Structure and interactions of the phloem lectin (phloem protein 2) Cus17 from Cucumis sativus

摘要

Phloem protein 2 (PP2) contributes crucially to phloem-based defense in plants by binding to carbohydrates displayed by pathogens. However, its three-dimensional structure and the sugar binding site remained un-explored. Here, we report the crystal structure of the dimeric PP2 Cus17 from Cucumis sativus in its apo form and complexed with nitrobenzene, N-acetyllactosamine, and chitotriose. Each protomer of Cus17 con-sists of two antiparallel four-stranded twisted (3 sheets, a (3 hairpin, and three short helices forming a (3 sand-wich architectural fold. This structural fold has not been previously observed in other plant lectin families. Structure analysis of the lectin-carbohydrate complexes reveals an extended carbohydrate binding site in Cus17, composed mostly of aromatic amino acids. Our studies suggest a highly conserved tertiary structure and a versatile binding site capable of recognizing motifs common to diverse glycans on plant pathogens/ pests, which makes the PP2 family suited for phloem-based plant defense.