Assembly of spherical viruses, which may involve hundreds of components, is not well understood on a biological or a physical basis. Recently, tremendous progress has been made with alphaviruses and retroviruses by efforts to capture intermediates in their respective assembly reactions. Using different strategies, investigators showed that when association was too weak to support assembly of monomers, assembly could be induced by dimerizing the capsid protein or by adsorbing capsid protein to a nucleic acid scaffold. In this review we will summarize those results and compare them to assembly of experimentally more tractable hepatitis B virus (HBV) and to mathematical models of assembly. These fundamental models show that in vitro dimerization of subunits is expected to favor capsid assembly, independent of the role of dimers in vivo. This analysis suggests that weak association energy may be a mechanism for in vivo regulation of assembly by, for example, dimerization factors and/or scaffold.
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