Plant biology

An ATPase for a Kinase

Science Signaling  04 May 2010:
Vol. 3, Issue 120, pp. ec130
DOI: 10.1126/scisignal.3120ec130

Many plant pattern recognition receptors (PRRs), which are involved in the rapid response to pathogen attack, contain an intrinsic kinase activity. XA21 is one such PRR in rice that mediates the immune response to infection by Xanthomonas and Xyella by binding the pathogen-associated molecular pattern Ax21. In a yeast two-hybrid screen, Chen et al. identified XB24 as a protein that interacted with XA21, and analysis of the protein sequence indicated that XB24 belonged to a class of ATPases for which little functional information is known. Further analysis in the yeast two-hybrid system showed that the interaction between XB24 and XA21 did not require the ATPase motif of XB24 but did require catalytic activity of XA21. Analysis in transgenic plants confirmed the interaction between XA21 and XB24. XB24 isolated from transgenic plants exhibited ATPase activity in vitro. When purified XA21 was incubated with purified XB24, the autophosphorylation of XA21 was enhanced; phosphorylation did not occur in a kinase-inactive mutant form of XA21, and XA21 phosphorylation was not enhanced by the addition of an ATPase-inactive mutant of XB24. Investigation of the response of various transgenic rice plants to Xanthomonas infection revealed that plants overexpressing the functional, but not the ATPase-inactive mutant, form of XB24 exhibited a reduced immune response and had larger lesions on the leaves and a higher bacterial load. After infection, the abundance of XA21 was decreased in the plants overexpressing XB24, whereas in control plants XA21 abundance increased after infection. In contrast, silencing of XB24 resulted in plants that were more resistant to Xanathomonas infection. Infected plants exhibited decreased interaction between XB24 and XA21 compared with the interaction in uninfected plants (based on coimmunoprecipitation experiments), suggesting that in response to infection, XB24 dissociates from XA21. The authors propose that XB24 stimulates an inhibitory autophosphorylation of XA21 that keeps the receptor’s activity low in the absence of infection and that, upon recognition of its ligand, XB24 dissociates, allowing XA21 to trigger the immune response.

X. Chen, M. Chern, P. E. Canlas, D. Ruan, C. Jiang, P. C. Ronald, An ATPase promotes autophosphorylation of the pattern recognition receptor XA21 and inhibits XA21-mediated immunity. Proc. Natl. Acad. Sci. U.S.A. 107, 8029–8034 (2010). [Abstract] [Full Text]