Cell-free co-synthesis of protein nanoassemblies: tubes, rings, and doughnuts

We used a cell-free transcription/translation system to synthesize structural proteins of the T4 bacteriophage. We focused on two proteins that participate in the formation of the virus tail tube assembly. Synthesized separately, the proteins assembled into their in vivo forms, namely one polymerized into rigid hollow nanotubes approximately 20 nm thick and hundreds of nanometers long, the other assembled into 10 nm tube-capping hexameric rings. Co-synthesis of the two proteins, however, revealed a novel structure of a nanodoughnut with an outer diameter of approximately 50 nm and thickness of approximately 20 nm. Cell-free co-synthesis and assembly of T4 structural proteins can be extended in a combinatorial fashion. The addition of other structural genes offers control of native nanoassemblies and may reveal ones not observable by mixing purified components.