virology blog
About viruses and viral disease
Jeremy joins the TWiVeroids to tell the amazing story of how the function of the HIV-1 protein called Nef was discovered and found to promote infection by excluding the host protein SERINC from virus particles.
You can find TWiV #409 at microbe.tv/twiv, or listen below.
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The illustration at left depicts a virion – the infectious particle that is designed for transmission of the nucleic acid genome among hosts or host cells. A virion is not the same as a virus. I define virus as a distinct biological entity with five different characteristics. Others believe that the virus is actually the infected host cell.
The idea that virus and virion are distinct was first proposed by Bandea in 1983. He suggested that a virus is an organism without a cohesive morphological structure, with subsystems that are not in structural continuity:
Viruses are presented as organisms which pass in their ontogenetic cycle through two distinctive phenotypic phases: (1) the vegetative phase and (2) the phase of viral particle or nucleic acid. In the vegetative phase, considered herein to be the ontogenetically mature phase of viruses, their component molecules are dispersed within the host cell. In this phase the virus shows the major physiological properties of other organisms: metabolism, growth, and reproduction.
According to Bandea’s hypothesis, the infected cell is the virus, while the virus particles are ‘spores’ or reproductive forms. His theory was largely ignored until the discovery of the giant mimivirus, which replicates its DNA genome and produces new virions in the cytoplasm within complex viral ‘factories’. Claverie suggested that the viral factory corresponds to the organism, whereas the virion is used to spread from cell to cell. He wrote that “to confuse the virion with the virus would be the same as to confuse a sperm cell with a human being”.
If we accept that the virus is the infected cell, then it becomes clear that most virologists have confused the virion and the virus. This is probably a consequence of the fact that modern virology is rooted in the study of bacteriophages that began in the 1940s. These viruses do not induce cellular factories, and disappear (the eclipse phase) early after cell entry. Contemporary examples of such confusion include the production by structural virologists of virus crystals, and the observation that viruses are the most abundant entities in the seas. In both cases it is the virion that is being studied. But virologists are not the only ones at fault – the media writes about the AIDS virus while showing an illustration of the virion.
Those who consider the virus to be the infected cell also believe that viruses are alive.
…one can conclude that infected eukaryotic cells in which viral factories have taken control of the cellular machinery became viruses themselves, the viral factory being in that case the equivalent of the nucleus. By adopting this viewpoint, one should finally consider viruses as cellular organisms. They are of course a particular form of cellular organism, since they do not encode their own ribosomes and cell membranes, but borrow those from the cells in which they live.
This argument leads to the assumption that viruses are living, according to the classical definition of living organisms as cellular organisms. Raoult and Forterre have therefore proposed that the living world should be divided into two major groups of organisms, those that encode ribosomes (archaea, bacteria and eukarya), and capsid-encoding organisms (the viruses).
BANDEA, C. (1983). A new theory on the origin and the nature of viruses Journal of Theoretical Biology, 105 (4), 591-602 DOI: 10.1016/0022-5193(83)90221-7
Forterre, P. (2010). Defining Life: The Virus Viewpoint Origins of Life and Evolution of Biospheres, 40 (2), 151-160 DOI: 10.1007/s11084-010-9194-1
Download: .wmv (394 MB) | .mp4 (110 MB)
Visit the virology W3310 home page for a complete list of course resources.
Influenza A viruses tend to garner most of the attention, but let’s not forget that there are two other virus types, B and C.
The enveloped influenza A virions have three membrane proteins (HA, NA, M2), a matrix protein (M1) just below the lipid bilayer, a ribonucleoprotein core (consisting of 8 viral RNA segments and three proteins: PA, PB1, PB2), and the NEP/NS2 protein. It would be difficult to distinguish influenza A and B viruses by electron microscopy, but there are differences. Influenza B virions have four proteins in the envelope: HA, NA, NB, and BM2. Like the M2 protein of influenza A virus, the BM2 protein is a proton channel that is essential for the uncoating process. The NB protein is believed to be an ion channel, but it is not required for viral replication in cell culture.
Influenza B viruses cause the same spectrum of disease as influenza A. However, influenza B viruses do not cause pandemics. This property may be a consequence of the limited host range of the virus – humans and seals – which limits the generation of new strains by reassortment. The virus causes significant morbidity: in the US in 2008, approximately one-third of all laboratory confirmed cases of influenza were caused by influenza B (as shown on the first graph on this CDC page). Consequently the seasonal trivalent influenza vaccine contains an influenza B virus component.
Influenza C viruses are somewhat different (there is a nice diagram here). The enveloped virions have hexagonal structures on the surface and form long (500 microns) cordlike structures as they bud from the cell (image below). Like the influenza A and B viruses, the core of influenza C viruses consists of a ribonucleoprotein made up of viral RNA and four proteins. The M1 protein lies just below the membrane, as in influenza A and B virions. A minor viral envelope protein is CM2, which functions as an ion channel. The major influenza C virus envelope glycoprotein is called HEF (hemagglutinin-esterase-fusion) because it has the functions of both the HA and the NA. Â Therefore the influenza virion contains 7 RNA segments, not 8 RNAs like influenza A and B viruses.
Nearly all adults have been infected with influenza C virus, which causes mild upper respiratory tract illness. Lower respiratory tract complications are rare. There is no vaccine against influenza C virus.
I know influenza B and C viruses quite well – I did my Ph.D. research on them. I showed that the influenza C virus genome consists of 7 RNA segments, and demonstrated reassortment among different influenza C virus strains.
Hatta, M., & Kawaoka, Y. (2003). The NB Protein of Influenza B Virus Is Not Necessary for Virus Replication In Vitro Journal of Virology, 77 (10), 6050-6054 DOI: 10.1128/JVI.77.10.6050-6054.2003
Racaniello VR, & Palese P (1979). Isolation of influenza C virus recombinants. Journal of Virology, 32 (3), 1006-14 PMID: 513198
We’ve briefly considered the structure of influenza virions and how the viral RNAs can encode one or more proteins. Now we’ll consider how influenza viruses multiply.
Viruses are obligate intracellular parasites: they cannot reproduce outside of a cell. The production of new infectious particles must take place within a cell. Upon entering cells, viruses parasitize the host machinery to produce new viral progeny. The sum total of all the events that take place in a virus-infected cell is called the infectious cycle, or viral replication. Virologists artificially divide the infectious cycle into steps to make it easier to study. The steps include attachment and entry of the virion, translation of mRNA into protein, genome replication (producing more RNA or DNA), assembly of new particles, and release of particles from the cell. We’ll consider each of these steps, and then move on to a discussion of how influenza virus infects us and causes disease.
Today we’ll focus on the first step, attachment of the virion to cells. Here is a typical cell. I’m sure everyone is familiar with it, but it doesn’t hurt to review.
You can see that there is a substantial barrier to anything getting into this cell – the plasma membrane. Viruses have evolved different ways to get around this. But what they all have in common is that virions must first attach to a receptor on the plasma membrane in order to enter the cell. Every virus has a specific receptor that it attaches to, and in turn there is a particular viral protein that binds this cell receptor. Here is an illustration of an influenza virion binding to its cell receptor.
You can see the individual ‘spikes’ on the virion binding to a structure on the cell. The influenza viral spike that attaches to the cell receptor is the HA protein – hemagglutinin. The cell receptor is sialic acid – a small sugar that is attached to many different proteins on the cell surface. Here’s what sialic acid looks like.
On the left is a drawing of a cell protein embedded in the plasma membrane. The interior of the cell – cytoplasm – is at the bottom. Part of the protein crosses the membrane, and there are also parts on the cytoplasmic and extracellular sides. The spheres are sugars that are attached to many proteins (protein + sugar = glycoprotein). Sialic acid is always the last sugar in a chain that is attached to a protein. On the right is the chemical structure of sialic acid; the next sugar, to the right, is galactose. Influenza virions attach to cells when the HA grabs onto the very small sialic acid.
The sugar is actually quite tiny compared to the HA – it fits into a small pocket on the top of the spike. Here is a molecular model showing the HA bound to an analog of sialic acid. The globular top of the HA is at the top of the image. The tiny red and white spheres show where sialic acid would be bound, in a pocket at the top of the HA.
So far we have docked the influenza virion onto the surface of the cell. It’s sitting there quite firmly, but it’s still on the outside of the cell. How does it get in – or more accurately, how do the viral RNAs get into the cell? Stay tuned.
Within the influenza A virion are eight segments of viral RNA. These molecules carry the all the information needed to make new influenza virus particles. These eight RNAs are shown schematically as olive green lines at the top of the illustration. RNAs are chains of four different nucleotides, A, C, G, U. In the case of influenza virus, the eight RNAs are a total of about 14,000 nucleotides in length. The nucleotides make up the genetic code – it is read by the cell’s translation machinery in groups of three, with each triplet specifying an amino acid.
There are two important aspects of these viral RNA that we must consider. First, you can see that the ends of the RNAs are labeled 3′ and 5′. Nucleic acids have polarity, in that one end of the chain is chemically different from the other. Such polarity is represented by 5′ or 3′. The second point is that when a nucleic acid is copied, or duplicated, by enzymes called polymerases, a strand of the complementary polarity is produced. Influenza viral RNAs are called (-), or negative strand RNAs, because they are the opposite polarity of the RNA that is translated to make protein. The RNA molecules that are templates for the synthesis of proteins are defined as having having (+), or positive polarity. Upon entering the cell, the (-) strand influenza viral RNAs must be copied into complementary (+) strands, so that they can serve as templates for proteins. The viral RNAs are copied by an enzyme – called RNA polymerase – that is carried into the cell with the virus.
In the above scheme, the olive green lines are the (-) strand RNAs found in the influenza virion. Once the virion enters the cell, these 8 RNAs are copied into (+) strand mRNAs. Finally, the mRNAs can serve as templates for the synthesis of proteins. The specific viral proteins that are produced by each viral mRNA are shown at the bottom of the illustration. From this picture we see that, for example, RNA segment 4 codes for the viral HA protein, and RNA segment 6 codes for the viral NA protein. Note also that some RNA segments encode for more than one protein. Both influenza A and B viruses have 8 RNA segments, while the influenza C viruses have 7.
Influenza viruses are called (-) strand RNA viruses because of the polarity of the RNA that is carried in the virion. Other RNA viruses – such as poliovirus – are (+) strand RNA viruses, because their genomic RNA can be translated into protein immediately upon entering the cell.
Any questions before we proceed?