B 12 -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 B 12 -dependent photoreceptor, CarH C . The resulting segments reassemble efficiently upon addition of cobalamin (AdoB 12 , MeB 12 , or CNB 12 ). Photolysis of the cofactors such as AdoB 12 and MeB 12 further leads to stable protein adducts harboring a bis-His-ligated B 12 . 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 B 12 . 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.