Structural modeling of a plant disease resistance gene product domain.

Dominant plant resistance genes are involved in the protection of plants against a wide variety of pathogens. Sequence analysis has revealed a variety of classes, often having domains in common. One commonly found region has come to be known as a putative nucleotide-binding site (NBS) due to the simple presence of sequence motifs. Until now, no experimental evidence has supported this idea. Here we suggest, as an alternative hypothesis, that part of this region is structurally homologous to the receiver domain common to many proteins of His-Asp phosphotransfer pathways. This conclusion is based on sequence analysis, threading experiments, and the construction of a molecular model of one domain that performs well against structure validation tools. The new hypothesis, in contrast to the NBS hypothesis, can explain the devastating effect of a Thr-->Ala mutation in a well-characterized resistance gene product. According to the new hypothesis, regions located N-terminal and C-terminal to the modeled portion, containing highly conserved sequence motifs, could form a separate domain.