Co-purification of Pea and Bean Leaf Soluble Auxin-binding Proteins with Ribulose-1,5-Bisphosphate Carboxylase

Soluble auxin-binding proteins (ABPs) were purified to constant specific activity from bean and pea leaves by a procedure involving (NH₄)₂SO₄ fractionation, anion exchange chromatography and gel filtration. Pea and bean ABPs exactly co-purify with ribulose-1,5-bisphosphate carboxylase (RuBPCase) in a variety of chromatographic separation procedures. The subunit compositions, electrophoretic purities and indole-3-acetic acid (IAA)-binding stoichiometries of the purified ABPs provide further evidence for the identity of RuBPCase and ABP. Pea ABP and bean ABP have dissociation constants for IAA of 0.8 and 1.3 micromolar, respectively, as determined by an (NH₄)₂SO₄ precipitation assay for IAA-binding to insolubilized ABP. IAA can bind to soluble bean and pea ABP (RuBPCase) as determined by equilibrium dialysis with affinities and stoichiometries similar to those determined for insolubilized ABP.