ORGANIZATION OF FUSICOCCIN RECEPTOR IN HIGHER-PLANT PLASMA-MEMBRANE -RELATIONSHIP BETWEEN AFFINITY AND MOLECULAR-MASS

Higher plant plasma membranes carry receptors of different affinity fo r the phytotoxin fusicoccin. Reception of fusicoccin involves proteins belonging to the highly conserved 14-3-3 family, but the complete str ucture of the fusicoccin receptor (FCR) is unknown. Using radiation in activation analysis, we estimated the molecular masses of low-affinity and high-affinity FCR at 63 +/- 7 and 130 +/- 15 kD, respectively. Th e dose dependences of receptor inactivation indicate that microsomal s pecimens contain ''silent'' FCRs of 420 +/- 90 kD in amounts commensur ate with that of the active FCRs. Both low- and high-affinity FCRs are inactivated by hydrolytic enzymes from the outer surface of the plasm a membrane and impairment of protoplast integrity causes an irreversib le transition of the low-affinity binding site into the high-affinity one. A scheme is proposed for the organization of different types of F CR in the plasma membrane, implying that the membrane affinity for fus icoccin reflects the interaction between proteins in the FCR complex.