Interaction of a photochromic UV sensor protein Rc-PYP with PYP-binding protein

Photoactive yellow protein (PYP) from Halorhodospira halophila is one of typical light sensor proteins. Although its photoreaction has been extensively studied, no downstream partner protein has been identified to date. In this study, the intermolecular interaction dynamics observed between PYP from Rhodobacter capsulatus (Rc-PYP) and a possible downstream protein, PYP-binding protein (PBP), were studied. It was found that UV light-induced a long-lived product (pUV*), which interacts with PBP to form a stable hetero-hexamer (Complex-II). The reaction scheme for this interaction was revealed using transient absorption and transient grating methods. Time-resolved diffusion detection showed that a hetero-trimer (Complex-I) is formed transiently, which produced Complex-II via a second-order reaction. Any other intermediates, including those from pBL do not interact with PBP. The reaction scheme and kinetics are determined. Interestingly, long-lived Complex-II dissociates upon excitation with blue light. These results demonstrate that Rc-PYP is a photochromic and new type of UV sensor, of which signaling process is similar to that of other light sensor proteins in the visible light region. The photochromic heterogeneous intermolecular interactions formed between PYP and PBP can be used as a novel and useful tool in optogenetics. ![Figure][1] ### Competing Interest Statement The authors have declared no competing interest. [1]: pending:yes