B₁₂-induced reassembly of split photoreceptor protein enables photoresponsive hydrogels with tunable mechanics

Although the tools based on split proteins have found broad applications, ranging from controlled biological signaling to advanced molecular architectures, many of them suffer from drawbacks such as background reassembly, low thermodynamic stability, and static structural features. Here, we present a chemically inducible protein assembly method enabled by the dissection of the carboxyl-terminal domain of a B₁₂-dependent photoreceptor, CarH_C. The resulting segments reassemble efficiently upon addition of cobalamin (AdoB₁₂, MeB₁₂, or CNB₁₂). Photolysis of the cofactors such as AdoB₁₂ and MeB₁₂ further leads to stable protein adducts harboring a bis-His-ligated B₁₂. Split CarH_C enables the creation of a series of protein hydrogels, of which the mechanics can be either photostrengthened or photoweakened, depending on the type of B₁₂. These materials are also well suited for three dimensional cell culturing. Together, this new protein chemistry, featuring negligible background autoassembly, stable conjugation, and phototunability, has opened up opportunities for designing smart materials.