Abstract
Sensory photoreceptor proteins mediate diverse responses to ambient light in all domains of life. Often distinct modules coupled by helical linkers enable light perception (sensor) and biological output function (effector). Rewiring different sensor and effector modules into photoreceptor chimeras allows using light to control target cellular processes with high spatiotemporal accuracy and minimal invasiveness for therapeutic or analytical purposes. Thereby, a major design challenge is fusing the linkers from both modules in a way that preserves signal transduction within the chimera. The present study tackles this issue and explores strategies for engineering photoreceptor chimeras. An initial rational-design approach guided by sequence and structure homology of the parent proteins was greatly hampered by insufficient knowledge of signaling mechanisms within these modular proteins. A novel and easy-to-use brute-force strategy, termed PATCHY (primer-aided truncation for the creation of hybrid enzymes) circumvents this problem by generating a complete library of fusion variants between target modules harboring all combinations of the parent linkers. Screening fusion libraries of a LOV (light-oxygen-voltage) blue-light sensor coupled to a histidine-kinase effector yielded light-induced and light-repressed chimeras, each group complying with a heptad periodicity of linker lengths. With less than 5% of all possible variants exhibiting light regulation, a delicate fine-tuning of linker sequence and protein function became evident. Thus, systematic testing of fusion variants with PATCHY not only facilitates the development of photoreceptor chimeras for manipulating cellular processes. Complementary to rational design, it also reveals molecular cues determining module compatibility and signal transduction in modular signal receptors.
