Soybean 4-kDa peptide, a hormone-like peptide, is a ligand for the 43-kDaprotein in legumes that functions as aprotein kinase and controls cell proliferation and differentiation. As this peptide stimulates proteinkinase activity, the interaction between the 4-kDa peptide (leginsulin) and the 43-kDa protein is considered important for signal transduction. However, the mechanism of interaction between the 4-kDa peptide and the 43-kDa protein is not clearly understood.