Multimode Observation of Virus Capsid Assembly

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Summary The capsid of the Hepatitis B Virus (HBV) is a 120-homodimer T=4 icosahedron. In vivo, it self-assembles, packages viral RNA, serves as a metabolic compartment for DNA synthesis, and trafficks within the cell. Its assembly and disassembly have become targets for potent antivirals. In the previous funding period, we characterized assembly and disassembly with purified protein using structural, single molecule, and bulk studies of assembly products and reactions. We characterized the allosteric transitions that activated assembly, demonstrated the importance of reversibility during self-association for fidelity, and identified roles of nucleation for directing the assembly path. In this proposal, we develop hypotheses to take advantage of these results to engineer virus-like particles with programmable assembly, cargo packaging, delivery, and release. In preliminary studies, we developed a method for targeting cargo to a capsid by linking the cargo to a small molecule that binds capsid with high affinity, essentially using an antiviral as a targeting device. This method can be applied to any cargo. In some cases, it is desirable to display cargo on the capsid exterior, in other cases it is desirable to package it within the capsid. In preliminary data, we demonstrate an approach to making “holey” capsids that expose the particle interior and can be re-sealed to enclose the contents. Using this same technology, we can make patches on the capsid surface; this can be used for displaying patches of receptor-binding ligands or cell-penetrating peptides. Cargo, packaged within a capsid, is not deliverable unless it can be released. In preliminary data, we developed techniques for triggering capsid disassembly in response to redox potential, taking advantage of chemically-induced metastability. This same approach can be applied to other triggering signals. The ultimate goal of these studies is to combine the approaches to a practical end: we propose to build two model biotech reagents, one to measure antibody levels and the other to deliver packaged cargo to specific cells. These approaches are each built on an understanding of the biochemistry and biophysics of HBV capsid assembly. HBV is one the smallest human pathogens. It is remarkably efficient at packaging its genome and delivering it to target cells. Based on our understanding of capsid assembly and capsid biophysics, we will develop approaches to specifically packaging cargo molecules and delivering these reagents intracellularly. The tools arising from this research will provide a means for man to take advantage of HBV.