Unidirectional Living Growth of Self-Assembled Protein Nanofibrils Revealed by Super Resolution Microscopy.

Protein-based nanofibrils are emerging as a promising class of materials providing unique propeties for applications in, e.g., biomedical and food engineering. Here, we use AFM and STORM imaging to elucidate the growth dynamics, exchange kinetics, and polymerization mechanism for fibrils composed of a de novo designed recombinant triblock protein polymer. This macromolecule features a silk-inspired self-assembling central block composed of GAGAGAGH repeats, which are known to fold into a beta roll with turns at each histidine, and once folded to stack forming a long, ribbon-like structure. We find several properties that allow the growth of patterned protein nanofibrils: the self-assembly takes place on only one side of the growing fibrils by the essentially irreversible addition of protein polymer subunits, and these fibril ends remain reactive indefinitely in the absence of monomer ("living ends"). Exploiting these characteristics, we can grow stable diblock protein nanofibrils by the sequential addition of differently labelled proteins. We establish control over the block length ratio by simply varying monomer feed conditions. Our results demonstrate the use of engineered protein polymers in creating precisely patterned protein nanofibrils, and open perspectives for the hierarchical self-assembly of functional biomaterials.