Designed Protein Cages: Theory and New Applications to Cryo-Electron Microscopy Scaffolding

By exploiting principles of symmetry that are shared by nearly all natural self-assembling structures, methods have been developed for engineering novel proteins that assemble to form a variety of complex, symmetric architectures. Recent successful designs include hollow protein cages composed of many identical subunits in cubic arrangements. Symmetric materials that extend by growth in two or three dimensions are also possible. These designed protein structures and materials offer new prospects for applications in fields ranging from synthetic biology to biomedical applications. In a new application area, we are exploiting designed protein cages for cryo-EM imaging. Recent breakthroughs in cryo-EM instrumentation and software have made it possible to image large protein assemblies at near-atomic resolution, but resolving the structures of proteins of ordinary size (typical of cellular proteins) has not been possible. Our new work shows how designed protein cages can overcome this critical barrier by serving as scaffolding to array other target proteins for imaging. Design principles and recent results will be discussed.