An EPR study of the lateral organization of electron carriers in chloroplast thylakoids

Abstract

Recent evidence on photosynthetic electron transport indicates the organization of electron carriers into 3 supramolecular intrinsic complexes: photosystem I (PSI); photosystem II (PSII); and the cytochrome b6-f complex [l]. The complexes are linked by mobile electron carriers, plastoquinone and plastocyanin. Most higher plant chloroplasts also contain two different types of thylakoid membranes: appressed membranes of the grana partitions: and stroma-exposed, non-appressed thylakoids. It has been proposed that there is a lateral heterogeneity of two of the electrontransfer complexes among the different regions of the thylakoid membrane with PSI1 complex primarily located in appressed thylakoids and PSI complex mainly located in non-appressed stroma thylakoids [2-51. The chloroplast cytochrome b6-f complex has been reported to show a more even distribution along these two membrane regions ]6,71. known as signal II [lo]. The cytochrome b6--f complex has been analyzed using the EPR signal of the reduced Rieske iron-sulfur center [ 111. Our results support the view that the PSI and PSI1 complexes are mainly spatially separated in different membrane regions, while the cytochrome complex is present in both appressed and non-appressed membranes.