Purification of the M flax-rust resistance protein expressed in Pichia pastoris

Animal NLRs provide structural insights into plant NLR function

BACKGROUND

The plant immune system employs intracellular NLRs (nucleotide binding [NB], leucine-rich repeat [LRR]/nucleotide-binding oligomerization domain [NOD]-like receptors) to detect effector proteins secreted into the plant cell by potential pathogens. Activated plant NLRs trigger a range of immune responses, collectively known as the hypersensitive response (HR), which culminates in death of the infected cell. Plant NLRs show structural and functional resemblance to animal NLRs involved in inflammatory and innate immune responses. Therefore, knowledge of the activation and regulation of animal NLRs can help us understand the mechanism of action of plant NLRs, and vice versa.

SCOPE

This review provides an overview of the innate immune pathways in plants and animals, focusing on the available structural and biochemical information available for both plant and animal NLRs. We highlight the gap in knowledge between the animal and plant systems, in particular the lack of structural information for plant NLRs, with crystal structures only available for the N-terminal domains of plant NLRs and an integrated decoy domain, in contrast to the more complete structures available for animal NLRs. We assess the similarities and differences between plant and animal NLRs, and use the structural information on the animal NLR pair NAIP/NLRC4 to derive a plausible model for plant NLR activation.

CONCLUSIONS

Signalling by cooperative assembly formation (SCAF) appears to operate in most innate immunity pathways, including plant and animal NLRs. Our proposed model of plant NLR activation includes three key steps: (1) initially, the NLR exists in an inactive auto-inhibited state; (2) a combination of binding by activating elicitor and ATP leads to a structural rearrangement of the NLR; and (3) signalling occurs through cooperative assembly of the resistosome. Further studies, structural and biochemical in particular, will be required to provide additional evidence for the different features of this model and shed light on the many existing variations, e.g. helper NLRs and NLRs containing integrated decoys.

An autoactive mutant of the M flax rust resistance protein has a preference for binding ATP, whereas wild-type M protein binds ADP

Resistance (R) proteins are key regulators of the plant innate immune system and are capable of pathogen detection and activation of the hypersensitive cell death immune response. To understand the molecular mechanism of R protein activation, we undertook a phenotypic and biochemical study of the flax nucleotide binding (NB)-ARC leucine-rich repeat protein, M. Using Agrobacterium-mediated transient expression in flax cotyledons, site-directed mutations of key residues within the P-loop, kinase 2, and MHD motifs within the NB-ARC domain of M were shown to affect R protein function. When purified using a yeast expression system and assayed for ATP and ADP, these mutated proteins exhibited marked differences in the quantity and identity of the bound nucleotide. ADP was bound to recombinant wild-type M protein, while the nonfunctional P-loop mutant did not have any nucleotides bound. In contrast, ATP was bound to an autoactive M protein mutated in the highly conserved MHD motif. These data provide direct evidence supporting a model of R protein function in which the "off" R protein binds ADP and activation of R protein defense signaling involves the exchange of ADP for ATP.

The AvrM effector from flax rust has a structured C-terminal domain and interacts directly with the M resistance protein

In plant immunity, recognition of pathogen effectors by plant resistance proteins leads to the activation of plant defenses and a localized cell death response. The AvrM effector from flax rust is a small secreted protein that is recognized by the M resistance protein in flax. Here, we investigate the mechanism of M-AvrM recognition and show that these two proteins directly interact in a yeast two-hybrid assay, and that this interaction correlates with the recognition specificity observed for each of the different AvrM variants. We further characterize this interaction by demonstrating that the C-terminal domain of AvrM is required for M-dependent cell death, and show that this domain also interacts with the M protein in yeast. We investigate the role of C-terminal differences among the different AvrM proteins for their involvement in this interaction and establish that M recognition is hindered by an additional 34 amino acids present at the C terminus of several AvrM variants. Structural characterization of recombinant AvrM-A protein revealed a globular C-terminal domain that dimerizes.