The slow ATP-induced decrease in chlorophyll fluorescence associated with the phosphorylation of LHC II seen in spinach thylakoids was inhibited by GTP and its non-hydrolysable analogues β, γ-imidoguanosine 5′-triphosphate and guanosine 5′-O-(3-thiotriphosphate), but not by other nucleotide triphosphates or diphosphates. Inhibition by guanosine 5′-O-(3-thiotriphosphate) appeared to require prior exposure of the thylakoid membranes to the guanine nucleotide under conditions where the protein kinase was active but unable to turnover. Binding studies with thylakoid membranes using [5′,8-3H]GTP or [35S]guanosine 5′-O-(3-thiotriphosphate) show that under similar conditions an increase in specific binding of these radiolabelled nucleotides occurs. The data presented provide evidence for the existence of a guanine nucleotide-binding regulatory protein that is able to interact with the thylakoid protein kinase.
