Structural requirements of endopolygalacturonase for the interaction with PGIP (polygalacturonase-inhibiting protein)

To invade a plant tissue, phytopathogenic fungi produce several cell wall-d egrading enzymes; among them, endopolygalacturonase (PG) catalyzes the frag mentation and solubilization of homogalacturonan. Polygalacturonase-inhibit ing proteins (PGIPs), found in the cell wall of many plants, counteract fun gal PGs by forming specific complexes with them. We report the crystal stru cture at 1.73 Angstrom resolution of PG from the phytopathogenic fungus Fus arium, moniliforme (FmPG). The structure of FmPG was useful to study the mo de of interaction of the enzyme with PGIP-2 from Phaseolus vulgaris. Severa l amino acids of FmPG were mutated, and their contribution to the formation of the complex with PGIP-2 was investigated by surface plasmon resonance. The residues Lys-269 and Arg-267, located inside the active site cleft, and His-188, at the edge of the active site cleft, are critical for the format ion of the complex, which is consistent with the observed competitive inhib ition of the enzyme played by PGIP-2. The replacement of His-188 with a pro line or the insertion of a tryptophan after position 270, variations that b oth occur in plant PGs, interferes with the formation of the complex. We su ggest that these variations are important structural requirements of plant PGs to prevent PGIP binding.