Flaviviruses are small, icosahedral enveloped viruses which include some the most important and implacable pathogens known to man. The virion encapsidates its RNA in an internal icosahedral capsid which is surrounded by an envelope containing two transmembrane proteins, M and E. Flavivirus structure remains poorly understood. We have determined the structure of the whole immature virion and found the positions of the E dimer and of M, which inhibits membrane fusion in the complex. Expression of the E protein together with the M protein leads to production of membranous particles which lack both a genome and a capsid. As this work proceeds to higher resolution, we should be able to define the contacts of the E proteins in the virus and examine the changes which accompany maturation and low pH mediated fusion.
Retroviruses are intriguing structural systems.
A great deal of evidence shows that these particles can form with variable diameters and contain variable amounts of nucleic acid. The conventional view of the particle is that it is icosahedral so that variable triangulation numbers must be invoked to explain the range of sizes. Expression of HIV p55gag proteins in a baculovirus system produces particles which resemble the authentic immature HIV particle. Cryo-electron micrographs of these particles showed that they were not icosahedral but rather comprised a set of ordered domains bounded by faults. Averaging of the protein subunits in the facets showed that they were arranged as a dimer of trimers on a net with angle near to that expected for hexagonal symmetry.