Purification and Properties of a Protein Which Binds Cytokinin-active 6-Substituted Purines

A protein which binds 6-substituted purines of the cytokinin type with relatively high affinity has been extensively purified from wheat germ. Conventional chromatographic techniques, as well as an affinity matrix to which a cytokinin was covalently coupled, were used in the purification. The wheat germ cytokinin-binding protein (CBF-1) has four unlike subunits and an apparent molecular weight of 183,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.CBF-1 is saturated at one cytokinin molecule per tetramer with a K(d) for 6-benzylaminopurine of 5 x 10(-7) molar. The protein exists both on the native wheat germ ribosome (1 molecule CBF-1 per 80S ribosome) and free in the cytosol with approximately three copies of the latter for each of the former. Data from affinity chromatography studies and cross-linking experiments strongly suggest that a specific binding site for CBF-1 occurs on the wheat germ ribosome.