Trends in Plant Science
OpinionNB-LRRs work a “bait and switch” on pathogens
Section snippets
NBLing away at pathogens
Plants employ a network of intertwined mechanisms to counter infection by pathogens and parasites. One such line of defense is based on dominant disease resistance (R) genes which mediate resistance to pathogens possessing corresponding avirulence (Avr) genes [1]. Although the protein products of Avr genes have little in common structurally, the majority of characterized R genes encode nucleotide binding leucine-rich repeat (NB-LRR) proteins (Box 1). NB-LRR-encoding genes make up one of the
A brief history of NB-LRR models
Molecular genetic studies of closely related NB-LRR proteins with differing recognition specificities indicate that the LRR domain is the primary determinant of NB-LRR protein recognition specificity 10, 11, 12, 13. Furthermore, the LRR domains of NB-LRR proteins appear to be under diversifying selection [14]. Taken together with the known role of LRR domains in protein–protein interactions, this originally led to the expectation that Avr and NB-LRR proteins might undergo receptor–ligand
The N-terminus: setting the bait
The central importance of the LRR domain in determining recognition specificity would lead to the expectation that either Avr proteins or recognition cofactors bind to this domain. At the same time, it has been generally assumed that the N-terminal domains of NB-LRR proteins function in the initiation of signaling owing to similarities in domain structure between plant NB-LRR and animal NACHT-LRR proteins 23, 24. However, all plant proteins with either confirmed or proposed roles as cofactors
From recognition to signaling: flipping the switch
If recognition involves cooperation between the LRR and N-terminal domains, what then is the nature of the mechanism or molecular switch that translates recognition into signal initiation? Structure–function analyses of the potato (Solanum tuberosum) CC-NB-LRR protein Rx indicate that this switch requires some assembly. One necessity for proper Rx function is a physical interaction between the LRR and ARC1 domains. This interaction, as well as a functional nucleotide binding pocket, is itself a
Evolutionary perspectives
All NB-LRR proteins presumably derive from a common ancestor [50], and this primordial NB-LRR protein could have functioned on the basis of either direct or indirect interaction with pathogen-derived molecules. Upon expansion of this protein family, NB-LRR proteins might have coopted various recognition cofactors which, combined with diversified LRR domains, might have offered a more complex recognition capacity. It has been suggested that a shift in recognition cofactors from guardees to
Acknowledgements
We are grateful to current and former laboratory members for stimulating discussions on NB-LRR protein function. We thank John McDowell and John Rathjen for insightful comments, suggestions and critical review of this manuscript. Work on NB-LRR proteins in the Moffett laboratory is supported by funding from the National Science Foundation (IOS-0744652) and by a National Science Foundation graduate research fellowship to S.M.C.
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