Investigating the influence of ionic concentrations and subunit interactions on the self-assembly of E2 protein

Understanding of self-assembly mechanism of viruslike protein cage is important in controlling release of molecular cargo for applications in drug delivery. E2 core protein is composed of 60 subunits which self-assemble into a hollow 25-nm porous protein cage. Due to its virus like dodecahedral structure without infectious capacity, we are interested in its potential application as nanocapsule in drug delivery. In our study, extrinsic and intrinsic factors that influence self-assembly were evaluated. Extrinsic factors, such as salts and denaturants, were introduced into E2 protein solution. The hydrodynamic diameter of the E2 core protein was used to monitor its disassembly or aggregation. We found that the protein size increased proportionally with the salt concentration while the size decreased as the denaturant concentration increased. To assess intrinsic factors that influence E2 self-assembly, we identified some key amino acids at interfaces of subunits and performed site-directed mutagenesis on them. Characterizations of each mutant for size and secondary structure contents were performed. We found that mutations at the inner surface have no apparent effects on both protein sizes and secondary structures. The mutations at intra-trimer interfaces changed the secondary structure contents but the protein sizes remained stable.