Permeability and Structural Changes Induced by Phytochrome in Lipid Vesicles (original) (raw)

This paper describes a method for rapidly monitoring early changes in electrolyte permeability induced by phytochrome from salt-loaded liposomes. The method allows for the continuous monitoring of low-level ion efflux from liposomes by measuring the conductivity of a liposome suspension medium which has osmotic and chemical potentials that promote a slow. passive efflux of the compartmented electrolytes. The addition of the far-red absorbing form of phytochrome (Pfr) to this system at 20°C immediately producesefflux rates which are 2-3 times greater than if the red-absorbing form (Pr) is added. This differential effect is not evident at 4°C and varies with the lipid composition of the liposomes. Under conditions in which Pfr induces a '-fold greater change in the electrolyte permeability of liposomes than Pr. only about 18% more "'I-labeled Pfr than 1251-labeled Pr binds to the liposomes. At equimolar concentrations. the photochromic small peptide of phytochrome (60 000 dalton monomer) and the more native 'large' phytochrome (120000 dalton subunits) induced equivalent changes in the electrolyte permeability of liposomes. N o differential leakage of ATP, glucose, or trvpsin from liposomes was observed after Pr and Pfr reacted with vesicles enclosing these substances. The Pfr form of phytochrome promoted greater turbidity in liposome suspensions and a greater degree of aggregation and/or vesiclc fusion than Pr. The kinetics of these changes suggested that they were not the hasis ot the differential permeability effects of Pr and Pfr.