Binding of phytochrome to liposomes and protoplasts.
The physiologically active form of oat phytochrome, Pfr, increases its binding to egg lecithin unilamellar liposomes with increasing ionic strength of the medium while the binding of Pr is almost constant. The preferential binding of Pfr is as much as twice that of Pr at KCl concentrations of above 0.2 M, in 0.1 M phosphate buffer (pH 7.2) at 27 degrees C. The binding of phytochrome to liposomes is also enhanced by approximately 80% at 27 degrees C compared to that at 3 degrees C. Thus, it appears that the binding between Pfr phytochrome and liposomes is hydrophobic in nature whereas the binding of Pr is not predominantly through hydrophobic interactions. The binding of both Pfr and Pr to multilamellar liposomes increases with increasing cholesterol content in the liposomes. The extent of phytochrome's binding is higher in the neutral pH region than above pH 7.5. It takes several hours to reach an equilibrium of binding. The photoreversion of liposome-bound Pfr is inhibited by 40% compared to that of free Pfr, while the phototransformation of liposome-bound Pr to Pfr is promoted by 30%. The rate of dithionite-accelerated dark reversion of liposome-bound Pfr is lower by 50% than that of the free form. These results are consistent with the proposal that the hydrophobic binding site involved results from a vacancy produced by the reorientation or displacement of the Pfr chromophore from the protein. Upon binding to phytochrome, unilamellar liposomes undergo fusion to form larger diameter liposomes. No preferential binding of the Pfr form was found with intact oat protoplasts in vitro.