Structure and Replication Cycle

Basic structure of Coronaviruses, depicting the four major envelope proteins found on all Coronaviruses. Note that the Hemagglutinin-esterase dimer is not represented on this diagram because not all Coronaviruses have one.

Spike glycoprotein trimer (S):

• FUNCTION: The S-Protein, the largest, most prominent structure on Coronaviruses, facilitates receptor-mediated entry into the cell. It mediates receptor attachment and cell membrane fusion. This large transmembrane protein assembles into trimers to form the distinctive spikes of Coronaviruses. It is heavily glycosylated, and is cleaved into two subunits, S1 and S2.

Membrane protein (M):

• FUNCTION: The M-Protein is a major, abundant dimer on Coronaviruses. It has two conformations, and allows membrane curvature, giving the virion envelope its shape. The M-Protein is also involved in organizing and packaging viral RNA into the virion.

Envelope protein (E):

• FUNCTION: The E-Protein, a small polypeptide that is not found in large quantities on Coronaviruses, is a transmembrane protein. It facilitates ion transport, virus assembly, viral budding, and release of the virus. Most significantly, it is involved in the pathogenesis of Coronaviruses.

Nucleocapsid protein (N):

• FUNCTION: Many N-Proteins bind to the Coronavirus' genomic ssRNA(+) in a beads-on-a-string fashion (N-Proteins = beads, RNA = string) to protect the genome from damage. This protein helps the virus bind to the replicase-transcriptase complex (RTC) to facilitate the packaging of the viral genome.

Hemagglutinin-esterase dimer protein (HE):

• FUNCTION: The HE fusion glycoprotein is present in the envelopes of several viruses, including Influenza C virus, and some Coronaviruses (particularly Betacoronaviruses of subgenus Embecovirus). This multi-functional protein enhances viral attachment, fusion, entry, and infection by binding sialic acids on surface Spike glycoproteins. It is involved in host cell receptor recognition, and destroys the receptor upon host cell entry and infection.