Cytokinin-Binding Proteins in Higher Plants

Abstract

In 1970 Matthysee and Abrams (13) described a cytokinin receptor protein from pea bud chromatin that, in the presence of an active cytokinin, increased RNA synthesis in an in vitro system (pea bud chromatin or DNA plus E. coli RNA polymerase). This stimulation of RNA synthesis could not be repeated by Berridge et al. (1, 2) who, however, described reversible, nonenzymatic binding of cytokinins to 80 S ribosomes from Brassica pekinensis and concluded that several binding sites, which could not be washed off with 0.5 M KCl, exist on the ribosome. These initial reports were not followed up by further studies until our own recent preliminary report (6) of a cytokinin-binding protein from higher plant ribosomes. Although we were able to confirm that cytokinins do indeed bind to higher plant ribosomes, the binding is much more complex than previously described. In addition to nuclear and ribosomal components, cytokinins have also been reported to interact with membrane-associated proteins. An allosteric effect of cytokinins on the binding of calcium to a glycoprotein on the membrane surface of fungal cells has been claimed (9, 10), and LeJohn recently reported (11) that this glycoprotein binds not only calcium, but also cytokinins, auxins, and the auxin analog tryptophane, and that the uptake of both auxins and tryptophane is regulated quite specifically by cytokinins.