ADP/ATP and protein phosphorylation dependence of phototransformable protochlorophyllide in isolated etioplast membranes

The effects of modulated ADP/ATP and NADPH/NADP(+) ratios, and of protein k inase inhibitors, on the in vitro reformation of phototransformable protoch lorophyllide, i.e. the aggregated ternary complexes between NADPH, protochl orophyllide, and NADPH-protochlorophyllide oxidoreductase (POR, EC 1.3.1.33 ), in etioplast membranes isolated from dark-grown wheat (Triticum aestivum ) were investigated. Low temperature fluorescence emission spectra (-196 de greesC) were used to determine the state of the pigments. The presence of s pectral intermediates of protochlorophyllide and the reformation of phototr ansformable protochlorophyllide were reduced at high ATP, but favoured by h igh ADP. Increased ADP level partly prevented the chlorophyllide blue-shift . The protein kinase inhibitor K252a prevented reformation of phototransfor mable protochlorophyllide without showing any effect on the chlorophyllide blue-shift. Addition of NADPH did not overcome the inhibition. The results indicate that protein phosphorylation plays a role in the conversion of the non-phototransformable protochlorophyllide to POR-associated phototransfor mable protochlorophyllide. The possible presence of a plastid ADP-dependent kinase, the activity of which favours the formation of PLBs, is discussed. Reversible protein phosphorylation is suggested as a regulatory mechanism in the prolamellar body formation and its light-dependent dispersal by affe cting the membrane association of POR. By the presence of a high concentrat ion of phototransformable protochlorophyllide, prolamellar bodies can act a s light sensors for plastid development. The modulation of plastid protein kinase and protein phosphatase activities by the NADPH/NADP(+) ratio is sug gested.