A gibberellin A (GA)-binding protein was identified from rice (Oryza sativa L.) leaves by a ligand-binding assay. The dissociation constant of GA-binding protein and GA complex was about 100 nM. This protein has a relative molecular mass of 47 000 and an isoelectric point of 5.1. The partial amino acid sequence of the protein was determined for 54 residues from both the N-terminal and internal regions. A sequence homology search indicated that the amino acid sequence of GA-binding protein was homologous to that of the ribulose-1,5-bisphosphate carboxylase/oxygenase activase from barley, Arabidopsis, spinach and Chlamydomonas. The GA-binding protein was immunologically detected in two polypeptides in the protein extract from leaves. The GA-binding protein identified was phosphorylated with Ca2+, Mg2+ and ATP in the leaf protein extracts of rice grown in the presence of exogenous GA.
