Plants have evolved intracellular immune receptors to detect pathogen proteins known as effectors. How these immune receptors detect effectors remains poorly understood. Here we describe the structural basis for direct recognition of AVR-Pik, an effector from the rice blast pathogen, by the rice intracellular NLR immune receptor Pik. AVR-PikD binds a dimer of the Pikp-1 HMA integrated domain with nanomolar affinity. The crystal structure of the Pikp-HMA/AVR-PikD complex enabled design of mutations to alter protein interaction in yeast and in vitro, and perturb effector-mediated response both in a rice cultivar containing Pikp and upon expression of AVR-PikD and Pikp in the model plant Nicotiana benthamiana. These data reveal the molecular details of a recognition event, mediated by a novel integrated domain in an NLR, which initiates a plant immune response and resistance to rice blast disease. Such studies underpin novel opportunities for engineering disease resistance to plant pathogens in staple food crops.DOI: http://dx.doi.org/10.7554/eLife.08709.001

Oxalate decarboxylase (EC 4.1.1.2) catalyzes the conversion of oxalate to formate and carbon dioxide and utilizes dioxygen as a cofactor. By contrast, the evolutionarily related oxalate oxidase (EC 1.2.3.4) converts oxalate and dioxygen to carbon dioxide and hydrogen peroxide. Divergent free radical catalytic mechanisms have been proposed for these enzymes that involve the requirement of an active site proton donor in the decarboxylase but not the oxidase reaction. The oxidase possesses only one domain and manganese binding site per subunit, while the decarboxylase has two domains and two manganese sites per subunit. A structure of the decarboxylase together with a limited mutagenesis study has recently been interpreted as evidence that the C-terminal domain manganese binding site (site 2) is the catalytic site and that Glu-333 is the crucial proton donor (Anand, R., Dorrestein, P. C., Kinsland, C., Begley, T. P., and Ealick, S. E. (2002) Biochemistry 41, 7659 -7669).The N-terminal binding site (site 1) of this structure is solvent-exposed (open) and lacks a suitable proton donor for the decarboxylase reaction. We report a new structure of the decarboxylase that shows a loop containing a 3 10 helix near site 1 in an alternative conformation. This loop adopts a "closed" conformation forming a lid covering the entrance to site 1. This conformational change brings Glu-162 close to the manganese ion, making it a new candidate for the crucial proton donor. Sitedirected mutagenesis of equivalent residues in each domain provides evidence that Glu-162 performs this vital role and that the N-terminal domain is either the sole or the dominant catalytically active domain.

Microbial cytochromes c¢ contain a 5-coordinate Hisligated heme that forms stable adducts with nitric oxide (NO) and carbon monoxide (CO), but not with dioxygen. We report the 1.95 and 1.35 A Ê resolution crystal structures of the CO-and NO-bound forms of the reduced protein from Alcaligenes xylosoxidans. NO disrupts the His±Fe bond and binds in a novel mode to the proximal face of the heme, giving a 5-coordinate species. In contrast, CO binds 6-coordinate on the distal side. A second CO molecule, not bound to the heme, is located in the proximal pocket. Since the unusual spectroscopic properties of cytochromes c¢ are shared by soluble guanylate cyclase (sGC), our ®ndings have potential implications for the activation of sGC induced by the binding of NO or CO to the heme domain.

Background: GlgE is a maltosyltransferase involved in bacterial ␣-glucan biosynthesis and is a genetically validated antituberculosis target. Results: We have determined the catalytic properties of Streptomyces coelicolor GlgE and solved its structure. Conclusion:The enzyme has the same catalytic properties as Mycobacterium tuberculosis GlgE and the structure reveals how GlgE functions. Significance: The structure will help guide the development of inhibitors with therapeutic potential.