Electrostatic control of chloroplast coupling factor binding to thylakoid membranes as indicated by cation effects of electron transport and reconstitution of photophosphorylation
Telfer, A.; Barber, J.; Jagendorf, A.T.
Biochimica et Biophysica Acta 591(2): 331-345
ISSN/ISBN: 0006-3002 PMID: 6772211 Accession: 005349998
Increase in electron transport rate and the decay rate of the 518 nm absorption change [pea plant chloroplasts] induced by EDTA treatment, is prevented by cations. The order of effectiveness is C3+ > C2+ > C+. In this respect methyl viologen is an effective divalent cation in addition to its action as an electron acceptor. Complete cation irreversible EDTA-induced uncoupling occurs in the dark in 2 min. Light greatly stimulates the rate of uncoupling by EDTA. The uncoupling is due to release of coupling factor I (CF1) from the thylakoid membrane. Binding of purified CF1 to CF1 depleted thylakoids can be achieved with any cation. The order of effectiveness is C3+ > C2+ > C+, reconstituted thylakoids are active in photophosphorylation regardless of the cation used for CF1 binding. The marked difference in the concentration requirements for cation effects on 9-aminoacridine fluorescence yield and for prevention of uncoupling by EDTA indicate that CF1 and its binding site have a lower surface charge density than the net surface charge density of the thylakoid membrane. It is concluded that CF1 binding only occurs when negative charges on CF1 and its binding site are electrostatically screened by cations. Previously reported examples of uncoupling by low ionic conditions are discussed in relation to the basic concepts of diffuse electrical layer theory.