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.

The 2.1 A structure of an elicitin-ergosterol complex: a recent addition to the Sterol Carrier Protein family.

Elicitins, produced by most of the phytopathogenic fungi of the genus Phytophthora, provoke in tobacco both remote leaf necrosis and the induction of a resistance against subsequent attack by various microorganisms. Despite the recent description of the three-dimensional crystal structure of cryptogein (CRY), the molecular basis of the interactions between Phytophthora and plants largely remains unknown. The X-ray crystal structure, refined at 2.1 A, of a ligand complexed, mutated CRY, K13H, is reported. Analysis of this structure reveals that CRY is able to encapsulate a ligand that induces only a minor conformational change in the protein structure. The ligand has been identified as an ergosterol by gas chromatographic analysis coupled with mass spectrometry analysis. This result is consistent with biochemical data that have shown that elicitins are a distinct class of Sterol Carrier Proteins (SCP). Data presented here provide the first structural description of the pertinent features of the elicitin sterol interaction and permit a reassessment of the importance of both the key residue 13 and the mobility of the omega loop for the accessibility of the sterol to the cavity. The biological implications thereof are discussed. This paper reports the first structure of a SCP/sterol complex.

Overexpression in Pichia pastoris and crystallization of an elicitor protein secreted by the phytopathogenic fungus, Phytophthora cryptogea.

A synthetic gene encoding beta-cryptogein, a member of the elicitin family, has been cloned into a vector for expression by the methylotrophic yeast, Pichia pastoris. Having first optimized the gene construction for secretion, we have overexpressed a modified beta-cryptogein in a secreted form. A purification scheme suited to this expression system has been developed and highly pure, biologically active protein has been obtained. For structural analysis of this recombinant beta-cryptogein, and new mutated forms thereof, optimal conditions for the crystallization of this protein have been determined and crystals that diffract to 2.2 A have been obtained.

Characterization of cryoglobulins by immunoblotting.

We adapted an immunoblotting technique for the immunochemical characterization of cryoglobulins. We first compared the results from 157 samples with results obtained by immunofixation and immunoelectrophoresis and then with clinical observations in 125 cases. Full identification was possible in 98% of the cases by immunoblotting, in 54% by immunofixation, and in 28% by immunoelectrophoresis. Novel microheterogeneity aspects were observed by immunoblotting and immunofixation in 13% and 6% of the cases studied, respectively. Our results confirm the frequency of autoimmune and infectious diseases associated with mixed cryoglobulinemias, whereas a lymphoproliferative syndrome was observed only in cases of cryoglobulinemia with a monoclonal constituent or a microheterogeneity aspect, which was not always revealed by immunofixation or immunoelectrophoresis. The pathophysiological importance and classification of microheterogeneity aspects is not clear. These observations justify using a sensitive and specific method for identifying cryoglobulins, even when present at low concentrations, and call for long-term studies of these patients.