Reversible Molecular Motional Switch Based on Circular Photoactive Protein Oligomers: Unexpected Photo-Induced Contraction

Molecular switches alterable between two stable states by environmental stimuli such as light and temperature offer the potential for controlling biological functions. Here, we report a photoswitchable protein complex made of multiple protein molecules that can rapidly and reversibly switch with significant conformational changes. The structural and photochromic properties of photoactive yellow protein (PYP) are harnessed to construct circular oligomer PYPs (coPYPs) of desired sizes. Considering the light-induced N-terminal protrusion of monomer PYP, we expected coPYPs would expand upon irradiation, but time-resolved x-ray scattering data reveal that the late intermediate has a light-induced contraction motion. This work not only provides an approach to engineering a novel protein-based molecular switch based on circular oligomers of a well-known protein unit but also demonstrates the importance of actually characterizing the structural dynamics of designed molecular switches.