The effect of solubilisation on the character of an ethylene-binding site from Phaseolus vulgaris L. cotyledons

The ethylene-binding site (EBS) from Phaseolus vulgaris cv. Canadian Wonder cotyledons can be solubilised from 96,000 g pelleted material by Triton X-100 or sodium cholate. Extraction of 96,000 g pellets with acetone, butanol or butanol and ether results in a total loss of ethylene-binding activity. Like the membrane-bound form, the solubilised EBS has an apparent K_D(liquid) of 10^-10 M at a concentration of 32 pmol EBS per gram tissue fresh weight. Propylene and acetylene act as competitive inhibitors, carbon dioxide appears to promote ethylene binding and ethane has no significant effect. The solubilised EBS is completely denatured affect. The solubilised EBS is completely denatured after 10 min at 70°C, by 1 mM mercaptoethanol and 0.1 mM dithiothreitol, but not by trypsin or chymotrypsin. However, solubilisation decreases the rate constant of association from 10³ M^-1 s^-1 to 10¹–10² M^-1 s^-1 and hence does not permit experimental determination of the rate constant of dissociation. The pH optimum for ethylene binding is altered from the range pH 7–10 in the membrane-bound form to the pH range 4–7 in the solubilised form. The EBS appears to be a hydrophobic, intergral membrane protein, which requires a hydrophobic environment to retain its activity. Partitioning of the EBS into polymer phases is determined by the detergent used for solubilisation indicating that when solubilised, the EBS forms a complex with detergent molecules.Abbreviations EBS ethylene-binding site - PEG polyethylene glycol