Studies on Protein Structures and Phosphorylation Site-Mutants of Plant Phytochromes

Phytochromes are important proteins for regulating photomorphogenesis as red and far-red photoreceptors. There are five phytochrome members (AtphyA to AtphyE) in <italic>Arabidopsis thaliana</italic>, among which far-red light-sensing AtphyA and red light-sensing AtphyB are extensively studied thus far. Structure-function studies are essential to understanding the regulatory roles of proteins, but only recently have the full-length phytochrome structures been reported using cryogenic electron microscopy. The atomic structures of AtphyA and AtphyB showed that N-terminal photosensory module (PSM) is arranged in a head-to-tail manner and the histidine kinase-related domain of C-terminal output module (OPM) associates in a head-to-head manner to form a dimer. In addition, while AtphyA shows a symmetric protein structure, AtphyB is an asymmetric dimer. This suggested that the protein structure of AtphyA is more stable than that of AtphyB. Structural features of phytochromes include N-terminal extension (NTE) region and hinge region that connects the PSM and OPM. Interestingly, several amino acid residues have been reported to be phosphorylated in the NTE and hinge regions of phytochromes. Subsequent analyses suggested that the phosphorylation of phytochrome A is important for the regulation of protein stability and protein-protein interactions. In the cases of AtphyB, AtphyD, and AtphyE, phosphorylation of the NTE is involved in regulating the rate of dark/thermal reversion, nuclear translocation and photobody formation, and protein-protein interactions. The presence of phosphorylation sites within the NTE of all phytochromes suggests the importance of regulation by their phosphorylation, and this functional similarity indicates the presence of a conserved regulatory mechanism within the phytochrome family.