A comparative analysis of the structural architecture of ssDNA viruses

Virus assembly, utilizing a limited number of viral coat protein (CP or VP) building blocks, is an excellent example of a directed macromolecular interaction occurring in nature. Two basic principles govern the assembly of spherical (icosahedral) viruses: (i) Genetic Economy - the encapsidated genome encodes a single or few CPs that assemble a protective shell (the viral capsid); and (ii) specificity - the CPs have to fold to recognize each other and form exact CP-CP interfacial interactions during the assembly pathway. Using a variety of biophysical techniques, including X-ray crystallography and cryo-electron microscopy, combined with homology model building, biochemistry and molecular biology, the nature of the interactions between protein-protein subunits and protein-nucleic acid that facilitate viral capsid assembly have been studied. This review discusses both the similarities and differences that have been elucidated for ssDNA Microviridae, Geminiviridae, and Parvoviridae virus families. The Microviridae represent a family of bacteriophages that utilize several CPs and scaffold proteins to assemble a T = 1 icosahedral capsid, the Geminiviridae plant viruses assemble a unique twinned quasi-isometric (geminate) pseudo T = 1 virion assembled from a single CP and the Parvoviridae represent animal viruses whose T = 1 capsids are formed from the common overlapping region of two to four VPs that have unique N-terminal extensions. A survey of the three-dimensional (3D) data available for these viruses shows that they utilize structural commonalities, facilitated by disparate CP/VP amino acid sequences for the successful assembly of mature infectious virions.