In Vitro Gibberellin A(1) Binding in Zea mays L

The first and second leaf sheaths of Zea mays L. cv Golden Jubilee were extracted and the extract centrifuged at 100,000g to yield a supernatant or cytosol fraction. Binding of [³H]gibberellin A₁ (GA₁) to a soluble macromolecular component present in the cytosol was demonstrated at 4°C by Sephadex G-200 chromatography. The binding component was of high molecular weight (HMW) and greater than 500 kilodaltons. The HMW component was shown to be a protein and the ³H-activity bound to this protein was largely [³H]GA₁ and not a metabolite. Binding was pH sensitive but only a small percentage (20%) appeared to be exchangeable on addition of unlabeled GA₁. Both biologically active and inactive GAs and non-GAs were able to inhibit GA₁ binding. [³H]GA₁ binding to an intermediate molecular weight (IMW) fraction (40-100 kilodaltons) was also detected, provided cytosol was first desalted using Sephadex G-200 chromatography. Gel filtration studies suggest that the HMW binding component is an aggregate derived from the IMW fraction. The HMW binding fraction can be separated into two components using anion exchange chromatography.