Dual regulatory roles of the extended N-terminus for activation of the tomato Mi-1.2 resistance protein

Plant resistance (R) proteins mediate race-specific immunity and initiate host defenses that are often accompanied by a localized cell-death response. Most R proteins belong to the NB-LRR protein family as they carry a central NB-ARC domain fused to an LRR domain. The CC domain at the N-terminus of some Solanaceous NB-LRR proteins is extended with a solanaceae domain (SD). Tomato Mi-1.2, which confers resistance against nematodes, white flies, psyllids and aphids, encodes a typical SD-CNL protein. Here, we analyzed the role of the extended N-terminus for Mi-1.2 activation. Removal of the first part of the N-terminus (Nt1) induced Mi-1.2-mediated cell death that could be suppressed by over-expression of the second half of the N-terminal region (Nt2). Yet, autoactivating NB-ARC-LRR mutants require in trans co-expression of the N-terminal region to induce cell death, indicating that the N-terminus functions both as a negative and a positive regulator. Based on secondary structure predictions we could link both functions to three distinct subdomains; a typical CC domain and two novel, structurally-conserved helical subdomains called SD1 and SD2. A negative regulatory function could be assigned to the SD1 whereas SD2 and the CC together function as positive regulators of Mi-1.2 mediated cell death