A PLANT HOMOLOG OF THE NEUTROPHIL NADPH OXIDASE GP91(PHOX) SUBUNIT GENE ENCODES A PLASMA-MEMBRANE PROTEIN WITH CA2+ BINDING MOTIFS

Rapid generation of O-2- and H2O2, which is reminiscent of the oxidati ve burst in neutrophils, is a central component of the resistance resp onse of plants to pathogen challenge. Here, we report that the Arabido psis rbohA (for respiratory burst oxidase homolog A) gene encodes a pu tative 108-kD protein, with a C-terminal region that shows pronounced similarity to the 69-kD apoprotein of the gp91(phox) subunit of the ne utrophil respiratory burst NADPH oxidase. The RbohA protein has a larg e hydrophilic N-terminal domain that is not present in gp91(phox). Thi s domain contains two Ca2+ binding EF hand motifs and has extended sim ilarity to the human RanGTPase-activating protein 1. rbohA, which is a member of a divergent gene family, generates transcripts of 3.6 and 4 .0 kb that differ only in their polyadenylation sites. rbohA transcrip ts are most abundant in roots, with weaker expression in aerial organs and seedlings. Antibodies raised against a peptide near the RbohA C t erminus detected a 105-kD protein that, unlike gp91(phox), does not ap pear to be highly glycosylated. Cell fractionation, two-phase partitio ning, and detergent extraction indicate that RbohA is an intrinsic pla sma membrane protein. We propose that plants have a plasma membrane en zyme similar to the neutrophil NADPH oxidase but with novel potential regulatory mechanisms for Ca2+ and G protein stimulation of O-2- and H 2O2 production at the cell surface.