Structure-function relationships of α and β elicitins, signal proteins involved in the plant-Phytophthora interaction

Elicitins form a family of 10-kDa holoproteins secreted by various Phytophthora species. The large-scale purification of parasiticein, a novel elicitin secreted by P. parasitica, led to the determination of its sequence. We have compared the necrotic activities and the primary and secondary structures (determined through circular dichroism) of four elicitins. On tobacco plants, they could be classified into two classes: a, comprising capsicein and parasiticein (less necrotic), and β, comprising cryptogein and cinnamomin (very toxic with a necrosis threshold of 0.1 μg per leaf). The features of elicitin structure which might be involved in the interaction of elicitins with the leaf target cells and that could explain the different necrosis-inducing properties of the two proteins are investigated. About 75% sequence identity was observed between the four elicitins: only two short terminal regions are heterologous, while the central core is mainly conserved. The circular-dichroism spectra showed that the secondary structure of the elicitins was largely conserved. All of them consisted of approx. 50% α-helix with little or no β-structure. Comparisons of the complete sequences, amino-acid compositions, isoelectric points, hydropathy indices and the secondary-structure predictions correlated with the necrotic classification. Alpha elicitins corresponded to acidic molecules with a valine residue at position 13, while β elicitins were basic with a lysine at this position, which appeared to be a putative active site responsible for necrosis induction.