Fluorescent anticytokinins as a probe for binding. Isolation of cytokinin-binding proteins from the soluble fraction and identification of a cytokinin-binding site on ribosomes of tobacco callus cells.

4-Substituted 2-methylthiopyrido[2,3-d]pyrimidines, a series of recently developed anticytokinins, have been found to fluoresce strongly in water and to be useful as probes for binding studies. The binding activity of the soluble proteins and particulate fraction of tobacco callus cells to the biologically most active member of the family, 4-n-butylamino-2-methylthiopyrido[2,3-d]pyrimidine (BAMPP), was studied fluorimetrically. We found that the binding activity is better monitored in terms of saturable binding rather than in terms of the amount of bound ligand, a conventional method used in isolation studies of hormone receptor proteins. Using this technique we isolated two kinds of high-affinity cytokinin-binding proteins from the soluble fraction and identified a high-affinity binding site on ribosomes.