A personal account of virus structure determination at the Indian Institute of Science, Bangalore

Abstract

Virus particles are excellent models for understanding the specificity of protein—protein and protein—nucleic acid interactions and mechanisms of biological assembly. At the Indian Institute of Science, we have carried out detailed investigations on the structure, stability and assembly of two isometric plant viruses, Sesbania mosaic virus (SeMV) and Physalis mottle virus (PhMV). The protein coat of SeMV consists of 180 protein subunits of molecular mass 29 kDa. It encapsidates a ss—RNA genome of 4149 bases. The genome of PhMV is a ss—RNA of size 6.67 kb encapsidated in an icosahedral shell of 180 identical coat protein (CP) subunits of molecular mass 20 kDa. The spatial arrangements of protein subunits in both the virus particles confirms to a T = 3 icosahedral lattice. The three dimensional X-ray structures of these viruses and a large number of their recombinant capsids have been determined. It was necessary to develop new algorithms and write a large number of programs for the determination of these structures. The stability of SeMV and PhMV particles are based on very different interactions. The capsid stability of SeMV depends on protein—protein, protein —nucleic acid and calcium mediated protein—protein interactions. In contrast, PhMV particles are stabilized predominantly by hydrophobic interactions between coat protein subunits. An analysis of the structural, biochemical and biophysical properties of the native SeMV, its recombinant capsid and several of its mutants has led to the understanding of the detailed pathway for the assembly of SeMV. Comparative structural analysis of the native and recombinant capsids of PhMV and studies on the assembly and stability properties of a large number of site specific and deletion mutants have suggested that the subunit folding and particle assembly are concerted events in this virus.