The photoreversible association of phytochrome with cellular membranes.

The characteristics of the repeatedly red/far-red light reversible association of phytochrome with membranes have been examined in etiolated Avena coleoptiles. This binding response, referred to as Association I, has been compared with the widely studied light-induced enhancement of phytochrome pelletability that is dependent on exogenous divalent cations (Association II). Association I is shown to be hydrophobic and pH independent, in contrast to Association II which shows a marked dependence on pH. Additionally Association I is Insensitive to a wide range of extraction conditions. By extracting In the presence of phenylmethylsulphonyl fluoride the possibility that proteolysis of Pr (the red light absorbing form of phytochrome) could account for the lower levels of phytochrome bound as Pr was discounted. Difference spectra of soluble and Association-I-bound phytochrome indicate that bound phytochrome is much less susceptible to spectral degradation than soluble phytochrome. Dose-response studies show that Association I obeys the Law of Reciprocity for irradiation times of up to 600s. Association I has a stoichiometry such that for each intrinsically- bound (dark-bound) phytochrome molecule three are bound after saturating red light. At saturation Association I comprises 7.5% of the total phytochrome. For subsaturating light treatments the occupancy of the PFr binding sites varies linearly with the tissue Pfr concentration. It is shown that the intrinsically-bound phytochrome controls the receptor sites. Light, therefore, controls both the number of ligands and the number of binding sites. Pr binding may be explained on the basis of phytochrome dimers. Preliminary investigations using saturating monochromatic irradiations have shown that a simple model with Pfr binding and Pr release is incomplete. The data can, however, be satisfactorily accounted for by small refinements to the model. The data are discussed in terms of the criteria that should be met by a physiologically significant binding response. The data are consistent with a scheme in which Pfr binds to a specific membrane- bound receptor site.