Motifs specific for the ADR1 NBS-LRR protein family in Arabidopsis are conserved among NBS-LRR sequences from both dicotyledonous and monocotyledonous plants

The activated disease resistance (ADR) 1 gene encodes a protein that possesses an N-terminal coiled-coil (CC) motif, nucleotide-binding site (NBS) and C-terminal leucine-rich repeat (LRR) domains. ADR1 belongs to a small, atypical Arabidopsis thaliana subclass containing four CC-NBS-LRR genes. The NBS region of most NBS-LRR proteins possesses numerous conserved motifs. In contrast, the LRR domain, which is subject to positive selection, is highly variable. Surprisingly, sequence analysis revealed that the LRR domain of the ADR1 sub-class was more conserved than the NBS region. Sequence analysis identified two novel conserved motifs, termed TVS and PKAE, specific for this CC-NBS-LRR sub-class. The TVS motif is adjacent to the P-loop, whereas the PKAE motif corresponded to the inter-domain region termed the NBS-LRR linker, which was conserved within the different CC-NBS-LRR classes but varied among classes. These ADR1-specific motifs were employed to identify putative ADR1 homologs in phylogenetically distant and agronomically important plant species. Putative ADR1 homologs were identified in 11 species including rice and in 3 further Poaceae species. The ADR1 subclass of CC-NBS-LRR proteins is therefore conserved in both monocotyledonous and dicotyledonous plant species.