BACTERIAL PHYTOTOXINS, SYRINGOMYCIN, SYRINGOSTATIN AND SYRINGOTOXIN, EXERT THEIR EFFECT ON THE PLASMA-MEMBRANE H+-ATPASE PARTLY BY A DETERGENT-LIKE ACTION AND PARTLY BY INHIBITION OF THE ENZYME

Syringostatin is a newly discovered phytotoxin produced by a phytopathogenic bacterium Pseudomonas syringae pv. syringae lilac isolate. The effects of syringostatin and the similar phytotoxins, syringomycin and syringotoxin, on H+-ATPase activity were investigated using cultured mung bean (Vigna radiata L. cv. Ryokuto) cells or plasma membrane vesicles isolated from mung bean hypocotyls. P-31-NMR analysis of cultured cells treated with syringostatin revealed that the cytoplasmic pH was decreased. When plasma membrane was prepared by a two-step method (Dextran gradient followed by a sucrose gradient), syringostatin, syringomycin and syringotoxin inhibited the H+-ATPase activity in a dose-dependent manner. In contrast, these toxins stimulated H+-ATPase activity when plasma membrane was prepared by a one-step method (sucrose gradient). While these toxins inhibited the H+-ATPase activity of inside-out plasma membrane vesicles, the H+-ATPase activity of right-side-out vesicles was stimulated. The detergent, Triton X-100, abolished this stimulatory effect of the toxins on the H+-ATPase of right-side-out vesicles and of one-step purified plasma membrane. The toxins also inhibited the activity of the plasma membrane H+-ATPase solubilized with deoxycholate and Zwittergent 3-14. Taken together, these results indicate that these toxins exert their effects partly by a detergent-like action on the plasma membrane and partly by inhibition of the enzyme.