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Abstract:
Resistance (R) proteins in plants confer specificity to the innate immune
system. Most R proteins have a centrally located NB-ARC (nucleotide-binding
adaptor shared by APAF-1, R proteins, and CED-4) domain. For two tomato
(Lycopersicon esculentum) R proteins, I-2 and Mi-1, we have previously shown
that this domain acts as an ATPase module that can hydrolyze ATP in vitro. To
investigate the role of nucleotide binding and hydrolysis for the function of
I-2 in planta, specific mutations were introduced in conserved motifs of the
NB-ARC domain. Two mutations resulted in autoactivating proteins that induce a
pathogen-independent hypersensitive response upon expression in planta. These
mutant forms of I-2 were found to be impaired in ATP hydrolysis, but not in ATP
binding, suggesting that the ATP- rather than the ADP-bound state of I-2 is the
active form that triggers defense signaling. In addition, upon ADP binding, the
protein displayed an increased affinity for ADP suggestive of a change of
conformation. Based on these data, we propose that the NB-ARC domain of I-2,
and likely of related R proteins, functions as a molecular switch whose state
(on/off) depends on the nucleotide bound (ATP/ADP).
