Geometric Defects and Icosahedral Viruses

We propose that viruses with geometric defects are not necessarily flawed viruses. A geometric defect may be a reactive site. Defects may facilitate assembly, dissociation, or accessibility of cellular proteins to virion components. In single molecule studies of hepadnavirus assembly, defects and overgrowth are common features. Icosahedral alphaviruses and flaviviruses, among others, have capsids with geometric defects. Similarly, immature retroviruses, which are non-icosahedral, have numerous "errors". In many viruses, asymmetric exposure of interior features allows for regulated genome release or supports intracellular trafficking. In these viruses, the defects likely serve a biological function. Commonly used approaches for spherical virus structure determination use symmetry averaging, which obscures defects. We suggest that there are three classes of asymmetry: regular asymmetry as might be found in a tailed phage, irregular asymmetry as found, for example, in defects randomly trapped during assembly, and dynamic asymmetry due to Brownian dynamics of virus capsids. Awareness of their presence and recent advances in electron microscopy will allow unprecedented investigation of capsid irregularities to investigate their biological relevance.