The principal objective is to determine the structural basis for the various functions of the protein core of togaviruses. Togaviruses consist of a lipid-enveloped core that contains a positive-stranded RNA genome. Eighty glycoprotein trimeric spikes penetrate the lipid and are anchored in the central capsid. The external spikes recognize cellular receptors and are the major targets for the host's neutralizing antibodies. Two major genera of togaviruses will be examined, namely alpha- and rubiviruses. Alphaviruses are arthropod-borne and can cause encephalitis, myositis and fever in humans. Rubella virus is the only member of the rubivirus genus, being primarily a mild childhood disease but able to cause severe fetal abnormalities on infecting pregnant women. The structure of Sindbis virus core protein (SCP), extracted from infectious virus, has been determined in our laboratory. Now Richard Kuhn has been able to produce two different recombinant forms of modified SCP which have also been crystallized. This makes it possible to examine mutated forms of the enzyme and to diffuse various ligands into crystals which are no longer clogged by the disordered basic amino-terminal domain. We plan to study the role of SCP in the assembly of mature virions (good looking crystals of whole cores are available), of its interaction with the glycoprotein spikes, of its interaction with its genomic RNA and also its binding to ribosomal RNA. We are also planning to look at mutations which alter the proteolytic activity of SCP utilized in its autocatalytic cleavage from the virally coded polyprotein. Crystals have also been grown of Semliki Forest virus core protein extracted from infectious virus. Richard Kuhn will be using the same techniques as he has developed for SCP to express the core proteins of Semliki Forest and Ross River virus. Semliki Forest and Ross River viruses are closely related alphaviruses. Mutational analysis and comparison of the structure of the core protein of these viruses with that of SCP will clarify the functionally important residues and structural properties. Rubella virus core protein is substantially different and does not act as a protease that modifies the translated polyprotein. Yet many of the structural properties of rubella virus are similar. It is hoped that it will be possible to determine the structure of rubella core protein expressed in E. coli in a manner now pioneered by Richard Kuhn for SCP.
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