Use of zinc ions to study thylakoid protein phosphorylation and the state 1-state 2 transition in vitro

At ATP concentrations less than 0.2 millimolar, zinc ions cause a marked stimulation of endogenous protein phosphorylation in thylakoid membranes isolated from tobacco (Nicotiana tabacum L. cv Turkish Samsun), pea (Pisum sativum L. cv Feltham First) and spinach (Spinacia oleracea L. cv Northland). The greatest stimulatory effect was observed at Zn²⁺ concentrations of 1 to 2 millimolar; higher concentrations were inhibitory. The stimulatory effect of Zn²⁺ was independent of Mg²⁺ concentration from 1 to 5 millimolar and thus does not appear to be due to the formation of a Zn²⁺ -ATP complex. Phosphorylation of histones IIA, an exogenous protein substrate, was inhibited by 2 millimolar Zn²⁺. At low levels of ATP, Zn²⁺ not only stimulates general endogenous protein phosphorylation, but also the phosphorylation of the apoproteins of the light-harvesting chlorophyll a/b-protein complex. However, under these conditions Zn²⁺ inhibits the ATP-induced quenching of photosystem II fluorescence and the increase in the ratio of photosystem I to photosystem II fluorescence which are both characteristic of the State 1-State 2 transition. These results suggest that phosphorylation of the light-harvesting chlorophyll a/b-protein complex may not directly bring about the State 1-State 2 transition.