Covering up a naked virus

Sabin type 2 poliovirusViruses can be broadly classified according to whether or not the particle is enveloped – surrounded by a membrane taken from the host cell – or naked. Some naked viruses apparently are more modest than we believed.

Members of the family Picornaviridae, which include Hepatitis A virus, poliovirus, and Coxsackieviruses, have non-enveloped particles that consist of a protein shell surrounding the viral RNA genome (poliovirus is illustrated). Examples of viruses that are enveloped include dengue virus, influenza virus, and measles virus.

Recently it was discovered that hepatitis A virus (HAV) particles are released from cells in membrane vesicles containing 1-4 virus particles. These membranous structures resemble exosomes, which are also released from uninfected cells and play roles in various biological processes. Enveloped hepatitis A virus particles are present in the blood of infected humans. However virus in the feces, which is transmitted to other hosts, is not enveloped.

Viral envelopes typically contain viral glycoproteins, such as the HA protein of influenza viruses, which serve important functions during replication, such as attachment to cell receptors. Envelope glycoproteins are also the target of antibodies that block viral infection. The presence of an envelope makes HAV resistant to neutralization with antibodies, because the membrane contains no viral proteins that can be blocked by antibodies.

Two other non-enveloped picornaviruses, Coxsackievirus B and poliovirus, are also released from cells within membrane vesicles. These virus particles are in vesicles derived from the autophagy pathway, which captures and recycles cytoplasmic contents by ejecting them from the cell.

What is the function of the membrane acquired by these naked viruses? Perhaps immune evasion: the presence of the cell membrane makes HAV and Coxsackievirus B virus particles resistant to neutralization with antibody. The ability to deliver multiple virus particles to a single cell might help to overcome genetic defects in the viral genome that are a consequence of the high mutation rates of these viruses.

An interesting problem is how these cloaked viruses enter cells, because there is no evidence that the membranes contain any viral proteins that could interact with a cell receptor. Nevertheless, entry of enveloped HAV and poliovirus into cells requires the known viral receptor. Perhaps the vesicles are taken into the cell by endocytosis, where viral particles are released from the vesicles, and then bind receptors to initiate escape of the genome.

Should HAV, poliovirus, and Coxsackievirus B be reclassified as enveloped viruses? Probably not, in part because the membranes surrounding these virus particles are not needed for infectivity. In contrast, removal of the membrane from influenza virus, dengue virus, or measles virus destroys their infectivity. Enveloped viruses acquire a membrane after the internal components have been assembled, whether they are helical or icosahedral nucleocapsids. In contrast, HAV, poliovirus, and Coxsackievirus B become fully infectious particles before they acquire an envelope.

Another argument against calling picornaviruses enveloped is that viral membranes contain viral glycoproteins that allow attachment to cell receptors and release of the viral genome into the cell. There is no evidence that the membranes of picornaviruses contain viral proteins.

The acquisition of a membrane may have taken place later in the evolution of picornaviruses, to allow more efficient infection or evasion of host responses. Alternatively, the membrane may simply be a by-product acquired when these viruses exit the cell by a non-lytic mechanism.

While the finding of membranes around picornavirus particles is intriguing, I am not yet convinced that these viruses should be considered to be enveloped. I would like to know if other non-enveloped viruses are similarly released from cells in membranous cloaks, and the function of this addition for viral replication in the host.

A new type of enveloped virus?

All known virus particles can be placed into one of two general categories: enveloped or non-enveloped. Viruses that fall into the former category are characterized by a lipid membrane derived from the host cell, and one or more nuclecapsid proteins that interact with the viral genome. A virus that infects an archaeal host may constitute a new category of enveloped viruses. It comprises a membrane vesicle that encloses a circular ssDNA genome which is devoid of nucleic acid-binding nucleoproteins.

Examples of enveloped virions that contain nucleoproteins are shown in the figure below. These include influenza virus (left), a simple retrovirus (center), and a togavirus (right).

The influenza virion contains segments of viral RNA bound to four different proteins. Retroviral RNA is bound to a nucleocapsid protein which in turn is enclosed in a capsid, while togavirus RNA is located within an icosahedral shell.

Until recently, it was believed that the genome of all other known enveloped DNA and RNA viruses is always associated with one or more viral proteins. This belief may be changed by the isolation, from a solar saltern in Trapani, Italy, of a virus that infects the archaeal species Halorubrum. Salterns are multi-pond systems in which sea water is evaporated to produce salt. In such hypersaline envrionments, Archaea predominate, and about 20 archaeal viruses have been isolated from these locations.

The virus isolated from the Italian saltern is called Halorubrum pleomorphic virus-1, or HRPV-1. Biochemical analyses of the virion show that it is composed of lipids and two structural proteins, VP3 and VP5. The genome is a circular ssDNA about 7 kb in length with nine open reading frames. The virion architecture is unique: it is composed of a flexible membrane (hence the designation pleomorphic) that contains external spikes of the VP4 protein, and is lined on the interior with VP3. The viral DNA is apparently not bound to any proteins in the virions.

At the upper left is my depiction of the appearance of HRPV-1. The diagram was produced by deleting the internal proteins and nucleic acid of a simple retrovirus and replacing these with a ssDNA genome. The HRPV-1 VP4 spikes and the internal VP3 proteins are present, but no proteins are bound to the viral genome. Whether or not the VP4 spikes are oligomeric as shown is unknown.

Most enveloped viruses acquire their lipid membrane by budding from the host cell, and a similar mechanism could account for the formation of HRPV-1 virions. In the absence of a nucleoprotein, it is not clear how the viral genome would be specifically incorporated into the budding envelope. Another condundrum is how the virions would pass through the proteinaceous layer that covers the archaeal host cell.

Whether HRPV-1 is representative of a new kind of virus lacking nucleocapsid protein will be revealed by the study of other pleomorphic enveloped viruses. Candidates include bacterial viruses that infect mycoplasmas, and another pleomorphic haloarchaeal virus isolated from a different Italian saltern, Haloarcula hispanica pleomorphic virus 1.

Pietila, M., Laurinavicius, S., Sund, J., Roine, E., & Bamford, D. (2009). The Single-Stranded DNA Genome of Novel Archaeal Virus Halorubrum Pleomorphic Virus 1 Is Enclosed in the Envelope Decorated with Glycoprotein Spikes Journal of Virology, 84 (2), 788-798 DOI: 10.1128/JVI.01347-09

Virology lecture #4: Structure of viruses

Download: .wmv (394 MB) | .mp4 (110 MB)

Visit the virology W3310 home page for a complete list of course resources.