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 wh ich are secreted by Phytophthora and Pythium fungi, parasites of many economically important crops, These proteins induce leaf necrosis in i nfected 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 structure s of this class of protein are available: The crystal structure determ ination of beta-cryptogein (CRY), secreted by Phytophthora cryptogea, was undertaken to identify structural features important for the necro tic activity of elicitins. Results: The structure of CRY was determine d using the multiwavelength anomalous diffraction technique and refine d to 2.2 Angstrom resolution. The overall structure has a novel fold c onsisting of six a helices and a beak-like motif,whose sequence is hig hly conserved within the family, composed of an antiparallel two-stran ded 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 dist inct binding sites seem to be correlated to the level of necrotic acti vity of elicitins. Conclusions: The determination of the crystal struc ture of a member of the elicitin family may make it possible to separa te 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.