Auxin-Modulated Protein Disulfide-Thiol-Interchange Activity from Soybean Plasma Membranes

The renaturation of scrambled (oxidized and inactive) RNase A is catalyzed by soybean (Glycine max cv Williams 82) plasma membranes. The catalysis is stimulated by the auxin herbicide 2,4-dichlorophenoxyacetic acid or by the natural auxin indole-3-acetic acid. The inactive auxin analog, 2,3-dichlorophenoxyacetic acid, is without effect. The activity occurs in the absence of external electron acceptors or donors and therefore appears to be a true disulfide-thiol-interchange activity between protein disulfides and thiols of RNase A and those of plasma membrane proteins. The activity is not affected by a mixture of reduced and oxidized glutathione. However, no auxin-stimulated activity was observed in the presence of either oxidized glutathione or reduced glutathione alone, a response characteristic of the previously described auxin-stimulated NADH oxidase activity of soybean plasma membranes. Taken together, the results suggest the operation in the plant plasma membrane of a protein disulfide-thiol-interchange activity that is stimulated by auxins. The auxin stimulations of the interchange activity are prevented by glutathione, reduced glutathione, and brefeldin A at concentrations that also prevent auxin stimulation of NADH oxidation by isolated plasma membranes and inhibit, as well, the auxin-stimulated elongation of excised segments of soybean hypocotyls.