SARS virus portrait. This graphic shows a virus particle (virion) in the foreground with elements of the viral life cycle (in a host cell) in the background. (original image is 3286 pixels across)
Coronaviruses get their name from their striking corona
(crown) of spikes (1). These are made of S glycoprotein (probably a trimer) drawn
here in red. These spikes bind to host cells and allow the virus to enter the cell.
The virus is enveloped (bluish green layer). This means it is cloaked in a membrane
derived from the host cell. In the case of coronaviruses, this membrane is derived
from inside the cell, not from the cell surface. M proteins insert into the membrane,
where they interact with the spikes (2). M proteins all seem to insert in the envelope
but many also connect to the nucleocapsid (3). Below the envelope is a gap (4). Deeper
still lies the core, drawn here as a purplish icosahedron (4). This seems to be composed
of M protein on the outside and contains the RNP (ribonucleoprotein). The RNP is
a complex of RNA (which carries the viruses genetic material) and N protein. This
nucleocapsid is drawn as a yellow rope like structure in the very middle of the virus. Although generally described as spherical, "photographs" (electron micrographs, especially cryoelectron micrographs (3)) of coronaviruses seem to show a clearly
faceted virion. This seems to be true not only of the core, which is probably icosahedral
(3), but also it seems of the overlying membrane (viral envelope) which seems to
hug the contours of the core reasonably closely. In the case of HIV, which is usually
depicted as having a spherical envelope, it would seem that the envelope must also
follow the shape of the underlying icosahedral matrix since the knobs, which project
through the envelope, interact with the matrix. This would seem to suggest that the
knobs pin the membrane down, effectively stitching it to the matrix. Please see our
illustration of the
HIV structure
CORONAVIRUSES AND SARS (Severe Acute Respiratory Syndrome):
A new type of coronavirus has been identified as the cause of the emergent disease called Severe Acute Respiratory Syndrome or SARS. The "SARS Virus" was officially declared the causative agent on 16th April 2003 by the
World Health Organisation. The genome for this novel virus was sequenced by various groups just prior to this announcement.
SARS (Severe Acute Respiratory Syndrome) LINKS:
REFERENCES
1. Michael M. C. Lai & Kathryn V. Holmes. Coronaviridae:
The Viruses and Their Replication. Field's Virology
2. de Haan CAM, Vennema H, Rottier PJM, Assembly of the
Coronavirus Envelope: Homotypic Interactions between the M Proteins. Journal of Virology
June 2000 : 4967-4978
3. Escors D, Camafeita E, Ortego J, Laude H, Enjuanes L,
Organisation of Two Transmissible Gastroenteritis Coronavirus Membrane Protein Topologies
within the Virion and Core. Journal of Virology Dec 2001: 12228 - 12240
4. Risco C. Anton IM, Enjuanes L, Carrascosa JL. The Transmissible
Gastroenteritis Coronavirus Contains a Spherical Core Shell Consisting of M and N
Proteins. Journal of Virology July 1996:70 : 4773 - 4777
