Plants have developed elaborate light-sensing pathways to be able to adequately react on the quality and quantity of light. These signal transduction pathways regulate among others growth directionality (phototropism), flowering, stomatal opening, and chloroplast movement. Light-signalling pathways in plants converge on the ubiquitin-protein ligase COP1 that regulates stability of many transcription factors involved in light-dependent responses. This puts regulated proteolysis by the ubiquitin proteasome system at the heart of light-depending signaling. Recently, Renicke et al., reported in Chemistry & Biology on the engineering of a synthetic tool that concentrates light-regulated proteolysis into a single construct. The module developed by the authors comprises the light-sensing LOV2 domain of Arabidopsis thaliana phot1 and the ornithine decarboxylase (ODC) like degradation sequence (degron) cODC1. Activity of this so-called photo-sensitive degron (psd) module depends on the presence of blue light: the conformational changes within the LOV2 domain expose the degron, which induces proteasomal degradation. In contrast to the majority of proteasomal substrates, the cODC1 degron is directly recognized by the proteasome, without polyubiquitylation by an ubiquitin-protein ligase. Thus, the psd module is a minimized plant light-signalling pathway that comprises a light-responsive domain fused with a degradation-inducing sequence. The module can be fused to target proteins that reside with their C-terminus in the nucleus or cytosol to regulate their stability by blue light.
Functionality of the design has been demonstrated in Saccharomyces cerevisiae through light-regulation of tester substrate stability, cell cycle regulators, generation of conditional mutants, light-based patterning of yeast growth and yeast photography. The experimental characterization of the degron was complemented by in silico modelling of psd-module behavior, which reproduced its basic features and indicated a strategy for further improvements of the psd-module. In summary, the psd-module implements the fundamentals of light-regulated protein degradation in non-plant organisms. The conservation of the chosen degron sequence among eukaryotes implies successful application in other organisms and versatile usage in many applications.