Hydrophobic Engineering of a Bacterial Nanodimensional Capsule Protein

Abstract

Recently, numerous protein-based nanomedicine platforms have been intensively explored. These systems are usually composed of self-assembling proteins that form controllable systems with different shapes and types. A particular type of the self-assembling proteins is the cage protein bacterioferritin (Bfr), a hollow protein that belongs to the ferritin superfamily of iron storage proteins. Bfr is composed of 24 identical protomers with 12 heme cofactors intercalated between adjacent subunits. Bfr has an 8 nm interior diameter and a 12 nm exterior diameter amenable for controllable modification. Previous investigations in the Honek laboratory involved engineering the Bfr cage proteins towards controlling the encapsulation of various guest molecules within its cavity. Polyhistidine amino acid sequences (His6-tags) were added to each of the C-termini of the Bfr protein subunits, which point towards the interior cavity of the protein, and successfully utilized as selective affinity interaction sites to bind several cargos such as a gold nanoparticle (AuNP), and a fluorescently labeled tetrameric protein, streptavidin. Additionally, the intrinsic heme cofactors were modified in different ways and reintroduced to the Bfr cage protein providing another approach for engineering this complex system. In the current study, the previously established engineering methodologies were utilized to investigate the possibility of engineering the Bfr cavity to be more hydrophobic and to examine its capability for encapsulating different hydrophobic molecules. The complexation between two different types of fatty acid-based molecules and the His6-Bfr cage protein was successfully achieved and analyzed. The two proof of concept guests studied in our laboratory were: a synthesized nonadecanoyl-nitrilotriacetic acid micelle and a commercially available lipid that has an NTA functionality preloaded with nickel ion. Also, another approach for labeling the heme propionate groups with long alkyl chains was investigated. Two long alkyl chain primary amines were successfully added to the heme; however, the poor aqueous solubility of these hydrophobic analogues acts like a barrier towards their successful incorporation into the Bfr cage. The results gained from the above investigations highlight novel applications of not only Bfr, but also other cage proteins that have similar characteristics.