A General, Symmetry-Based Approach for the Assembly of Proteins into Nanoscale Polyhedra.

Abstract

The assembly of individual protein subunits into large-scale symmetrical structures is widespread in Nature and confers unique biological properties which have potential applications in nano-technology and medicine. While efforts to functionalize and repurpose existing protein complexes have been mainly successful, designing well-defined de novo protein complexes remains an unsolved problem. A major challenge in engineering de novo symmetrical assemblies has been to design interactions between the protein subunits so that they specifically assemble into the desired structure. Prior de novo protein cages have been developed with moderate success, but suffer from a lack of generalizability and require significant computational effort and screening of mutant fusion proteins. The design and optimization of a simple, generalizable approach to designing novel fusion proteins which assemble into cage-like structures will be the subject of this dissertation. We show that by genetically fusing a C4-symmetric coiled-coil to the C-terminus of a C3-symmetric trimeric protein via a short, flexible linker, we can assemble a well-defined 24-subunit protein cage with octahedral symmetry. The flexible nature of these assemblies alleviates the need for rigorous interface modeling, requiring only minimal computation to determine the length of the linker sequence. This is the first de novo designed symmetrical protein complex to incorporate a C4 symmetry element, and we anticipate this method can be applied to a wider variety of proteins and symmetries, which may open up a new avenue of research into designer protein cages with unique, built-in functionalities.