Isolation and characterization of a cytokinin-binding protein from etiolated maize seedlings

A cytokinin-binding protein (CBP) was isolated from etiolated maize (Zea mays L.) seedlings by two protocols. Ammonium sulfate fractionation, hydrophobic chromatography on Toyopearl HW-60, and affinity chromatography on immobilized trans-zeatin were used for CBP isolation according to the first protocol. The second protocol included a combination of ion-exchange chromatography and gel filtration followed by affinity chromatography on zeatin riboside-Toyopearl as in the first protocol. In order to remove proteins that could be nonspecifically bound to the affinity matrix, chromatography on adenosine-Toyopearl preceded chromatography on zeatin riboside-Toyopearl. Following these procedures, a CBP with a molecular weight of 70 kD was isolated. This protein specifically and reversibly bound dihydrozeatin and, in the presence of trans-zeatin, activated in vitro RNA synthesis in a system containing chromatin-bound RNA polymerase I from barley leaves. The protein cross-reacted with anti-idiotypic antibodies isolated from antizeatin serum, and, therefore, these antibodies could be considered antibodies to zeatin-binding protein. The data thus obtained indicate that the isolated CBP is a cytokinin receptor.