Solubilization of the receptor for N-1-naphthylphthalamic Acid

A receptor protein for the auxin transport inhibitor, N-1-naphthylphthalamic acid (NPA), has been solubilized from corn coleoptile membranes using Triton X-100. [(3)H]NPA binding activity of the receptor was compared in soluble and membrane-bound states. Both activities are abolished by treatment with trypsin. Differences between the two are observed in pH optima and rates of heat inactivation.At pH 5, the membrane-bound and solubilized receptors have similar affinity for NPA (K(d) about 10(-7) molar). Solubilization results in a 10-fold increase in affinity for the low-affinity auxin analogs, alpha- and beta-naphthaleneacetic acid (K(d) about 10(-5) molar). There is no measurable competition by indoleacetic acid (K(d) > 10(-3) molar) for [(3)H]NPA binding to membranes, but there is competition with the solubilized receptor (K(d) for IAA about 10(-5) molar). There is no measurable competition with benzoic acid in either preparation. These observations of an affinity of auxin analogs for the solubilized NPA receptor raise the interesting possibility that the NPA binding site is one conformation of an auxin binding site involved in polar auxin transport.