Solution-phase dynamics of the Hepatitis B virus capsid : kinetics-based assays for study of supramolecular complexes
Hilmer, Jonathan Kyle
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Viruses are the most abundant form of life on the planet. Many forms are pathogenic and represent a major threat to human health, but viruses can also be useful nanoscale tools: as adjuvants, gene therapy agents, antimicrobials, or functionalized nanoscale building blocks. Viruses have historically been viewed as static and rigid delivery vehicles, but over the last few decades they have been recognized as flexible structures. Their structural dynamics are a crucial element of their functionality, and they represent a substantial target for antiviral strategies. To overcome the inherent problems of characterizing the biophysics of supramolecular complexes, we have developed a set of kinetic assays to probe capsid motion at several different amplitudes. The first assay, kinetic hydrolysis, works via the differential cleavage of folded versus unfolded proteins, and reports on large-scale conformational changes. The second assay, hydrogen-deuterium exchange, is a probe of small-amplitude dynamics. Both of these assays were used to study the solution-phase dynamics of the hepatitis B virus (HBV) capsid under the influence of assembly effectors and temperature. The results of these assays indicate that the HBV capsid adopts multiple conformations in response to the external environment. The dimeric subunit becomes primed for assembly via an entropically-driven process, but once assembled the capsid has reduced dependence on hydrophobic contacts. Depending on the assembly state, the subunit protein has varying response to assembly effectors, with changes to both small-amplitude and large-amplitude motions. The sum of the assay results indicate that the HBV capsid protein is capable of rotational translocations of the alpha-helices, while maintaining most of the secondary structure. Concerted structural shifts are implied, consistent with an allosteric model, which helps to explain previously observed allostery of capsid assembly and response to drugs.