Properties of a Solubilized Microsomal Auxin-binding Protein from Coleoptiles and Primary Leaves of Zea mays

An auxin-binding protein can be solubilized from microsomal membranes of Zea mays using either Triton X-100 extraction of the membranes or buffer extraction of the acetone-precipitated membranes. This paper describes the properties of the binding protein solubilized by these two methods. The binding is assayed by gel filtration chromatography in the presence of naphthalene [2-¹⁴C]acetic acid. Binding is rapid and reversible with an optimum at pH 5. Both preparations show similar molecular weights by gel filtration (80,000 daltons) at pH 7.6 and 0.1 molar NaCl, and both aggregate at low ionic strength. They appear to be the same active molecular species. The binding activity is destroyed by trypsin, pronase or para-chloromercuribenzoic acid, but not significantly reduced by phospholipase C, DNase, RNase, or dithioerythritol. Since saturating amounts of naphthalene acetic acid protect the molecule from inhibition by para-chloromercuribenzoic acid, it is concluded that the binding protein has a sulfhydryl group at the binding site, or protects such a group in its binding conformation. The dissociation constant of the protein for naphthalene acetic acid is 4.6 × 10⁻⁸ molar with 30 picomoles of sites per gram of tissue fresh weight. Binding constants were estimated for 13 other natural and synthetic auxins by competition with naphthalene[2-¹⁴C]acetic acid. Their dissociation constants are in general agreement with published values for their binding to intact membranes and their biological activity, although several exceptions were noted. A supernatant factor from the same tissue changes the apparent affinity of the protein for naphthalene acetic acid. This factor may be the same one as has been previously reported to alter the affinity of intact microsomes for auxin.