Integration of Calcium Responsiveness into Self-Healing Protein Hydrogels through Consensus Repeat Sequence Engineering

Calcium ions trigger numerous biological phenomena including bone growth, muscle contraction, and neurotransmitter release. Protein-based hydrogels that respond to calcium ions have shown promise as naturally derived therapeutic materials, including dynamic cell-culture scaffolds and injectable tissue adhesives. Recently, highly repetitive protein motifs were shown to fold and unfold in the presence and absence of calcium ions. These motifs comprise a nine-residue consensus sequence GGXGXDXUX, where G is glycine, D is aspartic acid, X is any amino acid, and U is an aliphatic amino acid. The sequence GGAGNDTLY represents a variation of the consensus repeat and is named a consensus β-roll tag (BRT) for its ability to form β rolls in the presence of calcium ions.

Here, we generate fusion proteins comprising calcium-responsive BRTs and crosslinking domains that promote hydrogel formation. A BRT mutation panel modifies the asparagine (N) residue in position 5; this residue is hypothesized to influence BRT responsiveness due to its proximity to the calcium-binding aspartic acid in position 6. The mutation panel explores the role of charge and hydrophobicity on calcium ion-actuated structural changes. Crosslinking domains consist of dynamic, associative motifs that give rise to self-healing and shear-thinning hydrogel properties. We investigate sequence–property relationships using circular dichroism and shear rheology, which enable the quantification of calcium-responsive stiffness. Overall, protein hydrogels containing calcium-responsive repeat motifs provide a tunable, modular, and naturally derived material platform with promise to mimic the dynamic chemo-mechanical environment of muscle and nerve tissues.