Delayed light studies on photosynthetic energy conversion. VIII. Evidence from millisecond emission of chloroplasts for two adenylate binding sites on membrane-bound coupling factor, CF1

Measurements of K+-driven Cl- uptake in thylakoids: inhibition of uptake by antibodies raised to the major polypeptides of the Cl(-)-efflux active particle(s)

The mechanism of a K+-driven Cl- accumulation against a concentration gradient was investigated by flow dialysis after addition of K+-Hepes. Non-specific chloride binding, measured in the presence of choline-Hepes, accounted for approximately 50% of the observed uptake in this system. The K+-Hepes-driven Cl- uptake was inhibited by poly-l-lysine and by two antibodies raised to the major polypeptides of the Cl(-)-efflux active particle. Poly-l-lysine had no effect on Cl- binding estimated with choline-Hepes.

Anion (and cation) requirements of the coupled electron flow in spinach thylakoids

Proton uptake as well as coupled electron flow of chloroplasts swollen in dilute impermeable buffers became dependent upon the addition of exogenous permeable anions. This dependence was observed with both cyclic and noncyclic electron acceptors, suggesting that this anion requirement is associated with the electrogenic proton uptake step rather than with the oxygen-evolving reactions of photosystem II.

Isolation of protein(s) containing chloride ion transport activity from thylakoid membranes

Extracts of detergent-treated thylakoids, when reconstituted into azolectin/cholesterol/dicetyl phosphate vesicles, stimulate chloride ion efflux as measured with a Cl--sensitive electrode. This stimulation is inhibited by piretanide, an active chloride ion transport inhibitor in fish intestinal epithelia. This stimulation is also abolished by pretreatment of extracted proteins with trypsin. Antiserum raised to efflux-active proteins inhibits a cation-driven Cl- influx in nonenergized thylakoids, as measured by a flow dialysis technique.

Chloride ion transport and its inhibition in thylakoid membranes

Cl- translocation across energized and nonenergized thylakoid membranes was found to be inhibited by piretanide, an inhibitor of active Cl- transport in fish intestinal epithelia. Piretanide has no effect on photophosphorylation catalyzed by phenazine methosulfate or on Ca2+-dependent ATPase activity of isolated chloroplast coupling factor (CF1). Light-dependent Cl- uptake, contrary to H+ uptake, is severalfold greater at pH 8.0 than at pH 6.7.

AMP photophosphorylation and binding by chloroplasts

Stoichiometric amounts of chloroplast thylakoids photophosphorylate free AMP to tightly bound ADP. Free ADP is a poor competitor for this AMP photoreaction, which saturates below 16 micronAMP. The inhibitor, diadenosine pentaphosphate, abolishes AMP photophosphorylation, and inhibits dark ADP binding. Taken together, these data imply that this photoreaction involves the high affinity nucleotide binding site(s) of chloroplast coupling factor CF1, and that little mixing with free nucleotides occurs.

Energy-dependent exchange of adenine nucleotides on chloroplast coupling factor (CF1)

1. [14C]ADP is incorporated into washed broken chloroplasts in the light. The bound labelled nucleotides which cannot be removed by washing are almost exclusively related to coupling factor CF1. [14C]ADP binding exhibits a monophasic concentration curve with a Km of 2 micronM. 2. By illumination of the chloroplasts, previously incorporated labelled nucleotides are released. A fast release is obtained in the presence of unlabelled ADP and ATP, indicating an energy-dependent exchange. A slow and incomplete release is induced by light in the absence of unlabelled adenine nucleotides. Obviously, under those conditions, an adenine nucleotide depleted CF1 conformation is established. 3. Re-binding of [14C]ADP by depleted membranes is an energy-independent process. Even after solubilization of adenylate-depleted CF1, [14C]ADP is incorporated into the protein. By re-binding of ADP in the dark, CF1 is converted to a non-exchangeable form. 4. Energy-dependent adenine nucleotide exchange on CF1 is suggested to include three different conformational states of the enzyme: (1) a stable, non-exchangeable form which contains firmly bound nucleotides, is converted to (2), an unstable form containing loosely bound adenine nucleotides. This conformation allows adenylate exchange; it is in equilibrium with (3) a metastable, adenylate-depleted form. The transition from state (1) to state (2) is the energy-requiring step.

Adenylate regulation of photosynthetic electron transport and the coupling sites of phosphorylation in spinach chloroplasts

The regulation by adenylates of activities of various partial electron transport systems in spinach chloroplasts was studied using systems from H2O to 2,5-dimethyl-p-benzoquinone, H2O to 2,6-dichlorophenolindophenol, reduced 2,6-dichlorophenolindophenol to methyl viologen, and H2O to methyl viologen or ferricyanide. Adenylates regulated all of them. The ratio of the amount of esterified Pi (P) to that of electrons transported (Deltae) in coupling with phosphorylation manifested that there are two phosphorylation sites: one between H2O and 2,5-dimethyl-p-benzoquinone or 2,6-dichlorophenolindophenol and another between reduced 2,6-dichlorophenolindophenol and methyl viologen, under the proposed stoichiometries, i.e., P/DeltaH+ = 0.5 and DeltaH+/Deltae = 1, where DeltaH+ is the amount of protons pumped by electron transport (= those translocated during phosphorylation), when the basal electron transport (the part not regulated by adenylates) was excluded. The effects of pH, phlorizin, and methylamine on the adenylate regulation of electron transport, and the stimulation profile of electron transport coupled with quasiarsenylation suggested no distinction between the two phosphorylation sites.