Optically Controlled Construction of Three-Dimensional Protein Arrays

Protein crystallization is an important tool for structural biology and nanostructure preparation. Here, we report on kinetic pathway-dependent protein crystals that are controlled by light. Photo-responsive crystallites are obtained by complexing the model proteins with cationic azobenzene dyes. The crystalline state is readily switched to a dispersed phase under ultra-violet light and restored by subsequent visible light illumination. The switching can be reversibly repeated for multiple cycles without noticeable structure deterioration. Importantly, the photo treatment not only significantly increases the crystallinity, but creates crystallites at conditions where no ordered lattices are observed upon directly mixing the components. Further control over the azobenzene isomerization kinetics produces protein single crystals of up to ~50 μm. This approach offers an intriguing method to fabricate metamaterials and study optically controlled crystallization.

Polyelectrolyte Encapsulation and Confinement within Protein Cage-Inspired Nanocompartments

Protein cages are nanocompartments with a well-defined structure and monodisperse size. They are composed of several individual subunits and can be categorized as viral and non-viral protein cages. Native viral cages often exhibit a cationic interior, which binds the anionic nucleic acid genome through electrostatic interactions leading to efficient encapsulation. Non-viral cages can carry various cargo, ranging from small molecules to inorganic nanoparticles. Both cage types can be functionalized at targeted locations through genetic engineering or chemical modification to entrap materials through interactions that are inaccessible to wild-type cages. Moreover, the limited number of constitutional subunits ease the modification efforts, because a single modification on the subunit can lead to multiple functional sites on the cage surface. Increasing efforts have also been dedicated to the assembly of protein cage-mimicking structures or templated protein coatings. This review focuses on native and modified protein cages that have been used to encapsulate and package polyelectrolyte cargos and on the electrostatic interactions that are the driving force for the assembly of such structures. Selective encapsulation can protect the payload from the surroundings, shield the potential toxicity or even enhance the intended performance of the payload, which is appealing in drug or gene delivery and imaging.