Abstract

Many plant disease resistance (R) genes encode proteins predicted to have an N-terminal coiled-coil (CC) domain, a central nucleotide-binding site (NBS) domain and a C-terminal leucine-rich repeat (LRR) domain. These CC–NBS–LRR proteins recognize specific pathogen-derived products and initiate a resistance response that often includes a type of cell death known as the hypersensitive response (HR). Co-expression of the potato CC–NBS–LRR protein Rx and its elicitor, the PVX coat protein (CP), results in a rapid HR. Surprisingly, co-expression of the LRR and CC–NBS as separate domains also resulted in a CP-dependent HR. Likewise, the CC domain complemented a version of Rx lacking this domain (NBS–LRR). Correspondingly, the LRR domain interacted physically in planta with the CC–NBS, as did CC with NBS–LRR. Both interactions were disrupted in the presence of CP. However, the interaction between CC and NBS–LRR was dependent on a wild-type P-loop motif, whereas the interaction between CC–NBS and LRR was not. We propose that activation of Rx entails sequential disruption of at least two intramolecular interactions.