Abstract
The role of cytokinin-binding proteins (CBPs) in hormone signal transduction remains unclear. A recent work by Mitsui et al. [1], based on cDNA amplification and sequence analysis, suggested that one of the subunits of a tobacco CBP complex might correspond to the enzyme S-adenosyl- l-homocysteine hydrolase (SAH hydrolase). We checked by direct biochemical assays whether the maize CBPs possess such an enzymatic activity. Using different types of fractionation and purification procedures, we showed that in extracts of maize shoots the major cytokinin-binding and SAH hydrolase activities can easily be separated by fast protein liquid chromatography (FPLC). On the other hand, no influence of cytokinins (dihydrozeatin and 6-benzyladenine) on the activity of maize SAH hydrolase was observed in vitro. From our results, it seems unlikely that the previously characterized 46-kDa major maize CBP possesses SAH hydrolase activity, indicating that any role it may have in the mechanism of hormone signal transduction does not involve methylation reactions.
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