Binding properties of the plant photoreceptor phytochrome to membranes

Phytochrome (P), a chromoprotein of 120,000 MW, occurs at low concentrations in all higher plants. The chromophore is an open tetrapyrrole. The pigment exists in two light‐absorbing forms: Pr, which absorbs at 660 nm, and Pfr, which absorbs at 730 nm. These forms are interconvertible by light. Pr, the physiologically inactive form, exists in dark‐grown plants; Pfr, the active form, appears after irradiation with red light, P‐mediated responses, of which about 80 are known, range from short‐time effects (sec) such as bioelectric potentials, to long‐time effects (hr) such as increases in enzymatic activity. Measurements of phototransformation in vivo with polarized light suggest that P is localized in the plasma membrane. Particulate cell fractions contain about 70% of total extractable P if Pfr is present and only 4% if Pr is present. Evidence indicates that the fraction containing Pfr may be the plasma membrane. One can isolate a partially solubilized membrane system, which can be reversibly reconstituted by adding Mg. The reformed vesicles bind Pfr in vitro. Pfr binding increases with decreasing pH and decreases with increasing monovalent cation concentration. Pfr is released from the membrane by far red light (Pr is formed) and by Triton X‐100. We suggest that Pfr binding to a membrane induces conformational changes; the functional properties of this membrane are altered, which might lead to the observed phytochrome‐mediated responses.