A soluble auxin‐binding protein from mung bean hypocotyls has indole‐3‐acetaldehyde reductase activity

Abstract

To clarify the roles of auxin‐binding proteins (ABPs) in the action of auxin, soluble auxin‐binding proteins were isolated from an extract of etiolated mung bean hypocotyls by affinity chromatography on 2,4‐dichlorophenoxyacetic acid (2,4‐D)‐linked Sepharose 4B. A 39‐kDa polypeptide was retained on the affinity column and eluted with a solution containing IAA or 2,4‐D, but not with a solution containing benzoic acid. The protein was then purified by several column‐chromatographic steps. The apparent molecular mass of the protein was estimated to be 77 kDa by gel filtration and 39 kDa by SDS‐PAGE. We designated this protein ABP39. The partial amino acid sequences of ABP39, obtained after chemical cleavage by CNBr, revealed high homology with alcohol dehydrogenase (ADH; EC 1.2.1.1). While the ABP39 was not capable of oxidizing ethanol, it did catalyze the reduction of indole‐3‐acetaldehyde (IAAld) to indole‐3‐ethanol (IEt) with an apparent Km of 22 μM. The IAAld reductase (EC 1.2.3.1) is specific for NADPH as a cofactor. The ABP39 also catalyzed the reduction of other aldehydes, such as acetaldehyde, benzaldehyde, phenylacetaldehyde and propionealdehyde. Indole‐3‐aldehyde was a poor substrate. The enzyme activity was inhibited by both indole‐3‐acetic acid and 2,4‐D in a competitive manner. Therefore, the enzyme is considered to be retained on the affinity column by recognition of auxin structure.