Engineering the orange carotenoid protein for applications in synthetic biology.

The Orange Carotenoid Protein (OCP) is a water-soluble protein that enables photoprotective energy quenching in cyanobacteria. Structurally, the OCP is modular, consisting of an all-helical N-terminal domain (NTD), and a mixed α-β C-terminal domain (CTD). A non-covalently associated carotenoid spans the two domains. Upon photoactivation, the carotenoid translocates into the NTD, causing a visible color change from orange to red, to activate the OCP as a quencher of excess captured energy. The recent identification of new families of the OCP with diverse activation and quenching properties, and the discovery of carotenoid-binding proteins that are homologs to either the discrete NTD or CTD, provide a tremendous repertoire of modules that can be used as building blocks for the design of custom photoswitchable proteins. Here based on recent foundational results, we describe potential ways that the OCP can be engineered to serve a wide range of applications in synthetic biology.