In tomato plants under attack by the bacterial pathogen Pseudomonas syringae, the plasma membrane serine/threonine kinase Pto binds the intracellularly injected bacterial effector proteins AvrPto and AvrPtoB, initiating a cell death response that limits spread of the infection. Although Pto substrates have been identified in vitro, the specific pathways mediating this "hypersensitive response" (HR) are unclear (see Eckardt). Wu et al. mutated individual residues in the Pto P +1 loop (which has been implicated in AvrPto binding and in regulating cell signaling), thereby creating Pto mutants that, when expressed in Nicotiana benthamiana, constitutively elicited the HR independently of AvrPto. All of these mutations involved amino acids exposed on the Pto surface, and the authors used a combination of site-directed mutagenesis and structural analysis to define a contiguous patch of surface-exposed residues involved in Pto signaling. Mutation of residues within this patch led to a constitutive HR; moreover, most mutants were unable to bind to AvrPto and AvRPtoB. Although kinase activity of PTO was required for the AvrPto-dependent HR, it was not required for activation of HR by mutants that elicited the HR independently. Thus, the authors propose a new model for Pto activation of the HR in which Avr binding displaces a negative regulator from the patch and stimulates Pto kinase activity. This leads to a conformational change (mimicked in the constitutively active mutants) that is the trigger for activation of the HR pathway.
A.-J. Wu, V. M. E. Andriotis, M. C. Durrant, J. P. Rathjen, A patch of surface-exposed residues mediates negative regulation of immune signaling by tomato Pto kinase. Plant Cell 16, 2809-2821 (2004). [Abstract] [Full Text]
N. A. Eckardt, Mechanism of Pto-mediated disease resistance: Structural analysis provides a new model. Plant Cell 16, 2543-2545 (2004). [Full Text]