bIcosahedral viral capsids are obligated to perform a thermodynamic balancing act. Capsids must be stable enough to protect the genome until a suitable host cell is encountered yet be poised to bind receptor, initiate cell entry, navigate the cellular milieu, and release their genome in the appropriate replication compartment. In this study, serotypes of adeno-associated virus (AAV), AAV1, AAV2, AAV5, and AAV8, were compared with respect to the physical properties of their capsids that influence thermodynamic stability. Thermal stability measurements using differential scanning fluorimetry, differential scanning calorimetry, and electron microscopy showed that capsid melting temperatures differed by more than 20°C between the least and most stable serotypes, AAV2 and AAV5, respectively. Limited proteolysis and peptide mass mapping of intact particles were used to investigate capsid protein dynamics. Active hot spots mapped to the region surrounding the 3-fold axis of symmetry for all serotypes. Cleavages also mapped to the unique region of VP1 which contains a phospholipase domain, indicating transient exposure on the surface of the capsid. Data on the biophysical properties of the different AAV serotypes are important for understanding cellular trafficking and is critical to their production, storage, and use for gene therapy. The distinct differences reported here provide direction for future studies on entry and vector production.

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Differential Scanning Fluorimetry (DSF) is a rapid, economical, and a straightforward technique for estimating the thermal stability of proteins. The principle involves the binding of a fluorescent dye to thermally exposed hydrophobic pockets of a protein. The dyes used in this technique are highly fluorescent in a non-polar environment and are quenched when exposed to aqueous solution. The change in fluorescence can be used to follow unfolding of proteins induced by temperature, pH, or chaotropic agents. The method is well characterized for monomeric proteins. Here, we extend the application to supramolecular protein and nucleo-protein complexes using virus particles as an example. SYPRO-orange™ dye is the dye of choice because it is matched for use with q-PCR instruments and the fluorescence response is stable across a wide range of pH and temperatures. Advantages of this technique over standard biophysical methods include the ability for high-throughput screening of biological and technical replicates and the high sensitivity.