| Virulent gram-negative bacterial phytopathogens have evolved a vast arsenal of biochemical effector proteins capable of subverting and suppressing plant defense mechanisms. In response, plants have evolved an advanced surveillance system, consisting of intracellular resistance "R" proteins that can detect effectors delivered by the bacterial Type-III secretion system. Recent evidence suggests that many R proteins recognize bacterial effectors indirectly by monitoring their effects on host proteins. Upon recognition, a signal transduction cascade is triggered, and host defenses are activated. Previous studies have established that mutations in the NDR1 gene of Arabidopsis thaliana suppress the resistance response of three R proteins, RPS2, RPM1, and RPS5. NDR1, a novel protein of unknown biochemical function, was previously assumed to operate downstream of the initial R-Avr protein interaction. The work presented in this dissertation suggests that this hypothesis should be revised to place NDR1 in tight association with the initial pathogen recognition event. NDR1 is a plasma membrane (PM) localized protein, and undergoes several post-translational modifications including carboxy-terminal processing and N-linked glycosylation. Biochemical and molecular evidence suggest that NDR1 is anchored to the PM via a C-terminal glycosylphosphatidylinositol (GPI) anchor. Unlike most GPI-modified proteins, however, NDR1 appears to exhibit an unusual "double anchor" topology, consisting of an N-terminal transmembrane domain in addition to the C-terminal GPI anchor. NDR1 interacts with a RIN4, a plant PM protein targeted by the Pseudomonas syringae Type-III effector molecules AvrRpt2, AvrB, and AvrRpm1. NDR1 may exist in R protein-containing complexes at the plasma membrane and participate directly in the initial stages of resistance. |
