Characterization of Naphthaleneacetic Acid Binding to Receptor Sites on Cellular Membranes of Maize Coleoptile Tissue

Abstract

Characteristics of and optimum conditions for saturable (specific) binding of [14C]naphthaleneacetic acid to sites located on membranous particles from maize (Zea mays L.) coleoptiles are described. Most or all of the specific binding appears to be due to a single kinetic class of binding sites having a Kd of 5-7 .times. 10-7 M for naphthalene-1-acetic acid (NAA). Binding of NAA is insensitive to high monovalent salt concentrations, indicating that binding is not primarily ionic. Specific binding is inhibited by Mg2+ or Ca2+ above 5 mM. Specific binding is improved by organic acids, especially citrate. Binding is heat-labile and is sensitive to agents that act either on proteins or on lipids. Specific binding is reversibly inactivated by reducing agents such as dithioerythritol; a reducible group, possibly a disulfide group, may be located at the binding site and required for its function. The affinity of the specific binding sites for auxins is modified by an unidentified dialyzable, heat-stable, apparently amphoteric, organic factor (supernatant factor) found in maize tissue.