We talk so much about the flu virus we thought we’d show you some nice pics that CDC has just put up. This is a review for many of you put reviews are always helpful. In these three pics, only one is the actual swine flu virus, the other two being “cartoon” depictions of a generic influenza virus. The cartoons are quite nice and helpful to see what you are looking at in the electron micrograph of influenza virions (virus particles), probably grown in tissue culture. I say “probably” because there is no other information on the site other than the micrograph was taken in the CDC Influenza Laboratory, but when the virus grows in your lungs it usually isn’t nice and spherical like this but assumes many shapes, often elongated and strand-like. Looking at the photomicrograph, though, you see the essentials. Here it is:
Large version here
There are six virus particles, spherical in shape, with a denser shell surrounding an interior space and a rough, brushy looking surface. Those brushy things are the surface proteins hemagglutinin (HA) and neuraminidase (NA). There are 16 broad kinds of HA (each with myriad lesser variations) and 9 kinds of NA (also each having lots of minor variations). That gives a total of 144 different broad combinations (16 times 9). The different kinds of HA and NA are unimaginatively named: just numbered H1 to H16 and N1 to N9. Any particular viral particle has only one of the 16 possible kinds of HA (called a subtype) and only one of the 9 possible kinds of NA and the particular combination gives the familiar subtype naming system, e.g., H1N1 or H3N2. Only a few of the combinations are found on flu viruses that infect humans. Most of the other subtype combinations are found in aquatic birds, the main reservoir for influenza viruses, but we know a number of other animals can be infected with one or another influenza virus, including pigs, cats, dogs and horses.
The two surface proteins (HA and NA) are usually portrayed as spikes sticking through a lipid bilayer membrane derived from the host cell that the virus dragged with it when its progeny budded off after infecting it. If you want to know more about lipid bilayers and the glycoproteins like HA and NA that stick through them we explained and illustrated a lot of it in a series of posts on the old site which we reprised in posts which start here. There are links to the others in the first one. But let’s just stay with the pics for now and you can go back and fill in the details later.
The first cartoon shows the viral particle from the outside, the HA (hemagglutinin) spikes in blue and the NA (neuriminidase) spikes in red (I had to reduce the size to fit on the blog page, but if you click on “larger version” you’ll see each more clearly):
Large version here
The spikes are sticking through the stolen host cell lipid membrane (shown as a brown base, now called an envelope; viruses that do this are called enveloped viruses, so influenza virus is an enveloped virus). The spikes are attached under it to a hard shell protein, designated M1. M1 gives the virus a more rigid structure. Embedded in and studding M1 and also sticking through the membrane is another protein called M2 or the ion channel protein. You can’t see it in the photomicrograph but it is shown in the two cartoon pics. There are more HA than NA on the surface and not much M2 visible. You’ll see two M2s looking like little buttons, one above and one below the equator towards the left. The brownish stuff is lipid bilayer and you can’t see the M1 protein on the first cartoon. The virus has genes that encode for HA, NA, M1 and M2 but not for the lipid bilayer which is scavenged from the host cell.
The HA and NA are involved with attaching to and entering the cell and again when the progeny leave it (more details here and here). The M2 ion channel protein is nicely explained by Vincent Rancaniello over at the Virology Blog, so head on over there for the details (it is involved in the antiviral adamantane class of drugs). You can get great explanations of many of the same things I’m talking about here (and much more) on the Virology Blog, written by a real virologist (not an epidemiologist). But we figure it’s always helpful to have more than one explanation, so that’s our excuse. That and the fact that we find it interesting and like to write about it.
So where are the viral genes whose sequences everyone talks about? They are in the interior of the shell. The second cartoon shows a cutaway view of the virus.
Large version here
Inside you see the eight genetic segments, labeled RNP. RNP stands for ribonucleoprotein, although the picture includes not only the protein but the RNA. So the squiggles are the virus’s RNA (its genetic blueprint) packaged structurally with a viral protein. Not shown is the polymerase complex, three proteins that work together to replicate the long strands of viral RNA. These three RNA-dependent replication machinery proteins are called PA, PB1 and PB2. They are also in the interior of the virus. More here.
Clearly there is a great deal more to the virus than this description of its physical landmarks. But the pics are nifty and it’s usually nice to put a face with a name, as they say. Reminder: The Virology Blog is a great place for details and recent news about the molecular biology. Highly recommended.