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J. Biol. Chem., Vol. 282, Issue 42, 30737-30744, October 19, 2007

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Pseudomonas syringae Type III Effector AvrPtoB Is Phosphorylated in Plant Cells on Serine 258, Promoting Its Virulence Activity^*

Fangming Xiao, Patrick Giavalisco, and Gregory B. Martin¹

From the Boyce Thompson Institute for Plant Research, Ithaca, New York 14853-1801 and the Department of Plant Pathology, Cornell University, Ithaca, New York 14853-4203

The Pseudomonas syringae pv. tomato protein AvrPtoB is translocated into plant cells via the bacterial type III secretion system. In resistant tomato leaves, AvrPtoB acts as an avirulence protein by interacting with the host Pto kinase and eliciting the host immune response. Pto-mediated immunity requires Prf, a Pto-interacting protein with a putative nucleotide-binding site and a region of leucine-rich repeats. In susceptible tomato plants, which lack either Pto or Prf, AvrPtoB acts as a virulence protein by promoting P. syringae pv. tomato growth and enhancing symptoms associated with bacterial speck disease. The N-terminal 307 amino acids of AvrPtoB (designated AvrPtoB_1–307) are sufficient for these virulence activities and for Pto-mediated avirulence. We report that AvrPtoB is phosphorylated by a Pto- and Prf-independent kinase activity that is conserved in several plant species, including tomato (Solanum lycopersicum), Nicotiana benthamiana, and Arabidopsis thaliana. AvrPtoB_1–307 was phosphorylated in tomato protoplasts, and mass spectrometry identified serine 258 as the major in vivo phosphorylation site of this protein. An alanine substitution of Ser²⁵⁸ resulted in the loss of virulence and the diminution of avirulence activity of AvrPtoB_1–307, whereas a phosphomimetic S258D mutant had activities similar to wild type AvrPtoB_1–307. These observations suggest that AvrPtoB has evolved to mimic a substrate of a conserved plant kinase, leading to enhancement of its virulence and avirulence activities in the host cell.

Received for publication, July 6, 2007 , and in revised form, August 17, 2007.

^* This work was supported by National Institutes of Health Grant R01GM078021 and United States Department of Agriculture-National Research Institute Grant 2005-35301-15675). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

¹ To whom correspondence should be addressed: Boyce Thompson Institute for Plant Research, Tower Rd., Ithaca, NY 14853-1801. Tel.: 607-254-1208; Fax: 607-255-6695; E-mail: gbm7{at}cornell.edu.

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