Gibberellic acid-Binding proteins from pea stems

The formation of complexes of gibberellic acid (GA3) and proteins under in vitro conditions was studied. It was shown that labelled GA3 binds to soluble cytoplasmic proteins, although a considerable amount of radioactivity remains in the pellet containing nuclei and cell debris. GA3-protein complexes are excluded from Sephadex G-10 column with the void volume. They sediment in linear sucrose density gradients as three distinct peaks, having higher S values than bovine serum albumin, used as a marker. Soluble GA3-protein complexes can be separated into four zones of radioactivity upon ion exchange chromatography on DEAE-Sephadex A-50 column, each of them eluting with a different KCl concentration. Agarose gel electrophoresis of GA3-protein complexes reveals two zones of radioactivity at the anodic part of the electrophoretogram. After extraction of the complex with ethanol, more than 90% of radioactivity is found in the ethanolic phase, which indicates that the binding is not covalent. GA4+7 and GA13 decrease the binding of GA3 to cytoplasmic proteins for 30%, suggesting that some common binding sites exist at the same binding proteins.