Cross-Linking and Bundling of Self-Assembled Protein-Based Polymer Fibrils via Heterodimeric Coiled Coils.

Previously, we developed triblock protein polymers that form fibrillar hydrogels at low protein polymer concentrations (denoted C-S-C). We here demonstrate that the structure of these hydrogels can be tuned via heterodimeric coiled coils that cross-link and bundle the self-assembled protein polymer fibrils. We fused well-characterized, 47 amino acids-long heterodimeric coiled coil "linkers" (D or D) to the C-terminus of the triblock polymer. The resulting C-S-C-D and C-S-C-D polymers, were successfully produced as secreted proteins in Pichia pastoris, with titers of purified protein in the order of g L of clarified broth. Atomic force microscopy showed that fibrils formed by either C-S-C-D or C-S-C-D alone already displayed extensive bundling, apparently as a result of homotypic (D/D and D/D) interactions. For fibrils prepared from protein polymers having no linkers, plus a small fraction of polymers containing either D or D linkers, no cross-linking and bundling was observed. At these same low concentrations of linkers, fibrils containing both the D and the D linkers did show cross-linking and bundling as a consequence of heterodimer formation. This work shows that we can control the extent of bundling and cross-linking of supramolecular fibrils by varying the density of heterodimerizing coiled coils in the fibrils, which is promising for the further development of materials that mimic the extracellular matrix.