Enveloped Virions

SOME virions additionally have:

• ENVELOPES, or lipid bilayer MEMBRANES: these always have viral proteins inserted in them (almost always GLYCOPROTEINS); these are often referred to as SPIKES. 
• There is usually an inner layer of protein (usually referred to as MATRIX protein) inside the envelope, usually in contact with the (nucleo)CAPSID protein.   See below for examples (West Nile virus, Flaviridae; influenza virus, Orthomyxoviridae; HIV, Retroviridae; SARS coronavirus; Hepatitis C flavivirus.  The nucleoprotein can be helical (Ortho- and Paramyxoviridae) or isometric

Cutaway depiction of West Nile flavivirus: note regular arrangement of envelope protein, underlying membrane and isometric capsid	Cutaway depiction of influenza virion: note helical nucleoprotein, matrix and envelope layers and spikes	Cutaway depiction of HIV virion: note conical nucleoprotein, isometric matrix and envelope layers and spikes
SARSCoV (SARS coronavirus).  Note elongated membrane spikes and underlying isometric capsid	Hepatitis C flavivirus (HCV): note elongated, regularly-spaced spikes and isometric structure	Herpesvirus: note many envelope glycoproteins, "tegument" inside membrane, and isometric capsid
copyright Russell Kightley (www.rkm.com.au)

Enveloped virions may additionally incorporate host membrane proteins in their envelopes: while this may be a side effect of the "budding" process by which viral nucleoprotein complexes acquire envelopes, it may also in some cases be a mechanism for avoidance of host immune systems (eg: hepatitis B virus incorporates host serum albumin into its capsid; HIV virions may incorporate host MHC proteins).

	Tutorial Contents
"What is a Virus?"	Isometric Virions	Helical Nucleocapsids	Compound structures

copyright June 1998, May 2008 by EP Rybicki unless otherwise stated