Plant NADPH oxidase (Rboh)
Rice (Oryza sativa Japonica Group)
Generally, the reactive oxygen species (ROS) are known as harmful reagents which cause impairment of cell functions. On the other hand, various groups of eukaryote, such as mammals and land plants, possess enzymes dedicated to ROS production, and use for many biological activities. One of the most important enzymes producing ROS is NADPH oxidase family.
Plant NADPH oxidase (Rboh) exists in plasma membrane, and reduce molecular oxygen to superoxide anion (O2-) by oxidation of cytosolic NADPH. ROS produced by Rboh plays multiple roles as signals that mediate immune response, abiotic stress, and developmental cues. Especially, importance of ROS production in immune response has been known from early on. Once an invasion of microbes is recognized, plant cell immediately generates ROS which trigger multiple immune responses. Rboh is regulated by Ca2+, small GTPase, protein kinase, and other factors. But, details of activation mechanism have not been understood. Rboh is composed of three distinguished segments : N-terminal regulatory region, Six transmembrane region, and C-terminal FAD/NADPH binding region. N-terminal region and C-terminal region exist in cytoplasm.
The structure shown here is a part of N-terminal regulatory region. This domain is predicted by amino acid sequence to contain two EF-hand motifs which are known as Ca2+ binding motif. Rboh forms homodimer by swapping its second EF-hand motif. The structure of swapped EF-hand motifs is quite unique compared to other EF-hand containing proteins. Separately from predicted EF-hand motifs, we noticed that Rboh possesses two additional EF-hand like motifs which were not predicted by amino acid sequence. The structure formed by these four EF-hand motifs, reveals structural homology to four EF-hand containing protein such as calcineurinB, recoverin. In Rboh, only one of these four EF-hand motifs, binds Ca2+. Because N-terminal regulatory region interacts with C-terminal FAD/NADPH binding region directly, conformational change caused by Ca2+ binding may regulate Rboh activity through this interaction.
Protein Data Bank (PDB)
author: Takashi Oda, Toshiyuki Shimizu