BOSTON - Researchers recently produced the first 3-D structures of the poliovirus in the moments after it attaches and enters a host cell by using cryoelectron microscopy and X-ray crystallography.
From the moment it attaches to its host, the poliovirus appears to make tiny adjustments in its protein shell that allow it to tightly grab its host receptor. Once bound, the virus creates temporary openings in its shell through which it throws out protein threads that embed in the host cell membrane, anchoring the virus to the cell and possibly creating pores for the viral ribonucleic acid (RNA) to enter, according to James Hogle, PhD, professor of biological chemistry and molecular pharmacology at Harvard Medical School and professor of biophysics at Harvard University.
Hogle's lab perfected a method for producing an intermediate form, 135S or A particle, that corresponds to the receptor induced structure by heating the virus. Using a combination of X-ray crystallography and electron microscopy, Hogle's group, in collaboration with Alasdair Steven, PhD, and his lab at the National Institutes of Health (NIH), Bethesda, MD, determined the structures of the 135S particle and a second intermediate form, the 80S particle, that lacks the viral RNA.
Images provide evidence that the 135S particle shell undergoes a series of tectonic-like movements during the conversion from the native virus to the 135S particle, which may open up gaps in the surface of the particle through which the 135S particle throws out coils of protein. The coils embed in the host cell membrane, tethering the 135S particle to the host. Once embedded, the protein coils may change orientation to create pores through which the viral contents may be emptied. The 80S appears to be what's left, said Hogle.
In a similar study, the Hogle and Steven laboratories, in collaboration with Vincent Racaniello, PhD, and his lab at Columbia University, New York City, determined the structure of the complex of poliovirus and its receptor.
The real advantage to working with poliovirus is the extraordinary wealth of biochemical, genetic and structural data available, said Hogle. "Polio was a major pediatric health problem at the beginning of the century, accelerating through the `30s, `40s and `50s, when it was arguably the AIDS of its time." As a result it became a major focus for virological studies.
Researchers believe the cell entry for poliovirus may be similar for other viruses including enteroviruses, echoviruses and Coxsackieviruses that are emerging as causes for childhood diseases, particularly in day care settings.
"The basic problem is the individuals who are most likely to be immunologically naive to enteroviruses are the young children who have never seen them before, especially those in early day care settings," said Hogle. "Their hands are in their mouths all the time. So, viruses passed by oral-oral and fecal-oral routes are passed quite readily. The babies bring the virus home and then mom, dad and siblings get sick as well."
According to Hogle, poliovirus is an archetype for a full family of viruses and how they enter cells. By understanding the fundamental process of how viruses work, researchers have another target for intervention. The payoff is new antiviral therapies. Hogle said pleconaril (ViroPharma), a new drug which binds the virus and blocks the receptor mediated structural changes, is nearing the end of clinical trials. The hydrophobic drug may treat viral meningitis, enterovirus respiratory disease, hand-foot-mouth disease and rhinovirus infection in asthmatics.
Structural modeling at low resolution requires a heavy dose of interpretation. The report of the virus-receptor complex from the Harvard-NIH-Columbia groups was published back-to-back with an independent study of the virus-receptor complex by researchers at Purdue University and State University of New York at Stoney Brook (Proc Natl Acad Sci USA 2000;97:79-84 and 73-78). On its face, both structures depict a mottled sphere, the virus, studded with 60 twiglike structures, and the receptors, each of which is buried deeply into a canyon in the viral surface. But the two groups differ in the way they orient the receptor at the point where it burrows into the virus (domain 1 in illustration).
The discrepancy hints at an even deeper difference in interpretation. The Purdue researchers believe the receptor binding sites are buried deeply in the canyons to make them inaccessible to antibodies.
Hogle and colleagues believe the receptor binds deep in the canyon in order to provide additional binding energy that can be used to induce the virus to undergo its conformational change into the 135S form. "Binding deep in a pocket or invagination is an efficient way to increase surface area and therefore energy of binding, allowing the system to use some of the energy of binding to induce structural changes," said Hogle.
For more information:
- Belnap D, Filman D, Trus B, et. al. Molecular tectonic model of virus structural transitions: The putative cell entry states of poliovirus. J Virol 2000;74(3):1342-1354.
- Belnap D, McDermott B, Filman D, et. al. Three-dimensional structure of poliovirus receptor bound to poliovirus. Proc Natl Acad Sci USA 2000;97(1):73-78.
You can express your views on this article, or other relevant themes, in the Infectious Diseases in Children Specialty Forums.