Binding of 124-kilodalton oat phytochrome to liposomes and chloroplasts

Binding of the radio-iodinated 124-kDa oat (Avena sativa L. cv. Garry) phytochrome to liposomes and chloroplasts was investigated as a model system in order to understand the molecular affinity of phytochrome toward cellular organelles in plants. The binding of intact (124 kDa) phytochrome to liposomes and chloroplasts is hydrophobic in nature, as in the case of the degraded (118/114 kDa) phytochrome, but electrostatic interactions play a greater role in the intact phytochrome. The physiologically active Pfr form of the intact phytochrome showed a binding preference over the inactive Pr form with neutral liposomes and chloroplasts. However, the Pfr form of intact phytochrome exhibits smaller binding preference than the Pfr form of degraded phytochrome over their respective Pr forms (see Kim, I.-S. and Song, P.-S. 1981, Biochemistry 20: 5482–5489, for degraded phytochrome binding). These results indicate that the 6/10 kDa N-terminus segment, which is lost in the degraded phytochrome, plays an important role in determining the protein surface properties of the intact phytochrome. A competitive binding study on phytochrome also suggested that the Pfr form had a greater binding affinity for chloroplasts than the Pr form. However, the physiological activity of the Pfr form may not be explained simply by the observed difference in binding affinity between the two forms of phytochrome.