ADP and ATP binding to noncatalytic sites of thiol-modulated chloroplast ATP synthase

A modified 'cold chase' technique was used to study tight [(14)C]ADP and [(14)C]ATP binding to noncatalytic sites of chloroplast ATP synthase (CF(0)F(1)). The binding was very low in the dark and sharply increased with light intensity. Dissociation of labeled nucleotides incorporated into noncatalytic sites of CF(0)F(1 )or CF(1) reconstituted with EDTA-treated thylakoid membranes was also found to be light-dependent. Time dependence of nucleotide dissociation is described by the first order equation with a k (d) of about 5 min(-1). The exposure of thylakoid membranes to 0.7-24.8 muM nucleotides leads to filling of up to two noncatalytic sites of CF(0)F(1). The sites differ in their specificity: one preferentially binds ADP, whereas the other - ATP. A much higher ATP/ADP ratio of nucleotides bound at noncatalytic sites of isolated CF(1) dramatically decreases upon its reconstitution with EDTA-treated thylakoid membranes. It is suggested that the decrease is caused by conformational changes in one of the alpha subunits induced by its interaction with the delta subunit and/or subunit I-II when CF(1) becomes bound to a thylakoid membrane.