In vivo Binding of Gibberellin A1 in Dwarf Pea Epicotyls 1

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Binding of [3H]gibberellin A1 (GA1) to extracts of dwarf pea epicotyls was investigated using sliced pea epicotyls (0.5-1.0 millimeter thick) that had been incubated in a solution containing [3H]GA1 at 0 C for 3 days. Gel filtration of a 100,000g supernatant indicated binding to a high (HMW) and an intermediate molecular weight (IMW) fraction with estimated molecular weights of 6 × 105 daltons and 4 to 7 × 104 daltons, respectively. The bound 3H-activity was [3H]GA1 and not a metabolite as deduced by thin layer chromatography. The bound label did not sediment during centrifugation at 100,000g for 2 hours; also, binding was not disrupted after treatment of a combined HMW and IMW fraction with DNase, RNase, or phospholipase A or C, but it was disrupted by protease or heat treatment. These facts suggest that binding of [3H]GA1 was occurring to a soluble protein(s). [3H]GA1 bound to a combined HMW and IMW fraction was not susceptible to changes in pH, nor could it be exchanged with a variety of GAs tested under in vitro conditions. Under in vivo equilibrium conditions, biologically active GAs, such as GA1, GA3, GA4, GA5, GA7, and keto GA1, could reduce the level of [3H]GA1 binding, whereas inactive GAs, such as iodo GA1 methyl ester, GA8, GA13, GA26, and non-GAs, such as (±)abscisic acid, had no effect. By varying the concentration of [3H]GA1 in the incubation medium, the specific binding of [3H]GA1 appeared to be due to two classes of binding sites having estimated Kd of 6 × 10−8 molar and 1.4 × 10−6 molar. The concentrations of the two sites were estimated to be 0.45 picomole per gram and 4.04 picomoles per gram on a fresh weight and 0.1 picomole per milligram and 0.9 picomole per milligram on a soluble protein basis, respectively.

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