Crystallization and preliminary X-ray diffraction studies of beta-cryptogein, a toxic elicitin secreted by the phytopathogenic fungus Phytophthora cryptogea

Crystal structure of a fungal elicitor secreted by Phytophthora cryptogea, a member of a novel class of plant necrotic proteins

BACKGROUND

Elicitins form a novel class of plant necrotic proteins which are secreted by Phytophthora and Pythium fungi, parasites of many economically important crops. These proteins induce leaf necrosis in infected plants and elicit an incompatible hypersensitive-like reaction, leading to the development of a systemic acquired resistance against a range of fungal and bacterial plant pathogens. No crystal structures of this class of protein are available. The crystal structure determination of beta-cryptogein (CRY), secreted by Phytophthora cryptogea, was undertaken to identify structural features important for the necrotic activity of elicitins.

RESULTS

The structure of CRY was determined using the multiwavelength anomalous diffraction technique and refined to 2.2 A resolution. The overall structure has a novel fold consisting of six alpha helices and a beak-like motif, whose sequence is highly conserved within the family, composed of an antiparallel two-stranded beta sheet and an omega loop. This motif is assumed to be a major recognition site for a putative receptor and/or ligand. Two other distinct binding sites seem to be correlated to the level of necrotic activity of elicitins.

CONCLUSIONS

The determination of the crystal structure of a member of the elicitin family may make it possible to separate the activity that causes leaf necrosis from that inducing systemic acquired resistance to pathogens, making it feasible to engineer a non-toxic elicitin that only elicits plant defences. Such studies should aid the development of non-toxic agricultural pest control.

Elicitins from Phytophthora and basic resistance in tobacco

Elicitins are a family of small proteins secreted by species of Phytophthora. They are thought to be major determinants of the resistance response of tobacco against these oomycetes, since purified elicitins, alone and at low concentrations, can induce vigorous defense responses in tobacco (i.e., hypersensitive cell death and resistance against subsequent pathogen attack), and in vitro elicitin production by Phytophthora isolates is strongly negatively correlated with their pathogenicity on tobacco plants. A number of elicitins have been purified and their amino acid sequences have been determined and found to be conserved. A three-dimensional structure for elicitin is emerging from nuclear magnetic resonance studies. Two structural classes, alpha and beta, are distinguished by their biological effects when applied to decapitated stems or petioles; the beta class causes more necrosis on leaves and provides better subsequent protection against pathogen attack. However, both these classes of elicitins will similarly cause necrosis when each is, instead, directly infiltrated into tobacco leaf panels. Effects of elicitins on tobacco cells include rapid electrolyte leakage, changes in protein phosphorylation and amounts of active oxygen species, and later production of ethylene and capsidiol. The sites of initial interaction with tobacco cells are unknown, but the interaction appears to induce general defense-related responses.

Local structural differences between alpha- and beta-elicitins shown by circular dichroism and ultraviolet difference spectroscopy

In order to investigate differences in the conformation of elicitins exhibiting different levels of activity (toxicity to tobacco plants), the environment of the tyrosyl residues in four elicitins has been compared by different spectroscopic methods (difference absorption and circular dichroism). We compared two alpha-elicitins (capsicein and parasiticein) and two beta-elicitins (beta-cryptogein and beta-cinnamomin), that are 50-100 times more toxic than the alpha-ones. Thermal difference UV spectroscopy and titration experiments clearly showed the exposure of Tyr-85 by comparison of parasiticein lacking Tyr-85 and the accessibility of its hydroxyl group to the solvent. The adjacent Tyr-87 was also suggested to be located at the surface. In beta-cryptogein, beta-cinnamomin and capsicein the pK was measured at between 10.5 and 10.8, while in parasiticein it is higher (11.5) owing to a difference in the local environment. Thermal difference UV spectroscopy showed one more exposed tyrosine in beta-elicitins than in alpha-ones. This difference was attributed to Tyr-12, considering the more hydrophilic characteristic of the sequence around residue 13 in beta-elicitins and the role of this region in the toxicity. However, no difference in titration behaviour was noted among elicitins concerning Tyr-12. The other two tyrosines also presented an abnormal pK of titration (> 12). In all elicitins Tyr-47 was probably exposed, while Tyr-33 was probably buried and not titrated, except in beta-cinnamomin at very alkaline pH.

Amino acid sequence of the alpha-elicitin secreted by Phytophthora cactorum

Elicitins are toxic, signalling proteins secreted by Phytophthora spp. responsible for the incompatible reaction and systemic leaf necroses. We have sequenced the least toxic known elicitin, which is secreted by Phytophthora cactorum and belongs to the alpha-elicitin class. This 98 residue sequence was compared to eight other known elicitins. In addition to the point mutation already observed to correlate with the differences in necrotic activities, we confirmed the existence of amino acid changes in the central region of the molecule, comparable to those already shown in one of the Phytophthora megasperma megasperma elicitins, to be correlated with toxicity.