Symmetry Assembled Supramolecular Protein Cages: Investigating a Strategy for Constructing New Biomaterials.

Abstract

In nature, the assembly of individual protein subunits into larger quaternary structures allows new biological properties to emerge as consequence of the higher order structure. The biological function and physical properties exhibited by these higher order structures differ widely from those of the individual subunits of which they are composed. Inspired by nature, my research objective was to develop and test new strategies for creating novel biomaterials with useful properties by the directed assembly of natural proteins into larger artificial protein structures. In particular, my research has explored the assembly of proteins into “cages”, which have potential applications in drug delivery or gene therapy by encapsulating therapeutic agents within the “cage”. The strategy for making such higher order protein structures in a specific and directed manner is to use protein building blocks (PBBs) with well defined symmetrical quaternary xi structures that can coordinate the assembly, and to utilize novel non-covalent cross-linking agents that bring the PBBs together and link them to form the “cage” structures. To demonstrate proof of concept, the protein KDPG aldolase from T. maritima was utilized as a PBB, which consists of three identical subunits that form a trimeric quaternary structure that closely resembles a triangle. Geometry dictates that triangles can form various highly symmetrical 3-D structures; for example tetrahedrons, octahedrons, and/or icosahedrons depending on the number of units in the assembly. As linkers, we exploited peptides designed de novo to form anti-parallel heterodimeric coiled coils. This method of cross-linking is useful because the heterodimeric nature allows only subunits with complementary peptides to be joined, providing control over assembly and maintaining adequate spatial arrangement of the PBBs. The results from our investigations show that fusion proteins of KDPG aldolase and peptide linkers assemble into cage structures, cage structures predicted to form based on the symmetry of the KDPG aldolase PBB, upon mixing of fusion proteins with complementary peptide linkers.