Curated Optogenetic Publication Database

Abstract: Many aspects of plant photoperception are mediated by the phytochrome (Phy) family of bilin-containing photoreceptors that reversibly interconvert between inactive Pr and active Pfr conformers1,2. Despite extensive biochemical studies, full understanding of plant Phy signalling has remained unclear due to the absence of relevant 3D models. Here we report a cryo-electron microscopy structure of Arabidopsis PhyB in the Pr state that reveals a topologically complex dimeric organization that is substantially distinct from its prokaryotic relatives. Instead of an anticipated parallel architecture, the C-terminal histidine-kinase-related domains (HKRDs) associate head-to-head, whereas the N-terminal photosensory regions associate head-to-tail to form a parallelogram-shaped platform with near two-fold symmetry. The platform is internally linked by the second of two internal Per/Arnt/Sim domains that binds to the photosensory module of the opposing protomer and a preceding 'modulator' loop that assembles tightly with the photosensory module of its own protomer. Both connections accelerate the thermal reversion of Pfr back to Pr, consistent with an inverse relationship between dimer assembly and Pfr stability. Lopsided contacts between the HKRDs and the platform create profound asymmetry to PhyB that might imbue distinct signalling potentials to the protomers. We propose that this unique structural dynamism creates an extensive photostate-sensitive surface for conformation-dependent interactions between plant Phy photoreceptors and their signalling partners.

Abstract: The superfamily of phytochrome (Phy) photoreceptors regulates a wide array of light responses in plants and microorganisms through their unique ability to reversibly switch between stable dark-adapted and photoactivated end states. Whereas the downstream signaling cascades and biological consequences have been described, the initial events that underpin photochemistry of the coupled bilin chromophore and the ensuing conformational changes needed to propagate the light signal are only now being understood. Especially informative has been the rapidly expanding collection of 3D models developed by x-ray crystallographic, NMR, and single-particle electron microscopic methods from a remarkably diverse array of bacterial Phys. These structures have revealed how the modular architecture of these dimeric photoreceptors engages the buried chromophore through distinctive knot, hairpin, and helical spine features. When collectively viewed, these 3D structures reveal complex structural alterations whereby photoisomerization of the bilin drives nanometer-scale movements within the Phy dimer through bilin sliding, hairpin reconfiguration, and spine deformation that ultimately impinge upon the paired signal output domains. When integrated with the recently described structure of the photosensory module from Arabidopsis thaliana PhyB, new opportunities emerge for the rational redesign of plant Phys with novel photochemistries and signaling properties potentially beneficial to agriculture and their exploitation as optogenetic reagents.

Abstract: Many aspects of plant photomorphogenesis are controlled by the phytochrome (Phy) family of bilin-containing photoreceptors that detect red and far-red light by photointerconversion between a dark-adapted Pr state and a photoactivated Pfr state. Whereas 3D models of prokaryotic Phys are available, models of their plant counterparts have remained elusive. Here, we present the crystal structure of the photosensing module (PSM) from a seed plant Phy in the Pr state using the PhyB isoform from Arabidopsis thaliana. The PhyB PSM crystallized as a head-to-head dimer with strong structural homology to its bacterial relatives, including a 5(Z)syn, 10(Z)syn, 15(Z)anti configuration of the phytochromobilin chromophore buried within the cGMP phosphodiesterase/adenylyl cyclase/FhlA (GAF) domain, and a well-ordered hairpin protruding from the Phy-specific domain toward the bilin pocket. However, its Per/Arnt/Sim (PAS) domain, knot region, and helical spine show distinct structural differences potentially important to signaling. Included is an elongated helical spine, an extended β-sheet connecting the GAF domain and hairpin stem, and unique interactions between the region upstream of the PAS domain knot and the bilin A and B pyrrole rings. Comparisons of this structure with those from bacterial Phys combined with mutagenic studies support a toggle model for photoconversion that engages multiple features within the PSM to stabilize the Pr and Pfr end states after rotation of the D pyrrole ring. Taken together, this Arabidopsis PhyB structure should enable molecular insights into plant Phy signaling and provide an essential scaffold to redesign their activities for agricultural benefit and as optogenetic reagents.