Comparison of the biological activity of fusicoccin in higher plants with its binding to plasma membranes.

The concentration dependences of the binding of fusicoccins (FCs) A, B, C, D, J and H to plasma membranes isolated from maize (Zea mays L.) roots have been studied in parallel with the effects of these compounds on elongation and (86)Rb transport in detached maize roots. The dissociation constants obtained showed a good correlation between the affinity of the FCs for the plasmalemma and their biological activity. However, the range of physiologically active FC concentrations proved to be about two orders of magnitude higher than that calculated from the dissociation constants. It was also shown that Vicia faba L. mesophyll protoplasts, unlike isolated plasma membranes, have two FC-binding sites, one with a K D similar to that of the isolated plasmalemma while the other has a substantially higher K D , apparently corresponding to the physiologically active state of the FC-binding proteins.