PERMEABILITY AND STRUCTURAL CHANGES INDUCED BY PHYTOCHROME IN LIPID VESICLES

Light-induced import of the chromoprotein, phytochrome, into mitochondria

Mitochondria extracted from plants that were irradiated with actinic light in vivo have associated with them the chromoprotein, phytochrome. This phytochrome retains its native subunit size of 124 kDa after proteolytic treatment of the mitochondria with trypsin and chymotrypsin. This result suggests that phytochrome is not exposed on the outer surface of the outer mitochondrial membrane. Phytochrome, so protected, is not found to be associated with mitochondria derived from unirradiated plants. The possibility that the photoactivation of phytochrome induces a conformational change in its structure which facilitates its transport into the mitochondrion is discussed.

Bluelight-induced, flavin-mediated transport of redox equivalents across artificial bilayer membranes

This paper continues our studies of physico-chemical properties of vesicle-bound flavins. Based on previous results, an advanced model system was designed in order to study the mechanisms underlying bluelight-induced redox transport across artificial membranes. The lumen of single-shelled vesicles was charged with cytochrome c, and amphiphilic flavin (AF1 3, AF1 10) was bound to the membrane. Upon bluelight irradiation redox equivalents are translocated from exogeneous 1e-(EDTA)-and 2e-(BH3CN-) donors across the membrane finally reducing the trapped cytochrome c both under aerobic and anaerobic conditions. The mechanisms involved are explored and evidence for the involvement of various redox states of oxygen, dihydroflavin and flavosemiquinone is presented.

The role of calcium ions in phytochrome-mediated germination of spores of Onoclea sensibilis L

Phytochrome is confirmed to be the photoreceptor pigment in the germination response of Onoclea sensibilis L. by demonstrating red-far-red (R-FR) photoreversibility. External Ca(2+) is required for this response with a threshold at a submicromolar concentration. Ethylene glycol-bis(β-amino-ethyl ether)-N,N,N',N'-tetraacetic acid, La(3+) and Co(2+) reversibly inhibit germination. Lanthanum only inhibits germination when applied before or during irradiation, indicating that the external Ca(2+) requirement is transient, although in the absence of Ca(2+) the R-stimulated system remains maximally poised to accept the ion for over 4 h after irradiation. The ability to respond to Ca(2+) 4.1 h after R-irradiation is not reversed by FR-irradiation, indicating that Ca(2+) transport has been uncoupled from phytochrome. Barium and Sr(2+), but not Mg(2+) can substitute for Ca(2+). Artificially increasing the concentration of intracellular free Ca(2+) with the ionophore A 23187 stimulates germination in the dark. The Ca(2+)-calmodulin antagonists, trifluoperizine and chlorpromazine, reversibly inhibit germination. Calcium is required in phytochrome-mediated fern spore germination; it may be acting as a second messenger.