was discovered in the 1930s coronaviruses gained particular notoriety when the severe acute respiratory syndrome (SARS) outbreak shook the world in 2002–2003. This review discusses coronavirus entry mechanisms focusing on the different triggers used by coronaviruses to initiate the conformational change of the S protein: receptor binding, low pH exposure and proteolytic activation. It is classified as a class I fusion protein, and is responsible for binding to the receptor on the host cell as well as mediating the fusion of host and viral membranes-A process driven by major conformational changes of the S protein. The spike glycoprotein (S) mediates virus entry and is a primary determinant of cell tropism and pathogenesis. To deliver their nucleocapsid into the host cell, they rely on the fusion of their envelope with the host cell membrane. Coronaviruses are enveloped positive-stranded RNA viruses that replicate in the cytoplasm.
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