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Viral self-assembly is of tremendous virological and biomedical importance. Although theoretical and crystallographic considerations suggest that controlled conformational change is a fundamental regulatory mechanism in viral assembly, direct proof that switching alters the thermodynamic attraction of self-assembling components has not been provided. Using the VP1 protein of polyomavirus, we report a new method to quantitatively measure molecular interactions under conditions of rapid protein self-assembly. We show, for the first time, that triggering virus capsid assembly through biologically relevant changes in Ca2concentration, or pH, is associated with a dramatic increase in the strength of protein molecular attraction as quantified by the second virial coefficient (B22). B22decreases from −2.3 × 10−4mol ml g−2(weak protein–protein attraction) to −2.4 × 10−3mol ml g−2(strong protein attraction) for metastable and Ca2-triggered self-assembling capsomeres, respectively. An assembly-deficient mutant (VP1CΔ63) is conversely characterized by weak protein–protein repulsion independently of chemical change sufficient to cause VP1 assembly. Concomitant switching of both VP1 assembly and thermodynamic attraction was also achieved by in vitrochanges in ammonium sulphate concentration, consistent with protein salting-out behaviour. The methods and findings reported here provide new insight into viral assembly, potentially facilitating the development of new antivirals and vaccines, and will open the way to a more fundamental physico-chemical description of complex protein self-assembly systems.

The article describes a disclosure about the addition of scan function to existing printers. The development of scan cartridge was made possible by the commercial availability of high resolution miniature complementary metal oxide semiconductors. The addition of scan function involves tricking the printer that what is being used is a print cartridge rather than a scan cartridge. The article also describes the role of the software on the host computer.