Proteins from plant cell walls inhibit polygalacturonases secreted by plant pathogens

Proteins extracted from the cell walls of Red Kidney bean hypocotyls, tomato stems, and suspension-cultured sycamore cells can completely inhibit the activity of the polygalacturonases (polygalacturonide hydrolases, EC 3.2.1.15) secreted by the fungal plant pathogens Colletotrichum lindemuthianum, Fusarium oxysporum, and Sclerotium rolfsii. The inhibitor of the C. lindemuthianum polygalacturonase, purified 560-fold from bean hypocotyl extracts, is 40 times as effective an inhibitor of the C. lindemuthianum polygalacturonase as of the F. oxysporum polygalacturonase, and does not demonstrably inhibit the S. rolfsii polygalacturonase. A crude hypocotyl extract that completely inhibits the three polygalacturonases does not inhibit C. lindemuthianum-secreted cellulase, xylanase, alpha-galactosidase, alpha-arabinofuranosidase, or alpha-galacturonosidase. The purified bean hypocotyl protein combines with the C. lindemuthianum polygalacturonase to form a complex with a dissociation constant of 2 x 10(-9) M or less. The physical properties of these inhibitors are similar to those of phytohemagglutinins and of the plant glycoproteins capable of agglutinating transformed animal cells.